Hi You probably could put a couple of cheap DAC's on a board with a FPGA and reduce the data on the fly. I'd guess that would be be in the same $100 range as a half way decent sound card. Clock the DAC's off of a 10 MHz reference and eliminate the cal issue. If you are down around 10 Hz or worse yet 1 Hz, the AC coupling of the sound card will get in the way, even with a bandpass approach. You really don't know what they may have in there at the low end. Build it yourself and that stuff's not an issue. Bob On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: > If one has a high end sound card then it could be used to implement the bandpass filter and replace the zero crossing detector. > It may be necessary to insert a pilot tone to calibrate the sound card sampling clock frequency. > A noise floor of about 1E-13/Tau should be achievable. > This simplifies the DMTD system by replacing the zero crossing detector with a low gain linear preamp. > > If one analyses the resultant data off line then one can also try out different techniques such as a Costas receiver rather than a simple bandpass filter plus zero crossing detector. > However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS will result in a file with a size of at least 1.15GB. > > Bruce > > > Bruce Griffiths wrote: >> If one were to use a bandpass filter with a Q of 10 to filter the beat frequency output of the mixer, then if the input frequency is 10MHz and the filter component tempco is 100ppm/C then the resultant phase shift tempco is about 16ps/C referred to the mixer input frequency. >> >> This phase shift tempco is certainly low enough not to have significant impact when measuring the frequency stability of a typical 10811A if the temperature fluctuations are kept small enough during the run. >> >> The effect of using a bandpass filter with too narrow a bandwidth is to artificially reduce ADEV for small Tau, so it may be prudent to use a higher beat frequency that 1Hz or even 10Hz and not calculate ADEV for Tau less than say 10(??) times the beat frequency period. A trade off between this and the effect of aliasing is required. >> >> Bruce >> >> Bob Camp wrote: >>> Hi >>> >>> With most 10811 range oscillators the impact of a simple bandpass filter is low enough to not be a major issue. That's for normal lab temperatures with the circuitry in a conventional die cast box. No guarantee if you open the window and let the fresh air blow in during the run. >>> >>> That's true with a heterodyne. I can see no obvious reason it would not be true on DMTD. >>> >>> Bob >>> >>> >>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: >>> >>>> The only major issue with DMTD systems is that they undersample the phase fluctuations and hence are subject to aliasing effects. >>>> The low pass filter has to have a bandwidth of the same order as the beat frequency or the beat frequency signal will be significantly attenuated. >>>> Since the phase is only sampled once per beat frequency period the phase fluctuations are undersampled. >>>> Various attempts to use both zero crossings have not been successful. >>>> >>>> In principle if one can overcome the increased phase shift tempco associated with a bandpass filter, using a bandpass filter can in principle ensure that the phase fluctuations are oversampled. >>>> >>>> >>>> Bruce >>>> >>>> Bob Camp wrote: >>>>> Hi >>>>> >>>>> A straight heterodyne system will get you to the floor of most 10811's with a very simple (2 stage) limiter. As with the DMTD, the counter requirements aren't really all that severe. >>>>> >>>>> Bob >>>>> >>>>> >>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >>>>> >>>>> >>>>>> >>>>>>> "It's possible / likely for injection lock ... to be a problem ..." >>>>>>> >>>>>> Something I certainly worried about and tested for. >>>>>> What I found (for MY case) is that injection lock is NOT a problem. >>>>>> The reason being is that unlike most other ways, where the two OSC have to be completely independent, >>>>>> The tight loop approach forces the Two Osc to "Lock with something like 60 + db gain, >>>>>> so a little stray -80db injection lock coupling that would very much limit other systems has >>>>>> no measurable effect at e-13. Just one of the neat little side effects that make the tight loop approach so simple. >>>>>> >>>>>> >>>>>>> "then a part in 10^14 is going to be at the 100 of nanovolts level." >>>>>>> >>>>>> For that example, just need to put a simple discrete 100 to 1 resistor divider >>>>>> in-between the control voltage and the EFC and now you have a nice workable 10uv. >>>>>> BUT the bigger point is, probable not needed, cause you are NOT going to do any better than the stability of the OSC with a grounded shorted EFC input. >>>>>> >>>>>> as you said and I agree is so true: >>>>>> >>>>>>> "There is no perfect way to do any of this, only a lot of compromises ... you need to watch out for". >>>>>>> >>>>>> But you did not offer any easier way to do it, which is what the original request was for and my answer addressed. >>>>>> This is the cheapest easiest way BY FAR to get high performance, at low tau, ADEV numbers that I've seen. >>>>>> >>>>>> ws >>>>>> *************** >>>>>> >>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>> Sent: Saturday, February 06, 2010 12:09 PM >>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>> >>>>>> >>>>>> >>>>>>> Hi >>>>>>> >>>>>>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer. >>>>>>> >>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting. >>>>>>> >>>>>>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for. >>>>>>> >>>>>>> Bob >>>>>>> >>>>>>> -------------------------------------------------- >>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>> >>>>>>> >>>>>>>> Peat said: >>>>>>>> >>>>>>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements. >>>>>>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level. >>>>>>>>> >>>>>>>> If you accept that the measurement is going to limited by the Reference Osc, >>>>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level, >>>>>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability". >>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>>>> >>>>>>>> >>>>>>>> By replacing the "Voltage to freq converter, Freq counter& Printer with a Radio shack type PC data logging DVM, >>>>>>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed. >>>>>>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work >>>>>>>> and a higher speed oversampling ADC data logger and a good offset voltage. >>>>>>>> >>>>>>>> I must add this is not a popular solution (Or a general Purpose one) but >>>>>>>> IF you know analog and have a GOOD osc with EFC to use for the reference, >>>>>>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance. >>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >>>>>>>> >>>>>>>> Basic modified NIST Block Diag attached: >>>>>>>> The NIST paper sums it up quite nicely: >>>>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution >>>>>>>> so one has excellent precision capabilities with this system.' >>>>>>>> >>>>>>>> This does not address your other question of ADEV vs MDEV, >>>>>>>> What I've described is just a simple way to get the Low cost, GOOD Raw data. >>>>>>>> What you then do with that Data is a different subject. >>>>>>>> >>>>>>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice. >>>>>>>> >>>>>>>> >>>>>>>> Have fun >>>>>>>> ws >>>>>>>> >>>>>>>> ************* >>>>>>>> >>>>>>>> [time-nuts] ADEV vs MDEV >>>>>>>> Pete Rawson peterawson at earthlink.net >>>>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>>>> >>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate >>>>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>>>>> provide an appropriate measure of stability in this time range, or is >>>>>>>> the ADEV estimate a more correct answer? >>>>>>>> >>>>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>>>> other counters behave similarly. >>>>>>>> >>>>>>>> I would appreciate any comments or observations on this topic. >>>>>>>> My motivation is to discover the simplest scheme for making >>>>>>>> stability measurements at this performance level; this is NOT >>>>>>>> even close to the state-of-the-art, but can still be useful. >>>>>>>> >>>>>>>> Pete Rawson >>>>>>>> >>>>>>>> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

In a dual mixer system isolation is more critical to avoid locking between the 2 sources being compared. Isolation amplifiers with 80dB or more reverse isolation and low phase noise are inexpensive requiring 3 or 4 transistors (depending on the desired isolation) if you build your own. One can replace the counter with an inexpensive FPGA and eliminate the picket fence source. Bruce Bob Camp wrote: > Hi > > Here's my heterodyne vs tight loop logic: > > 1) With a heterodyne at around 8 Hz your isolation amplifiers are not as critical. Injection locking is a lot less likely at 8 Hz than dead on frequency. > 2) You eliminate the need for any form of phase lock. > 3) You do need a mixer and a couple of OP-27/37's. > 4) Resistors, capacitors, power supplies are likely the same between the two. > 5) For time tagging you'll need a picket fence source - not much money there. You could also decide not to time tag. > > > So far the Hetrodyne is as cheap as the tight lock. Probably a bit cheaper. Most of the parts are the same between the two setups, they simply get wired differently. > > The only real question is weather your DVM or my junk counter cost more money. My counter can be pretty bad and still not "get in the way". My old (and long gone) Beckman EPUT meter could probably handle the task. Tough to get data out of a vacuum tube counter though. > > Most 10811's are not 1x10^-13 at 1 second. A few might be. The vast majority are up around 0.8 to 2x10^-12 at one second. They were only specified to make 5x10^-12 at one second. Both systems are equally limited by the performance of the reference oscillator. > > Bob > > > On Feb 6, 2010, at 5:42 PM, WarrenS wrote: > > >> Bob >> >> So, if your point is that there are other ways to do it. ...We Agree >> (And the reason for the advanced methods is so that the counter resolution is not the limiting factor) >> >> Or are you saying a Tight Phase-Lock Loop" is not the simplest and cheapest way to get 1e13 resolution at 1 sec? >> That I'd have to see something new to believe it. >> >> Just so things do not get too far off the original topic, here is a reminder: >> >>>>>> "I would appreciate any comments or observations on the SIMPLEST scheme for making stability measurements at 1e-13 in one sec." >>>>>> >> ws Answer) Try the "Tight Phase-Lock Loop Method" >> >> May want to compare the blocks and equipment needed for A straight heterodyne system, or a DMTD, compared to the Analog "Tight Phase-Lock Loop" Method, AND then see what added problems there are because of injection locking, Osc coupling, Phase noise, ETC, ETC. >> >> >> ws >> >> *************** >> >> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >> Sent: Saturday, February 06, 2010 2:03 PM >> Subject: Re: [time-nuts] ADEV vs MDEV >> >> >> Hi >> >> A straight heterodyne system will get you to the floor of most 10811's with a very simple (2 stage) limiter. >> As with the DMTD, the counter requirements aren't really all that severe. >> >> Bob >> ********************* >> >> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >> >> >>> >>>> "It's possible / likely for injection lock ... to be a problem ..." >>>> >>> Something I certainly worried about and tested for. >>> What I found (for MY case) is that injection lock is NOT a problem. >>> The reason being is that unlike most other ways, where the two OSC have to be completely independent, >>> The tight loop approach forces the Two Osc to "Lock with something like 60 + db gain, >>> so a little stray -80db injection lock coupling that would very much limit other systems has >>> no measurable effect at e-13. Just one of the neat little side effects that make the tight loop approach so simple. >>> >>> >>>> "then a part in 10^14 is going to be at the 100 of nanovolts level." >>>> >>> For that example, just need to put a simple discrete 100 to 1 resistor divider >>> in-between the control voltage and the EFC and now you have a nice workable 10uv. >>> BUT the bigger point is, probable not needed, cause you are NOT going to do any better than the stability of the OSC with a grounded shorted EFC input. >>> >>> as you said and I agree is so true: >>> >>>> "There is no perfect way to do any of this, only a lot of compromises ... you need to watch out for". >>>> >>> But you did not offer any easier way to do it, which is what the original request was for and my answer addressed. >>> This is the cheapest easiest way BY FAR to get high performance, at low tau, ADEV numbers that I've seen. >>> >>> ws >>> *************** >>> >>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>> Sent: Saturday, February 06, 2010 12:09 PM >>> Subject: Re: [time-nuts] ADEV vs MDEV >>> >>> >>> >>>> Hi >>>> >>>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer. >>>> >>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting. >>>> >>>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for. >>>> >>>> Bob >>>> >>>> -------------------------------------------------- >>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>> Sent: Saturday, February 06, 2010 2:19 PM >>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>> >>>> >>>>> Peat said: >>>>> >>>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements. >>>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level. >>>>>> >>>>> >>>>> If you accept that the measurement is going to limited by the Reference Osc, >>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level, >>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability". >>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>> >>>>> >>>>> By replacing the "Voltage to freq converter, Freq counter& Printer with a Radio shack type PC data logging DVM, >>>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed. >>>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work >>>>> and a higher speed oversampling ADC data logger and a good offset voltage. >>>>> >>>>> I must add this is not a popular solution (Or a general Purpose one) but >>>>> IF you know analog and have a GOOD osc with EFC to use for the reference, >>>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance. >>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >>>>> >>>>> Basic modified NIST Block Diag attached: >>>>> The NIST paper sums it up quite nicely: >>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution >>>>> so one has excellent precision capabilities with this system.' >>>>> >>>>> This does not address your other question of ADEV vs MDEV, >>>>> What I've described is just a simple way to get the Low cost, GOOD Raw data. >>>>> What you then do with that Data is a different subject. >>>>> >>>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice. >>>>> >>>>> >>>>> Have fun >>>>> ws >>>>> >>>>> ************* >>>>> >>>>> [time-nuts] ADEV vs MDEV >>>>> Pete Rawson peterawson at earthlink.net >>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>> >>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate >>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>> provide an appropriate measure of stability in this time range, or is >>>>> the ADEV estimate a more correct answer? >>>>> >>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>> other counters behave similarly. >>>>> >>>>> I would appreciate any comments or observations on this topic. >>>>> My motivation is to discover the simplest scheme for making >>>>> stability measurements at this performance level; this is NOT >>>>> even close to the state-of-the-art, but can still be useful. >>>>> >>>>> Pete Rawson >>>>> >>>>> >>>

Even better is to toss out the mixers and sample the RF signals directly. However suitable ADCs cost $US100 or more each. To which one has to add an FPGA and an interface to a PC with sufficient throughput to handle the down converted I + Q samples. Bob Camp wrote: > Hi > > You probably could put a couple of cheap DAC's (ADCs are preferable as it avoids having to implement the conversion logic plus comparator required when using a DAC.) > on a board with a FPGA and reduce the data on the fly. I'd guess that would be be in the same $100 range as a half way decent sound card. Clock the DAC's off of a 10 MHz reference and eliminate the cal issue. > > If you are down around 10 Hz or worse yet 1 Hz, the AC coupling of the sound card will get in the way, even with a bandpass approach. You really don't know what they may have in there at the low end. Build it yourself and that stuff's not an issue. > > Bob > > My sound card has a 1Hz cutoff RC high pass input filter plus an internal high pass digital filter. Its not too difficult to measure the sound card frequency response using a white noise source for example. Bruce > On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: > > >> If one has a high end sound card then it could be used to implement the bandpass filter and replace the zero crossing detector. >> It may be necessary to insert a pilot tone to calibrate the sound card sampling clock frequency. >> A noise floor of about 1E-13/Tau should be achievable. >> This simplifies the DMTD system by replacing the zero crossing detector with a low gain linear preamp. >> >> If one analyses the resultant data off line then one can also try out different techniques such as a Costas receiver rather than a simple bandpass filter plus zero crossing detector. >> However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS will result in a file with a size of at least 1.15GB. >> >> Bruce >> >> >> Bruce Griffiths wrote: >> >>> If one were to use a bandpass filter with a Q of 10 to filter the beat frequency output of the mixer, then if the input frequency is 10MHz and the filter component tempco is 100ppm/C then the resultant phase shift tempco is about 16ps/C referred to the mixer input frequency. >>> >>> This phase shift tempco is certainly low enough not to have significant impact when measuring the frequency stability of a typical 10811A if the temperature fluctuations are kept small enough during the run. >>> >>> The effect of using a bandpass filter with too narrow a bandwidth is to artificially reduce ADEV for small Tau, so it may be prudent to use a higher beat frequency that 1Hz or even 10Hz and not calculate ADEV for Tau less than say 10(??) times the beat frequency period. A trade off between this and the effect of aliasing is required. >>> >>> Bruce >>> >>> Bob Camp wrote: >>> >>>> Hi >>>> >>>> With most 10811 range oscillators the impact of a simple bandpass filter is low enough to not be a major issue. That's for normal lab temperatures with the circuitry in a conventional die cast box. No guarantee if you open the window and let the fresh air blow in during the run. >>>> >>>> That's true with a heterodyne. I can see no obvious reason it would not be true on DMTD. >>>> >>>> Bob >>>> >>>> >>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: >>>> >>>> >>>>> The only major issue with DMTD systems is that they undersample the phase fluctuations and hence are subject to aliasing effects. >>>>> The low pass filter has to have a bandwidth of the same order as the beat frequency or the beat frequency signal will be significantly attenuated. >>>>> Since the phase is only sampled once per beat frequency period the phase fluctuations are undersampled. >>>>> Various attempts to use both zero crossings have not been successful. >>>>> >>>>> In principle if one can overcome the increased phase shift tempco associated with a bandpass filter, using a bandpass filter can in principle ensure that the phase fluctuations are oversampled. >>>>> >>>>> >>>>> Bruce >>>>> >>>>> Bob Camp wrote: >>>>> >>>>>> Hi >>>>>> >>>>>> A straight heterodyne system will get you to the floor of most 10811's with a very simple (2 stage) limiter. As with the DMTD, the counter requirements aren't really all that severe. >>>>>> >>>>>> Bob >>>>>> >>>>>> >>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >>>>>> >>>>>> >>>>>> >>>>>>> >>>>>>>> "It's possible / likely for injection lock ... to be a problem ..." >>>>>>>> >>>>>>>> >>>>>>> Something I certainly worried about and tested for. >>>>>>> What I found (for MY case) is that injection lock is NOT a problem. >>>>>>> The reason being is that unlike most other ways, where the two OSC have to be completely independent, >>>>>>> The tight loop approach forces the Two Osc to "Lock with something like 60 + db gain, >>>>>>> so a little stray -80db injection lock coupling that would very much limit other systems has >>>>>>> no measurable effect at e-13. Just one of the neat little side effects that make the tight loop approach so simple. >>>>>>> >>>>>>> >>>>>>> >>>>>>>> "then a part in 10^14 is going to be at the 100 of nanovolts level." >>>>>>>> >>>>>>>> >>>>>>> For that example, just need to put a simple discrete 100 to 1 resistor divider >>>>>>> in-between the control voltage and the EFC and now you have a nice workable 10uv. >>>>>>> BUT the bigger point is, probable not needed, cause you are NOT going to do any better than the stability of the OSC with a grounded shorted EFC input. >>>>>>> >>>>>>> as you said and I agree is so true: >>>>>>> >>>>>>> >>>>>>>> "There is no perfect way to do any of this, only a lot of compromises ... you need to watch out for". >>>>>>>> >>>>>>>> >>>>>>> But you did not offer any easier way to do it, which is what the original request was for and my answer addressed. >>>>>>> This is the cheapest easiest way BY FAR to get high performance, at low tau, ADEV numbers that I've seen. >>>>>>> >>>>>>> ws >>>>>>> *************** >>>>>>> >>>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>> Sent: Saturday, February 06, 2010 12:09 PM >>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>>> Hi >>>>>>>> >>>>>>>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer. >>>>>>>> >>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting. >>>>>>>> >>>>>>>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for. >>>>>>>> >>>>>>>> Bob >>>>>>>> >>>>>>>> -------------------------------------------------- >>>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> Peat said: >>>>>>>>> >>>>>>>>> >>>>>>>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements. >>>>>>>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level. >>>>>>>>>> >>>>>>>>>> >>>>>>>>> If you accept that the measurement is going to limited by the Reference Osc, >>>>>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level, >>>>>>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability". >>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>>>>> >>>>>>>>> >>>>>>>>> By replacing the "Voltage to freq converter, Freq counter& Printer with a Radio shack type PC data logging DVM, >>>>>>>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed. >>>>>>>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work >>>>>>>>> and a higher speed oversampling ADC data logger and a good offset voltage. >>>>>>>>> >>>>>>>>> I must add this is not a popular solution (Or a general Purpose one) but >>>>>>>>> IF you know analog and have a GOOD osc with EFC to use for the reference, >>>>>>>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance. >>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >>>>>>>>> >>>>>>>>> Basic modified NIST Block Diag attached: >>>>>>>>> The NIST paper sums it up quite nicely: >>>>>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution >>>>>>>>> so one has excellent precision capabilities with this system.' >>>>>>>>> >>>>>>>>> This does not address your other question of ADEV vs MDEV, >>>>>>>>> What I've described is just a simple way to get the Low cost, GOOD Raw data. >>>>>>>>> What you then do with that Data is a different subject. >>>>>>>>> >>>>>>>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice. >>>>>>>>> >>>>>>>>> >>>>>>>>> Have fun >>>>>>>>> ws >>>>>>>>> >>>>>>>>> ************* >>>>>>>>> >>>>>>>>> [time-nuts] ADEV vs MDEV >>>>>>>>> Pete Rawson peterawson at earthlink.net >>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>>>>> >>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>>>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate >>>>>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>>>>>> provide an appropriate measure of stability in this time range, or is >>>>>>>>> the ADEV estimate a more correct answer? >>>>>>>>> >>>>>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>>>>> other counters behave similarly. >>>>>>>>> >>>>>>>>> I would appreciate any comments or observations on this topic. >>>>>>>>> My motivation is to discover the simplest scheme for making >>>>>>>>> stability measurements at this performance level; this is NOT >>>>>>>>> even close to the state-of-the-art, but can still be useful. >>>>>>>>> >>>>>>>>> Pete Rawson >>>>>>>>> >>>>>>>>> >>>>>>>>> >>

Could it be another reason?

Well ONE of us certainly has something backward.

WarrenS wrote: > > And still another way of looking at the problem is: > One wants to know the Average freq over each of the 1 sec Tau sample > intervals, > and do it in such a way that there is no dead spots and no overlapping > information. (within the stated bandwidth) > Can be done with Phase (which tends to have limited resolution at low > tau) > BUT Easy enough to do with a properly set up "Tight Phase-Lock Loop". Stein doesnt believe so, particularly for long Tau (greater than a few times the PLL inverse bandwidth). The relevant paper is on the Symmetricom site. > Now send that raw data to "PLOTTER" and tell it that it is Freq data > and let it do its thing. > NO phase need be involved at the raw data, and I don't think 'Plotter' > is first turning Freq Data into phase to get ADEV Freq data. > but that is a Ulrich question. > >> "However its easier just to use the sampled phases in the alternative >> formula." > Are we off topic again? > For me, It is just as easy to have "Plotter" use either formula, It > does not complain about which is harder. > > BTW, using a ADEV program I wrote in excel. When using Freq data, it > was a little easer than using Phase data. > Both give the same answer, If there is enough accurate Phase > resolution, i.e 0.1 pS. for low taus faster than 1 sec. > >> "One cannot use EFC samples spaced at intervals of Tau directly ..." > Right, I hope all know that. Needs to be integrated (averaged) Freq > over the sampled Tau period, > NOT instantaneous freq at some random points along the way. > The NIST VtoF converter did that integration and so will a simple RC > filter with oversampling and a PC. > > Still don't see where it needs to reconstruct the Phase evolution. What do you think that your DVM is attempting to do? There is (hopefully) no significant difference between the DVM output and a sequence of phase samples. The only concern is (with either method of reconstructing the phase evolution) the effect of integrating small errors. The tight PLL requires that an integrator (that introduces noise) of some form is needed to produce the average frequency samples (or equivalent phase samples). Average frequency is proportional to the phase difference between start and end of the averaging time. So the phase differences and the average frequency samples are equivalent to within a known scale factor. > On the contrary the phase is used to reconstruct the Average Freq. > AND one of the BIG problems is that is very hard to do accurately if a > Phase TI is being used at Taus below 100 ms. (10Hz) > > Just so things do not get too far off the original topic, here is a > reminder: >>>>> "I would appreciate any comments or observations on the SIMPLEST >>>>> scheme for making stability measurements at 1e-13 in one sec." > > > ws Bruce > > ************** > ----- Original Message ----- From: "Bruce Griffiths" > <bruce.griffiths@xtra.co.nz> > To: "Discussion of precise time and frequency measurement" > <time-nuts@febo.com> > Sent: Saturday, February 06, 2010 2:24 PM > Subject: Re: [time-nuts] ADEV vs MDEV > > >> Another way of looking at the problem is: >> One has to reconstruct the phase evolution with time by integrating >> the instantaneous frequency. >> Then if the resultant phase evolution is sampled every Tau seconds >> and the first differences taken and divided by Tau the result is a >> sequence of average frequency samples required by the AVAR formula. >> However its easier just to use the sampled phases in the alternative >> formula. >> >> With the tight PLL method one has a sequence of frequency samples >> averaged over an interval on the order of the inverse PLL loop >> bandwidth. >> One then has to use these samples to reconstruct the phase evolution >> over time. >> >> One cannot use EFC samples spaced at intervals of Tau directly in the >> ADEV formula which requires a sequence of frequency averages over an >> interval of Tau. >> If one ignores this requirement the resultant stability measure is >> not ADEV. >> >> Bruce >> >> Bruce Griffiths wrote: >>> The tight PLL method doesn't directly produce the average frequency >>> over Tau. >>> As explained in (see snapshot of relevant section): >>> NIST special Publication 1065 Handbook of Frequency Stability >>> Analysis <http://tf.nist.gov/timefreq/general/pdf/2220.pdf> >>> the average frequency deviations for averaging time Tau are needed >>> for the calculation. >>> You need to sample at a sufficiently high rate to avoid aliasing and >>> average (ie integrate) the individual EFC samples. >>> >>> If one uses phase measures then the fluctuations in the frequency >>> averages can easily and directly calculated from the difference >>> between the phase measured at time intervals separated by Tau. >>> >>> Bruce >>> >>> WarrenS wrote: >>>> Bruce said: >>>> >>>>> Thus NIST and others quietly dropped this method several decades ago. >>>> Could it be another reason? >>>> I'll bet that was after they wanted to do better than 1e14 >>>> resolution AND had unlimited amounts of time and Money, >>>> Something most time Nuts are not blessed with. I Never said it was >>>> the BEST way. >>>> JUST given the goal, which was 1e13 in one second, there is not a >>>> simpler and cheaper way to do it. >>>> And nothing you said counter that point. >>>> >>>> >>>>> The frequency measures need to be integrated (either implicitly or >>>>> explicitly) to produce phase measures which can then be used to >>>>> calculate ADEV, MDEV etc. >>>> >>>> Well ONE of us certainly has something backward. >>>> To calculate ADEV, MDEV etc. YOU need Freq Differences. >>>> The first thing that happens when phase is used is that it is >>>> turned into Freq by taking the difference between each sample. >>>> Integrated Freq data, which is what "Tight Phase-Lock Loop Method" >>>> gives you directly (no Phase conversion needed), >>>> Need not FIRST turned into Phase so that it can then be turned back >>>> into Freq. >>>> BUT in any case there is no difference in the noise, for a given >>>> bandwidth, If you don't run out of digits and You have enough >>>> resolution. >>>> The "Tight Phase-Lock Loop Method" can EASY get sub pS resolution, >>>> which is better than most other ways. >>>> AND don't need filters and slue rate control and multistage >>>> limiters and on & on to do it, an RC works fine to replace all the >>>> stuff. >>>> >>>> ws >>>> >>>> ***************** >>>> >>>> ----- Original Message ----- From: "Bruce Griffiths" >>>> <bruce.griffiths@xtra.co.nz> >>>> To: "Discussion of precise time and frequency measurement" >>>> <time-nuts@febo.com> >>>> Sent: Saturday, February 06, 2010 12:11 PM >>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>> >>>> >>>>> Sounds good but you still haven't found its Achilles heel: >>>>> >>>>> The frequency measures need to be integrated (either implicitly or >>>>> explicitly) to produce phase measures which can then be used to >>>>> calculate ADEV, MDEV etc. >>>>> The major problem is that integration amplifies the small errors >>>>> that are inevitably present. >>>>> In practice (except for very noisy sources) the technique isnt >>>>> particularly useful for Tau more than a few times the inverse PLL >>>>> bandwidth. >>>>> >>>>> Thus NIST and others quietly dropped this method several decades ago. >>>>> This is alluded to in Steins recent paper availble on the >>>>> Symmetricom website: >>>>> >>>>> *The Allan Variance – Challenges and Opportunities* >>>>> >>>>> >>>>> Bruce >>>>> >>>>> WarrenS wrote: >>>>>> Peat said: >>>>>> >>>>>>> I would appreciate any comments or observations on the topic of >>>>>>> apparatus with demonstrated stability measurements. >>>>>>> My motivation is to discover the SIMPLEST scheme for making >>>>>>> stability measurements at the 1E-13 in 1s performance level. >>>>>>> >>>>>> >>>>>> If you accept that the measurement is going to limited by the >>>>>> Reference Osc, >>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at >>>>>> that level, >>>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock >>>>>> Loop Method of measuring Freq stability". >>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>> >>>>>> >>>>>> By replacing the "Voltage to freq converter, Freq counter& >>>>>> Printer with a Radio shack type PC data logging DVM, >>>>>> It can be up and running from scratch in under an Hr, with no >>>>>> high end test equipment needed. >>>>>> If you want performance that exceeds the best of most DMTD at low >>>>>> Tau it takes a little more work >>>>>> and a higher speed oversampling ADC data logger and a good offset >>>>>> voltage. >>>>>> >>>>>> I must add this is not a popular solution (Or a general Purpose >>>>>> one) but >>>>>> IF you know analog and have a GOOD osc with EFC to use for the >>>>>> reference, >>>>>> as far as I've been able to determine it is the BEST SIMPLE >>>>>> answer that allows High performance. >>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in >>>>>> 0.1 sec (at 30 Hz Bandwidth) >>>>>> >>>>>> Basic modified NIST Block Diag attached: >>>>>> The NIST paper sums it up quite nicely: >>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 >>>>>> per Hz resolution >>>>>> so one has excellent precision capabilities with this system.' >>>>>> >>>>>> This does not address your other question of ADEV vs MDEV, >>>>>> What I've described is just a simple way to get the Low cost, >>>>>> GOOD Raw data. >>>>>> What you then do with that Data is a different subject. >>>>>> >>>>>> You can run the raw data thru one of the many ADEV programs out >>>>>> there, 'Plotter' being my choice. >>>>>> >>>>>> >>>>>> Have fun >>>>>> ws >>>>>> >>>>>> ************* >>>>>> >>>>>> [time-nuts] ADEV vs MDEV >>>>>> Pete Rawson peterawson at earthlink.net >>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>> >>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>>> existing TI counters can reach this goal in 10s. (using MDEV >>>>>> estimate >>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>>> provide an appropriate measure of stability in this time range, >>>>>> or is >>>>>> the ADEV estimate a more correct answer? >>>>>> >>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>> other counters behave similarly. >>>>>> >>>>>> I would appreciate any comments or observations on this topic. >>>>>> My motivation is to discover the simplest scheme for making >>>>>> stability measurements at this performance level; this is NOT >>>>>> even close to the state-of-the-art, but can still be useful. >>>>>> >>>>>> Pete Rawson >>>>>>

Bruce Griffiths wrote: > If one has a high end sound card then it could be used to implement the > bandpass filter and replace the zero crossing detector. > It may be necessary to insert a pilot tone to calibrate the sound card > sampling clock frequency. > A noise floor of about 1E-13/Tau should be achievable. > This simplifies the DMTD system by replacing the zero crossing detector > with a low gain linear preamp. > > If one analyses the resultant data off line then one can also try out > different techniques such as a Costas receiver rather than a simple > bandpass filter plus zero crossing detector. > However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS > will result in a file with a size of at least 1.15GB. First step would be to implement a real-time first-line receiver processing. Should not be too hard. Costas-loop isn't too complex to implement, but for full repeatability using the same signals for different set of parameters... but then again, it is fairly straight-forward to implement. Using either a separate channel or FDM a reference 1 kHz (from say TADD-2 divider) should work for reference tone. A good board can be word-clock locked without too much hazzle. Not too happy about the quality of the real-time spectrum apps I have seen. Cheers, Magnus

Hi My main concern with the low frequency pole in the sound card is the quality of the R/C used. You can certainly model what ever you have. If they used an aluminum electrolytic for the "C" it may not be the same next time you check it .... On a 10 Hz system, a 1 Hz pole is probably not an issue. It might get in the way with a 1 Hz beat note. Another thing I have only seen in passing: "Sigma Delta's have poor low frequency noise characteristics". I haven't dug into it to see if that's really true or not. If you buy your own ADC's, you certainly would not be restricted to a Sigma Delta. Even with a cheap pre-built FPGA board, you could look into higher sample rates than a conventional sound card. You would drop back to 16 bits, but it might be worth it. Bob On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote: > Even better is to toss out the mixers and sample the RF signals directly. > However suitable ADCs cost $US100 or more each. > To which one has to add an FPGA and an interface to a PC with sufficient throughput to handle the down converted I + Q samples. > > Bob Camp wrote: >> Hi >> >> You probably could put a couple of cheap DAC's > > (ADCs are preferable as it avoids having to implement the conversion logic plus comparator required when using a DAC.) > >> on a board with a FPGA and reduce the data on the fly. I'd guess that would be be in the same $100 range as a half way decent sound card. Clock the DAC's off of a 10 MHz reference and eliminate the cal issue. >> >> If you are down around 10 Hz or worse yet 1 Hz, the AC coupling of the sound card will get in the way, even with a bandpass approach. You really don't know what they may have in there at the low end. Build it yourself and that stuff's not an issue. >> >> Bob >> >> > My sound card has a 1Hz cutoff RC high pass input filter plus an internal high pass digital filter. > Its not too difficult to measure the sound card frequency response using a white noise source for example. > > Bruce >> On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: >> >> >>> If one has a high end sound card then it could be used to implement the bandpass filter and replace the zero crossing detector. >>> It may be necessary to insert a pilot tone to calibrate the sound card sampling clock frequency. >>> A noise floor of about 1E-13/Tau should be achievable. >>> This simplifies the DMTD system by replacing the zero crossing detector with a low gain linear preamp. >>> >>> If one analyses the resultant data off line then one can also try out different techniques such as a Costas receiver rather than a simple bandpass filter plus zero crossing detector. >>> However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS will result in a file with a size of at least 1.15GB. >>> >>> Bruce >>> >>> >>> Bruce Griffiths wrote: >>> >>>> If one were to use a bandpass filter with a Q of 10 to filter the beat frequency output of the mixer, then if the input frequency is 10MHz and the filter component tempco is 100ppm/C then the resultant phase shift tempco is about 16ps/C referred to the mixer input frequency. >>>> >>>> This phase shift tempco is certainly low enough not to have significant impact when measuring the frequency stability of a typical 10811A if the temperature fluctuations are kept small enough during the run. >>>> >>>> The effect of using a bandpass filter with too narrow a bandwidth is to artificially reduce ADEV for small Tau, so it may be prudent to use a higher beat frequency that 1Hz or even 10Hz and not calculate ADEV for Tau less than say 10(??) times the beat frequency period. A trade off between this and the effect of aliasing is required. >>>> >>>> Bruce >>>> >>>> Bob Camp wrote: >>>> >>>>> Hi >>>>> >>>>> With most 10811 range oscillators the impact of a simple bandpass filter is low enough to not be a major issue. That's for normal lab temperatures with the circuitry in a conventional die cast box. No guarantee if you open the window and let the fresh air blow in during the run. >>>>> >>>>> That's true with a heterodyne. I can see no obvious reason it would not be true on DMTD. >>>>> >>>>> Bob >>>>> >>>>> >>>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: >>>>> >>>>> >>>>>> The only major issue with DMTD systems is that they undersample the phase fluctuations and hence are subject to aliasing effects. >>>>>> The low pass filter has to have a bandwidth of the same order as the beat frequency or the beat frequency signal will be significantly attenuated. >>>>>> Since the phase is only sampled once per beat frequency period the phase fluctuations are undersampled. >>>>>> Various attempts to use both zero crossings have not been successful. >>>>>> >>>>>> In principle if one can overcome the increased phase shift tempco associated with a bandpass filter, using a bandpass filter can in principle ensure that the phase fluctuations are oversampled. >>>>>> >>>>>> >>>>>> Bruce >>>>>> >>>>>> Bob Camp wrote: >>>>>> >>>>>>> Hi >>>>>>> >>>>>>> A straight heterodyne system will get you to the floor of most 10811's with a very simple (2 stage) limiter. As with the DMTD, the counter requirements aren't really all that severe. >>>>>>> >>>>>>> Bob >>>>>>> >>>>>>> >>>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>> >>>>>>>>> "It's possible / likely for injection lock ... to be a problem ..." >>>>>>>>> >>>>>>>>> >>>>>>>> Something I certainly worried about and tested for. >>>>>>>> What I found (for MY case) is that injection lock is NOT a problem. >>>>>>>> The reason being is that unlike most other ways, where the two OSC have to be completely independent, >>>>>>>> The tight loop approach forces the Two Osc to "Lock with something like 60 + db gain, >>>>>>>> so a little stray -80db injection lock coupling that would very much limit other systems has >>>>>>>> no measurable effect at e-13. Just one of the neat little side effects that make the tight loop approach so simple. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> "then a part in 10^14 is going to be at the 100 of nanovolts level." >>>>>>>>> >>>>>>>>> >>>>>>>> For that example, just need to put a simple discrete 100 to 1 resistor divider >>>>>>>> in-between the control voltage and the EFC and now you have a nice workable 10uv. >>>>>>>> BUT the bigger point is, probable not needed, cause you are NOT going to do any better than the stability of the OSC with a grounded shorted EFC input. >>>>>>>> >>>>>>>> as you said and I agree is so true: >>>>>>>> >>>>>>>> >>>>>>>>> "There is no perfect way to do any of this, only a lot of compromises ... you need to watch out for". >>>>>>>>> >>>>>>>>> >>>>>>>> But you did not offer any easier way to do it, which is what the original request was for and my answer addressed. >>>>>>>> This is the cheapest easiest way BY FAR to get high performance, at low tau, ADEV numbers that I've seen. >>>>>>>> >>>>>>>> ws >>>>>>>> *************** >>>>>>>> >>>>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>> Sent: Saturday, February 06, 2010 12:09 PM >>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> Hi >>>>>>>>> >>>>>>>>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer. >>>>>>>>> >>>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting. >>>>>>>>> >>>>>>>>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for. >>>>>>>>> >>>>>>>>> Bob >>>>>>>>> >>>>>>>>> -------------------------------------------------- >>>>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> Peat said: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements. >>>>>>>>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level. >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> If you accept that the measurement is going to limited by the Reference Osc, >>>>>>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level, >>>>>>>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability". >>>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> By replacing the "Voltage to freq converter, Freq counter& Printer with a Radio shack type PC data logging DVM, >>>>>>>>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed. >>>>>>>>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work >>>>>>>>>> and a higher speed oversampling ADC data logger and a good offset voltage. >>>>>>>>>> >>>>>>>>>> I must add this is not a popular solution (Or a general Purpose one) but >>>>>>>>>> IF you know analog and have a GOOD osc with EFC to use for the reference, >>>>>>>>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance. >>>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >>>>>>>>>> >>>>>>>>>> Basic modified NIST Block Diag attached: >>>>>>>>>> The NIST paper sums it up quite nicely: >>>>>>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution >>>>>>>>>> so one has excellent precision capabilities with this system.' >>>>>>>>>> >>>>>>>>>> This does not address your other question of ADEV vs MDEV, >>>>>>>>>> What I've described is just a simple way to get the Low cost, GOOD Raw data. >>>>>>>>>> What you then do with that Data is a different subject. >>>>>>>>>> >>>>>>>>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice. >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> Have fun >>>>>>>>>> ws >>>>>>>>>> >>>>>>>>>> ************* >>>>>>>>>> >>>>>>>>>> [time-nuts] ADEV vs MDEV >>>>>>>>>> Pete Rawson peterawson at earthlink.net >>>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>>>>>> >>>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>>>>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate >>>>>>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>>>>>>> provide an appropriate measure of stability in this time range, or is >>>>>>>>>> the ADEV estimate a more correct answer? >>>>>>>>>> >>>>>>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>>>>>> other counters behave similarly. >>>>>>>>>> >>>>>>>>>> I would appreciate any comments or observations on this topic. >>>>>>>>>> My motivation is to discover the simplest scheme for making >>>>>>>>>> stability measurements at this performance level; this is NOT >>>>>>>>>> even close to the state-of-the-art, but can still be useful. >>>>>>>>>> >>>>>>>>>> Pete Rawson >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

Bob Camp wrote: > Hi > > My main concern with the low frequency pole in the sound card is the quality of the R/C used. You can certainly model what ever you have. If they used an aluminum electrolytic for the "C" it may not be the same next time you check it .... > > One should at least calibrate the effect before and after each run. It would be even better to embed such a calibration within each run. > On a 10 Hz system, a 1 Hz pole is probably not an issue. It might get in the way with a 1 Hz beat note. > > Another thing I have only seen in passing: "Sigma Delta's have poor low frequency noise characteristics". I haven't dug into it to see if that's really true or not. If you buy your own ADC's, you certainly would not be restricted to a Sigma Delta. > > The input noise spectrum for my sound card does indeed rise significantly at low frequencies. A beat note of around 100Hz or 1KHz note would be more suitable than lower frequencies. With a Costas receiver the equivalent timing noise isn't strongly dependent on the beat frequency. > Even with a cheap pre-built FPGA board, you could look into higher sample rates than a conventional sound card. You would drop back to 16 bits, but it might be worth it. > > Bob > > > 1 bit of a 16 bit accurate ADC corresponds to a timing resolution of around 0.48ps referred to a 10MHz mixer input signal. The beat frequency amplitude has to be amplified to just below the ADC full scale input. Bruce > On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote: > > >> Even better is to toss out the mixers and sample the RF signals directly. >> However suitable ADCs cost $US100 or more each. >> To which one has to add an FPGA and an interface to a PC with sufficient throughput to handle the down converted I + Q samples. >> >> Bob Camp wrote: >> >>> Hi >>> >>> You probably could put a couple of cheap DAC's >>> >> (ADCs are preferable as it avoids having to implement the conversion logic plus comparator required when using a DAC.) >> >> >>> on a board with a FPGA and reduce the data on the fly. I'd guess that would be be in the same $100 range as a half way decent sound card. Clock the DAC's off of a 10 MHz reference and eliminate the cal issue. >>> >>> If you are down around 10 Hz or worse yet 1 Hz, the AC coupling of the sound card will get in the way, even with a bandpass approach. You really don't know what they may have in there at the low end. Build it yourself and that stuff's not an issue. >>> >>> Bob >>> >>> >>> >> My sound card has a 1Hz cutoff RC high pass input filter plus an internal high pass digital filter. >> Its not too difficult to measure the sound card frequency response using a white noise source for example. >> >> Bruce >> >>> On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: >>> >>> >>> >>>> If one has a high end sound card then it could be used to implement the bandpass filter and replace the zero crossing detector. >>>> It may be necessary to insert a pilot tone to calibrate the sound card sampling clock frequency. >>>> A noise floor of about 1E-13/Tau should be achievable. >>>> This simplifies the DMTD system by replacing the zero crossing detector with a low gain linear preamp. >>>> >>>> If one analyses the resultant data off line then one can also try out different techniques such as a Costas receiver rather than a simple bandpass filter plus zero crossing detector. >>>> However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS will result in a file with a size of at least 1.15GB. >>>> >>>> Bruce >>>> >>>> >>>> Bruce Griffiths wrote: >>>> >>>> >>>>> If one were to use a bandpass filter with a Q of 10 to filter the beat frequency output of the mixer, then if the input frequency is 10MHz and the filter component tempco is 100ppm/C then the resultant phase shift tempco is about 16ps/C referred to the mixer input frequency. >>>>> >>>>> This phase shift tempco is certainly low enough not to have significant impact when measuring the frequency stability of a typical 10811A if the temperature fluctuations are kept small enough during the run. >>>>> >>>>> The effect of using a bandpass filter with too narrow a bandwidth is to artificially reduce ADEV for small Tau, so it may be prudent to use a higher beat frequency that 1Hz or even 10Hz and not calculate ADEV for Tau less than say 10(??) times the beat frequency period. A trade off between this and the effect of aliasing is required. >>>>> >>>>> Bruce >>>>> >>>>> Bob Camp wrote: >>>>> >>>>> >>>>>> Hi >>>>>> >>>>>> With most 10811 range oscillators the impact of a simple bandpass filter is low enough to not be a major issue. That's for normal lab temperatures with the circuitry in a conventional die cast box. No guarantee if you open the window and let the fresh air blow in during the run. >>>>>> >>>>>> That's true with a heterodyne. I can see no obvious reason it would not be true on DMTD. >>>>>> >>>>>> Bob >>>>>> >>>>>> >>>>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: >>>>>> >>>>>> >>>>>> >>>>>>> The only major issue with DMTD systems is that they undersample the phase fluctuations and hence are subject to aliasing effects. >>>>>>> The low pass filter has to have a bandwidth of the same order as the beat frequency or the beat frequency signal will be significantly attenuated. >>>>>>> Since the phase is only sampled once per beat frequency period the phase fluctuations are undersampled. >>>>>>> Various attempts to use both zero crossings have not been successful. >>>>>>> >>>>>>> In principle if one can overcome the increased phase shift tempco associated with a bandpass filter, using a bandpass filter can in principle ensure that the phase fluctuations are oversampled. >>>>>>> >>>>>>> >>>>>>> Bruce >>>>>>> >>>>>>> Bob Camp wrote: >>>>>>> >>>>>>> >>>>>>>> Hi >>>>>>>> >>>>>>>> A straight heterodyne system will get you to the floor of most 10811's with a very simple (2 stage) limiter. As with the DMTD, the counter requirements aren't really all that severe. >>>>>>>> >>>>>>>> Bob >>>>>>>> >>>>>>>> >>>>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> "It's possible / likely for injection lock ... to be a problem ..." >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>> Something I certainly worried about and tested for. >>>>>>>>> What I found (for MY case) is that injection lock is NOT a problem. >>>>>>>>> The reason being is that unlike most other ways, where the two OSC have to be completely independent, >>>>>>>>> The tight loop approach forces the Two Osc to "Lock with something like 60 + db gain, >>>>>>>>> so a little stray -80db injection lock coupling that would very much limit other systems has >>>>>>>>> no measurable effect at e-13. Just one of the neat little side effects that make the tight loop approach so simple. >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> "then a part in 10^14 is going to be at the 100 of nanovolts level." >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>> For that example, just need to put a simple discrete 100 to 1 resistor divider >>>>>>>>> in-between the control voltage and the EFC and now you have a nice workable 10uv. >>>>>>>>> BUT the bigger point is, probable not needed, cause you are NOT going to do any better than the stability of the OSC with a grounded shorted EFC input. >>>>>>>>> >>>>>>>>> as you said and I agree is so true: >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> "There is no perfect way to do any of this, only a lot of compromises ... you need to watch out for". >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>> But you did not offer any easier way to do it, which is what the original request was for and my answer addressed. >>>>>>>>> This is the cheapest easiest way BY FAR to get high performance, at low tau, ADEV numbers that I've seen. >>>>>>>>> >>>>>>>>> ws >>>>>>>>> *************** >>>>>>>>> >>>>>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>>> Sent: Saturday, February 06, 2010 12:09 PM >>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> Hi >>>>>>>>>> >>>>>>>>>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer. >>>>>>>>>> >>>>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting. >>>>>>>>>> >>>>>>>>>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for. >>>>>>>>>> >>>>>>>>>> Bob >>>>>>>>>> >>>>>>>>>> -------------------------------------------------- >>>>>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> Peat said: >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements. >>>>>>>>>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>> If you accept that the measurement is going to limited by the Reference Osc, >>>>>>>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level, >>>>>>>>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability". >>>>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> By replacing the "Voltage to freq converter, Freq counter& Printer with a Radio shack type PC data logging DVM, >>>>>>>>>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed. >>>>>>>>>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work >>>>>>>>>>> and a higher speed oversampling ADC data logger and a good offset voltage. >>>>>>>>>>> >>>>>>>>>>> I must add this is not a popular solution (Or a general Purpose one) but >>>>>>>>>>> IF you know analog and have a GOOD osc with EFC to use for the reference, >>>>>>>>>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance. >>>>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >>>>>>>>>>> >>>>>>>>>>> Basic modified NIST Block Diag attached: >>>>>>>>>>> The NIST paper sums it up quite nicely: >>>>>>>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution >>>>>>>>>>> so one has excellent precision capabilities with this system.' >>>>>>>>>>> >>>>>>>>>>> This does not address your other question of ADEV vs MDEV, >>>>>>>>>>> What I've described is just a simple way to get the Low cost, GOOD Raw data. >>>>>>>>>>> What you then do with that Data is a different subject. >>>>>>>>>>> >>>>>>>>>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice. >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> Have fun >>>>>>>>>>> ws >>>>>>>>>>> >>>>>>>>>>> ************* >>>>>>>>>>> >>>>>>>>>>> [time-nuts] ADEV vs MDEV >>>>>>>>>>> Pete Rawson peterawson at earthlink.net >>>>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>>>>>>> >>>>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>>>>>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate >>>>>>>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>>>>>>>> provide an appropriate measure of stability in this time range, or is >>>>>>>>>>> the ADEV estimate a more correct answer? >>>>>>>>>>> >>>>>>>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>>>>>>> other counters behave similarly. >>>>>>>>>>> >>>>>>>>>>> I would appreciate any comments or observations on this topic. >>>>>>>>>>> My motivation is to discover the simplest scheme for making >>>>>>>>>>> stability measurements at this performance level; this is NOT >>>>>>>>>>> even close to the state-of-the-art, but can still be useful. >>>>>>>>>>> >>>>>>>>>>> Pete Rawson >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>> >>

A sound-card back end has always seemed like a pretty reasonable approach to me, if you're inclined to go the DMTD route. I wouldn't send a 'baseband' signal to the sound card, though -- I'd upconvert it to a few kHz to get away from the numerous bad things that sound cards do near DC. -- john, KE5FX > Hi > > My main concern with the low frequency pole in the sound card is > the quality of the R/C used. You can certainly model what ever > you have. If they used an aluminum electrolytic for the "C" it > may not be the same next time you check it .... > > On a 10 Hz system, a 1 Hz pole is probably not an issue. It might > get in the way with a 1 Hz beat note. > > Another thing I have only seen in passing: "Sigma Delta's have > poor low frequency noise characteristics". I haven't dug into it > to see if that's really true or not. If you buy your own ADC's, > you certainly would not be restricted to a Sigma Delta. > > Even with a cheap pre-built FPGA board, you could look into > higher sample rates than a conventional sound card. You would > drop back to 16 bits, but it might be worth it. > > Bob > > > On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote: > > > Even better is to toss out the mixers and sample the RF signals > directly. > > However suitable ADCs cost $US100 or more each. > > To which one has to add an FPGA and an interface to a PC with > sufficient throughput to handle the down converted I + Q samples. > > > > Bob Camp wrote: > >> Hi > >> > >> You probably could put a couple of cheap DAC's > > > > (ADCs are preferable as it avoids having to implement the > conversion logic plus comparator required when using a DAC.) > > > >> on a board with a FPGA and reduce the data on the fly. I'd > guess that would be be in the same $100 range as a half way > decent sound card. Clock the DAC's off of a 10 MHz reference and > eliminate the cal issue. > >> > >> If you are down around 10 Hz or worse yet 1 Hz, the AC > coupling of the sound card will get in the way, even with a > bandpass approach. You really don't know what they may have in > there at the low end. Build it yourself and that stuff's not an issue. > >> > >> Bob > >> > >> > > My sound card has a 1Hz cutoff RC high pass input filter plus > an internal high pass digital filter. > > Its not too difficult to measure the sound card frequency > response using a white noise source for example. > > > > Bruce > >> On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: > >> > >> > >>> If one has a high end sound card then it could be used to > implement the bandpass filter and replace the zero crossing detector. > >>> It may be necessary to insert a pilot tone to calibrate the > sound card sampling clock frequency. > >>> A noise floor of about 1E-13/Tau should be achievable. > >>> This simplifies the DMTD system by replacing the zero > crossing detector with a low gain linear preamp. > >>> > >>> If one analyses the resultant data off line then one can also > try out different techniques such as a Costas receiver rather > than a simple bandpass filter plus zero crossing detector. > >>> However 1000 seconds of data for 2 channels of 24 bit samples > at 192KSPS will result in a file with a size of at least 1.15GB. > >>> > >>> Bruce > >>> > >>> > >>> Bruce Griffiths wrote: > >>> > >>>> If one were to use a bandpass filter with a Q of 10 to > filter the beat frequency output of the mixer, then if the input > frequency is 10MHz and the filter component tempco is 100ppm/C > then the resultant phase shift tempco is about 16ps/C referred to > the mixer input frequency. > >>>> > >>>> This phase shift tempco is certainly low enough not to have > significant impact when measuring the frequency stability of a > typical 10811A if the temperature fluctuations are kept small > enough during the run. > >>>> > >>>> The effect of using a bandpass filter with too narrow a > bandwidth is to artificially reduce ADEV for small Tau, so it may > be prudent to use a higher beat frequency that 1Hz or even 10Hz > and not calculate ADEV for Tau less than say 10(??) times the > beat frequency period. A trade off between this and the effect of > aliasing is required. > >>>> > >>>> Bruce > >>>> > >>>> Bob Camp wrote: > >>>> > >>>>> Hi > >>>>> > >>>>> With most 10811 range oscillators the impact of a simple > bandpass filter is low enough to not be a major issue. That's for > normal lab temperatures with the circuitry in a conventional die > cast box. No guarantee if you open the window and let the fresh > air blow in during the run. > >>>>> > >>>>> That's true with a heterodyne. I can see no obvious reason > it would not be true on DMTD. > >>>>> > >>>>> Bob > >>>>> > >>>>> > >>>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: > >>>>> > >>>>> > >>>>>> The only major issue with DMTD systems is that they > undersample the phase fluctuations and hence are subject to > aliasing effects. > >>>>>> The low pass filter has to have a bandwidth of the same > order as the beat frequency or the beat frequency signal will be > significantly attenuated. > >>>>>> Since the phase is only sampled once per beat frequency > period the phase fluctuations are undersampled. > >>>>>> Various attempts to use both zero crossings have not been > successful. > >>>>>> > >>>>>> In principle if one can overcome the increased phase shift > tempco associated with a bandpass filter, using a bandpass filter > can in principle ensure that the phase fluctuations are oversampled. > >>>>>> > >>>>>> > >>>>>> Bruce > >>>>>> > >>>>>> Bob Camp wrote: > >>>>>> > >>>>>>> Hi > >>>>>>> > >>>>>>> A straight heterodyne system will get you to the floor of > most 10811's with a very simple (2 stage) limiter. As with the > DMTD, the counter requirements aren't really all that severe. > >>>>>>> > >>>>>>> Bob > >>>>>>> > >>>>>>> > >>>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>>> > >>>>>>>>> "It's possible / likely for injection lock ... to be a > problem ..." > >>>>>>>>> > >>>>>>>>> > >>>>>>>> Something I certainly worried about and tested for. > >>>>>>>> What I found (for MY case) is that injection lock is NOT > a problem. > >>>>>>>> The reason being is that unlike most other ways, where > the two OSC have to be completely independent, > >>>>>>>> The tight loop approach forces the Two Osc to "Lock with > something like 60 + db gain, > >>>>>>>> so a little stray -80db injection lock coupling that > would very much limit other systems has > >>>>>>>> no measurable effect at e-13. Just one of the neat > little side effects that make the tight loop approach so simple. > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>>> "then a part in 10^14 is going to be at the 100 of > nanovolts level." > >>>>>>>>> > >>>>>>>>> > >>>>>>>> For that example, just need to put a simple discrete 100 > to 1 resistor divider > >>>>>>>> in-between the control voltage and the EFC and now you > have a nice workable 10uv. > >>>>>>>> BUT the bigger point is, probable not needed, cause you > are NOT going to do any better than the stability of the OSC with > a grounded shorted EFC input. > >>>>>>>> > >>>>>>>> as you said and I agree is so true: > >>>>>>>> > >>>>>>>> > >>>>>>>>> "There is no perfect way to do any of this, only a lot > of compromises ... you need to watch out for". > >>>>>>>>> > >>>>>>>>> > >>>>>>>> But you did not offer any easier way to do it, which is > what the original request was for and my answer addressed. > >>>>>>>> This is the cheapest easiest way BY FAR to get high > performance, at low tau, ADEV numbers that I've seen. > >>>>>>>> > >>>>>>>> ws > >>>>>>>> *************** > >>>>>>>> > >>>>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> > >>>>>>>> To: "Discussion of precise time and frequency > measurement"<time-nuts@febo.com> > >>>>>>>> Sent: Saturday, February 06, 2010 12:09 PM > >>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>>> Hi > >>>>>>>>> > >>>>>>>>> It's possible / likely to injection lock with the tight > loop approach and get data that's much better than reality. A lot > depends on the specific oscillators under test and the buffers > (if any) between the oscillators and mixer. > >>>>>>>>> > >>>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt > then a part in 10^14 is going to be at the 100 of nanovolts > level. Certainly not impossible, but it does present it's own set > of issues. Lab gear to do it is available, but not all that > common. DC offsets and their temperature coefficients along with > thermocouple effects could make things exciting. > >>>>>>>>> > >>>>>>>>> There is no perfect way to do any of this, only a lot > of compromises here or there. Each approach has stuff you need to > watch out for. > >>>>>>>>> > >>>>>>>>> Bob > >>>>>>>>> > >>>>>>>>> -------------------------------------------------- > >>>>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> > >>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM > >>>>>>>>> To: "Discussion of precise time and frequency > measurement"<time-nuts@febo.com> > >>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV > >>>>>>>>> > >>>>>>>>> > >>>>>>>>> > >>>>>>>>>> Peat said: > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>>> I would appreciate any comments or observations on > the topic of apparatus with demonstrated stability measurements. > >>>>>>>>>>> My motivation is to discover the SIMPLEST scheme for > making stability measurements at the 1E-13 in 1s performance level. > >>>>>>>>>>> > >>>>>>>>>>> > >>>>>>>>>> If you accept that the measurement is going to limited > by the Reference Osc, > >>>>>>>>>> for Low COST and SIMPLE, with the ability to measure > ADEVs at that level, > >>>>>>>>>> Can't beat a simple analog version of NIST's "Tight > Phase-Lock Loop Method of measuring Freq stability". > >>>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> By replacing the "Voltage to freq converter, Freq > counter& Printer with a Radio shack type PC data logging DVM, > >>>>>>>>>> It can be up and running from scratch in under an Hr, > with no high end test equipment needed. > >>>>>>>>>> If you want performance that exceeds the best of most > DMTD at low Tau it takes a little more work > >>>>>>>>>> and a higher speed oversampling ADC data logger and a > good offset voltage. > >>>>>>>>>> > >>>>>>>>>> I must add this is not a popular solution (Or a > general Purpose one) but > >>>>>>>>>> IF you know analog and have a GOOD osc with EFC to > use for the reference, > >>>>>>>>>> as far as I've been able to determine it is the BEST > SIMPLE answer that allows High performance. > >>>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than > 1e-12 in 0.1 sec (at 30 Hz Bandwidth) > >>>>>>>>>> > >>>>>>>>>> Basic modified NIST Block Diag attached: > >>>>>>>>>> The NIST paper sums it up quite nicely: > >>>>>>>>>> 'It is not difficult to achieve a sensitivity of a > part in e14 per Hz resolution > >>>>>>>>>> so one has excellent precision capabilities with this system.' > >>>>>>>>>> > >>>>>>>>>> This does not address your other question of ADEV vs MDEV, > >>>>>>>>>> What I've described is just a simple way to get the > Low cost, GOOD Raw data. > >>>>>>>>>> What you then do with that Data is a different subject. > >>>>>>>>>> > >>>>>>>>>> You can run the raw data thru one of the many ADEV > programs out there, 'Plotter' being my choice. > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> Have fun > >>>>>>>>>> ws > >>>>>>>>>> > >>>>>>>>>> ************* > >>>>>>>>>> > >>>>>>>>>> [time-nuts] ADEV vs MDEV > >>>>>>>>>> Pete Rawson peterawson at earthlink.net > >>>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 > >>>>>>>>>> > >>>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with > >>>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It > seems that > >>>>>>>>>> existing TI counters can reach this goal in 10s. > (using MDEV estimate > >>>>>>>>>> or 100+s. using ADEV estimate). The question is; does > the MDEV tool > >>>>>>>>>> provide an appropriate measure of stability in this > time range, or is > >>>>>>>>>> the ADEV estimate a more correct answer? > >>>>>>>>>> > >>>>>>>>>> The TI performance I'm referring to is the 20-25 ps, > single shot TI, > >>>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I > have data > >>>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the > >>>>>>>>>> other counters behave similarly. > >>>>>>>>>> > >>>>>>>>>> I would appreciate any comments or observations on this topic. > >>>>>>>>>> My motivation is to discover the simplest scheme for making > >>>>>>>>>> stability measurements at this performance level; this is NOT > >>>>>>>>>> even close to the state-of-the-art, but can still be useful. > >>>>>>>>>> > >>>>>>>>>> Pete Rawson > >>>>>>>>>> > >>>>>>>>>> > >>>>>>>>>> > >>> > > > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.

Hi Take the outputs of the ADC's, bandpass filter them, do a straight line fit to the 100 or so points nearest each zero crossing. Report the result to the PC for each zero crossing. Not much data to the PC. Easy to do it all in a fairly small FPGA. Staying down at low frequencies opens up the range of offset oscillators available. The higher you go, the tougher it will be to select a good reference. Bob On Feb 6, 2010, at 7:57 PM, Bruce Griffiths wrote: > Bob Camp wrote: >> Hi >> >> My main concern with the low frequency pole in the sound card is the quality of the R/C used. You can certainly model what ever you have. If they used an aluminum electrolytic for the "C" it may not be the same next time you check it .... >> >> > One should at least calibrate the effect before and after each run. > It would be even better to embed such a calibration within each run. >> On a 10 Hz system, a 1 Hz pole is probably not an issue. It might get in the way with a 1 Hz beat note. >> >> Another thing I have only seen in passing: "Sigma Delta's have poor low frequency noise characteristics". I haven't dug into it to see if that's really true or not. If you buy your own ADC's, you certainly would not be restricted to a Sigma Delta. >> >> > The input noise spectrum for my sound card does indeed rise significantly at low frequencies. > A beat note of around 100Hz or 1KHz note would be more suitable than lower frequencies. > With a Costas receiver the equivalent timing noise isn't strongly dependent on the beat frequency. >> Even with a cheap pre-built FPGA board, you could look into higher sample rates than a conventional sound card. You would drop back to 16 bits, but it might be worth it. >> >> Bob >> >> >> > 1 bit of a 16 bit accurate ADC corresponds to a timing resolution of around 0.48ps referred to a 10MHz mixer input signal. > The beat frequency amplitude has to be amplified to just below the ADC full scale input. > > Bruce >> On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote: >> >> >>> Even better is to toss out the mixers and sample the RF signals directly. >>> However suitable ADCs cost $US100 or more each. >>> To which one has to add an FPGA and an interface to a PC with sufficient throughput to handle the down converted I + Q samples. >>> >>> Bob Camp wrote: >>> >>>> Hi >>>> >>>> You probably could put a couple of cheap DAC's >>>> >>> (ADCs are preferable as it avoids having to implement the conversion logic plus comparator required when using a DAC.) >>> >>> >>>> on a board with a FPGA and reduce the data on the fly. I'd guess that would be be in the same $100 range as a half way decent sound card. Clock the DAC's off of a 10 MHz reference and eliminate the cal issue. >>>> >>>> If you are down around 10 Hz or worse yet 1 Hz, the AC coupling of the sound card will get in the way, even with a bandpass approach. You really don't know what they may have in there at the low end. Build it yourself and that stuff's not an issue. >>>> >>>> Bob >>>> >>>> >>>> >>> My sound card has a 1Hz cutoff RC high pass input filter plus an internal high pass digital filter. >>> Its not too difficult to measure the sound card frequency response using a white noise source for example. >>> >>> Bruce >>> >>>> On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: >>>> >>>> >>>> >>>>> If one has a high end sound card then it could be used to implement the bandpass filter and replace the zero crossing detector. >>>>> It may be necessary to insert a pilot tone to calibrate the sound card sampling clock frequency. >>>>> A noise floor of about 1E-13/Tau should be achievable. >>>>> This simplifies the DMTD system by replacing the zero crossing detector with a low gain linear preamp. >>>>> >>>>> If one analyses the resultant data off line then one can also try out different techniques such as a Costas receiver rather than a simple bandpass filter plus zero crossing detector. >>>>> However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS will result in a file with a size of at least 1.15GB. >>>>> >>>>> Bruce >>>>> >>>>> >>>>> Bruce Griffiths wrote: >>>>> >>>>> >>>>>> If one were to use a bandpass filter with a Q of 10 to filter the beat frequency output of the mixer, then if the input frequency is 10MHz and the filter component tempco is 100ppm/C then the resultant phase shift tempco is about 16ps/C referred to the mixer input frequency. >>>>>> >>>>>> This phase shift tempco is certainly low enough not to have significant impact when measuring the frequency stability of a typical 10811A if the temperature fluctuations are kept small enough during the run. >>>>>> >>>>>> The effect of using a bandpass filter with too narrow a bandwidth is to artificially reduce ADEV for small Tau, so it may be prudent to use a higher beat frequency that 1Hz or even 10Hz and not calculate ADEV for Tau less than say 10(??) times the beat frequency period. A trade off between this and the effect of aliasing is required. >>>>>> >>>>>> Bruce >>>>>> >>>>>> Bob Camp wrote: >>>>>> >>>>>> >>>>>>> Hi >>>>>>> >>>>>>> With most 10811 range oscillators the impact of a simple bandpass filter is low enough to not be a major issue. That's for normal lab temperatures with the circuitry in a conventional die cast box. No guarantee if you open the window and let the fresh air blow in during the run. >>>>>>> >>>>>>> That's true with a heterodyne. I can see no obvious reason it would not be true on DMTD. >>>>>>> >>>>>>> Bob >>>>>>> >>>>>>> >>>>>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>> The only major issue with DMTD systems is that they undersample the phase fluctuations and hence are subject to aliasing effects. >>>>>>>> The low pass filter has to have a bandwidth of the same order as the beat frequency or the beat frequency signal will be significantly attenuated. >>>>>>>> Since the phase is only sampled once per beat frequency period the phase fluctuations are undersampled. >>>>>>>> Various attempts to use both zero crossings have not been successful. >>>>>>>> >>>>>>>> In principle if one can overcome the increased phase shift tempco associated with a bandpass filter, using a bandpass filter can in principle ensure that the phase fluctuations are oversampled. >>>>>>>> >>>>>>>> >>>>>>>> Bruce >>>>>>>> >>>>>>>> Bob Camp wrote: >>>>>>>> >>>>>>>> >>>>>>>>> Hi >>>>>>>>> >>>>>>>>> A straight heterodyne system will get you to the floor of most 10811's with a very simple (2 stage) limiter. As with the DMTD, the counter requirements aren't really all that severe. >>>>>>>>> >>>>>>>>> Bob >>>>>>>>> >>>>>>>>> >>>>>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> "It's possible / likely for injection lock ... to be a problem ..." >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> Something I certainly worried about and tested for. >>>>>>>>>> What I found (for MY case) is that injection lock is NOT a problem. >>>>>>>>>> The reason being is that unlike most other ways, where the two OSC have to be completely independent, >>>>>>>>>> The tight loop approach forces the Two Osc to "Lock with something like 60 + db gain, >>>>>>>>>> so a little stray -80db injection lock coupling that would very much limit other systems has >>>>>>>>>> no measurable effect at e-13. Just one of the neat little side effects that make the tight loop approach so simple. >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> "then a part in 10^14 is going to be at the 100 of nanovolts level." >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> For that example, just need to put a simple discrete 100 to 1 resistor divider >>>>>>>>>> in-between the control voltage and the EFC and now you have a nice workable 10uv. >>>>>>>>>> BUT the bigger point is, probable not needed, cause you are NOT going to do any better than the stability of the OSC with a grounded shorted EFC input. >>>>>>>>>> >>>>>>>>>> as you said and I agree is so true: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> "There is no perfect way to do any of this, only a lot of compromises ... you need to watch out for". >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> But you did not offer any easier way to do it, which is what the original request was for and my answer addressed. >>>>>>>>>> This is the cheapest easiest way BY FAR to get high performance, at low tau, ADEV numbers that I've seen. >>>>>>>>>> >>>>>>>>>> ws >>>>>>>>>> *************** >>>>>>>>>> >>>>>>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>>>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>>>> Sent: Saturday, February 06, 2010 12:09 PM >>>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> Hi >>>>>>>>>>> >>>>>>>>>>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer. >>>>>>>>>>> >>>>>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting. >>>>>>>>>>> >>>>>>>>>>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for. >>>>>>>>>>> >>>>>>>>>>> Bob >>>>>>>>>>> >>>>>>>>>>> -------------------------------------------------- >>>>>>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>>>>>> To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com> >>>>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>> Peat said: >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements. >>>>>>>>>>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level. >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>> If you accept that the measurement is going to limited by the Reference Osc, >>>>>>>>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level, >>>>>>>>>>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability". >>>>>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> By replacing the "Voltage to freq converter, Freq counter& Printer with a Radio shack type PC data logging DVM, >>>>>>>>>>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed. >>>>>>>>>>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work >>>>>>>>>>>> and a higher speed oversampling ADC data logger and a good offset voltage. >>>>>>>>>>>> >>>>>>>>>>>> I must add this is not a popular solution (Or a general Purpose one) but >>>>>>>>>>>> IF you know analog and have a GOOD osc with EFC to use for the reference, >>>>>>>>>>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance. >>>>>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >>>>>>>>>>>> >>>>>>>>>>>> Basic modified NIST Block Diag attached: >>>>>>>>>>>> The NIST paper sums it up quite nicely: >>>>>>>>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution >>>>>>>>>>>> so one has excellent precision capabilities with this system.' >>>>>>>>>>>> >>>>>>>>>>>> This does not address your other question of ADEV vs MDEV, >>>>>>>>>>>> What I've described is just a simple way to get the Low cost, GOOD Raw data. >>>>>>>>>>>> What you then do with that Data is a different subject. >>>>>>>>>>>> >>>>>>>>>>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> Have fun >>>>>>>>>>>> ws >>>>>>>>>>>> >>>>>>>>>>>> ************* >>>>>>>>>>>> >>>>>>>>>>>> [time-nuts] ADEV vs MDEV >>>>>>>>>>>> Pete Rawson peterawson at earthlink.net >>>>>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>>>>>>>> >>>>>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that >>>>>>>>>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate >>>>>>>>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool >>>>>>>>>>>> provide an appropriate measure of stability in this time range, or is >>>>>>>>>>>> the ADEV estimate a more correct answer? >>>>>>>>>>>> >>>>>>>>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI, >>>>>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data >>>>>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>>>>>>>> other counters behave similarly. >>>>>>>>>>>> >>>>>>>>>>>> I would appreciate any comments or observations on this topic. >>>>>>>>>>>> My motivation is to discover the simplest scheme for making >>>>>>>>>>>> stability measurements at this performance level; this is NOT >>>>>>>>>>>> even close to the state-of-the-art, but can still be useful. >>>>>>>>>>>> >>>>>>>>>>>> Pete Rawson >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>> >>> > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >

Which just leaves the minor problem of the offset oscillator. One option is to use a phase truncation spur free output frequency from a DDS. If one is using the Costas receiver approach the beat frequency need not be a nice round number like 1.0000KHz. Another method is to use a crystal whose frequency is offset a few kHz from 10MHz. Yet another is the classical method of dividing 10MHz by 100 and subtracting (using an LSB mixer) the resultant 100KHz from 10MHz to produce 9.9MHz, then divide the 9.9MHz signal by 100 and add (using a USB mixer) the resultant 99kHz signal to the 9.99Mhz signal to produce a 9.999MHz output. Bruce John Miles wrote: > A sound-card back end has always seemed like a pretty reasonable approach to > me, if you're inclined to go the DMTD route. I wouldn't send a 'baseband' > signal to the sound card, though -- I'd upconvert it to a few kHz to get > away from the numerous bad things that sound cards do near DC. > > -- john, KE5FX > > > >> Hi >> >> My main concern with the low frequency pole in the sound card is >> the quality of the R/C used. You can certainly model what ever >> you have. If they used an aluminum electrolytic for the "C" it >> may not be the same next time you check it .... >> >> On a 10 Hz system, a 1 Hz pole is probably not an issue. It might >> get in the way with a 1 Hz beat note. >> >> Another thing I have only seen in passing: "Sigma Delta's have >> poor low frequency noise characteristics". I haven't dug into it >> to see if that's really true or not. If you buy your own ADC's, >> you certainly would not be restricted to a Sigma Delta. >> >> Even with a cheap pre-built FPGA board, you could look into >> higher sample rates than a conventional sound card. You would >> drop back to 16 bits, but it might be worth it. >> >> Bob >> >> >> On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote: >> >> >>> Even better is to toss out the mixers and sample the RF signals >>> >> directly. >> >>> However suitable ADCs cost $US100 or more each. >>> To which one has to add an FPGA and an interface to a PC with >>> >> sufficient throughput to handle the down converted I + Q samples. >> >>> Bob Camp wrote: >>> >>>> Hi >>>> >>>> You probably could put a couple of cheap DAC's >>>> >>> (ADCs are preferable as it avoids having to implement the >>> >> conversion logic plus comparator required when using a DAC.) >> >>> >>>> on a board with a FPGA and reduce the data on the fly. I'd >>>> >> guess that would be be in the same $100 range as a half way >> decent sound card. Clock the DAC's off of a 10 MHz reference and >> eliminate the cal issue. >> >>>> If you are down around 10 Hz or worse yet 1 Hz, the AC >>>> >> coupling of the sound card will get in the way, even with a >> bandpass approach. You really don't know what they may have in >> there at the low end. Build it yourself and that stuff's not an issue. >> >>>> Bob >>>> >>>> >>>> >>> My sound card has a 1Hz cutoff RC high pass input filter plus >>> >> an internal high pass digital filter. >> >>> Its not too difficult to measure the sound card frequency >>> >> response using a white noise source for example. >> >>> Bruce >>> >>>> On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote: >>>> >>>> >>>> >>>>> If one has a high end sound card then it could be used to >>>>> >> implement the bandpass filter and replace the zero crossing detector. >> >>>>> It may be necessary to insert a pilot tone to calibrate the >>>>> >> sound card sampling clock frequency. >> >>>>> A noise floor of about 1E-13/Tau should be achievable. >>>>> This simplifies the DMTD system by replacing the zero >>>>> >> crossing detector with a low gain linear preamp. >> >>>>> If one analyses the resultant data off line then one can also >>>>> >> try out different techniques such as a Costas receiver rather >> than a simple bandpass filter plus zero crossing detector. >> >>>>> However 1000 seconds of data for 2 channels of 24 bit samples >>>>> >> at 192KSPS will result in a file with a size of at least 1.15GB. >> >>>>> Bruce >>>>> >>>>> >>>>> Bruce Griffiths wrote: >>>>> >>>>> >>>>>> If one were to use a bandpass filter with a Q of 10 to >>>>>> >> filter the beat frequency output of the mixer, then if the input >> frequency is 10MHz and the filter component tempco is 100ppm/C >> then the resultant phase shift tempco is about 16ps/C referred to >> the mixer input frequency. >> >>>>>> This phase shift tempco is certainly low enough not to have >>>>>> >> significant impact when measuring the frequency stability of a >> typical 10811A if the temperature fluctuations are kept small >> enough during the run. >> >>>>>> The effect of using a bandpass filter with too narrow a >>>>>> >> bandwidth is to artificially reduce ADEV for small Tau, so it may >> be prudent to use a higher beat frequency that 1Hz or even 10Hz >> and not calculate ADEV for Tau less than say 10(??) times the >> beat frequency period. A trade off between this and the effect of >> aliasing is required. >> >>>>>> Bruce >>>>>> >>>>>> Bob Camp wrote: >>>>>> >>>>>> >>>>>>> Hi >>>>>>> >>>>>>> With most 10811 range oscillators the impact of a simple >>>>>>> >> bandpass filter is low enough to not be a major issue. That's for >> normal lab temperatures with the circuitry in a conventional die >> cast box. No guarantee if you open the window and let the fresh >> air blow in during the run. >> >>>>>>> That's true with a heterodyne. I can see no obvious reason >>>>>>> >> it would not be true on DMTD. >> >>>>>>> Bob >>>>>>> >>>>>>> >>>>>>> On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>> The only major issue with DMTD systems is that they >>>>>>>> >> undersample the phase fluctuations and hence are subject to >> aliasing effects. >> >>>>>>>> The low pass filter has to have a bandwidth of the same >>>>>>>> >> order as the beat frequency or the beat frequency signal will be >> significantly attenuated. >> >>>>>>>> Since the phase is only sampled once per beat frequency >>>>>>>> >> period the phase fluctuations are undersampled. >> >>>>>>>> Various attempts to use both zero crossings have not been >>>>>>>> >> successful. >> >>>>>>>> In principle if one can overcome the increased phase shift >>>>>>>> >> tempco associated with a bandpass filter, using a bandpass filter >> can in principle ensure that the phase fluctuations are oversampled. >> >>>>>>>> >>>>>>>> Bruce >>>>>>>> >>>>>>>> Bob Camp wrote: >>>>>>>> >>>>>>>> >>>>>>>>> Hi >>>>>>>>> >>>>>>>>> A straight heterodyne system will get you to the floor of >>>>>>>>> >> most 10811's with a very simple (2 stage) limiter. As with the >> DMTD, the counter requirements aren't really all that severe. >> >>>>>>>>> Bob >>>>>>>>> >>>>>>>>> >>>>>>>>> On Feb 6, 2010, at 4:24 PM, WarrenS wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> >>>>>>>>>>> "It's possible / likely for injection lock ... to be a >>>>>>>>>>> >> problem ..." >> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> Something I certainly worried about and tested for. >>>>>>>>>> What I found (for MY case) is that injection lock is NOT >>>>>>>>>> >> a problem. >> >>>>>>>>>> The reason being is that unlike most other ways, where >>>>>>>>>> >> the two OSC have to be completely independent, >> >>>>>>>>>> The tight loop approach forces the Two Osc to "Lock with >>>>>>>>>> >> something like 60 + db gain, >> >>>>>>>>>> so a little stray -80db injection lock coupling that >>>>>>>>>> >> would very much limit other systems has >> >>>>>>>>>> no measurable effect at e-13. Just one of the neat >>>>>>>>>> >> little side effects that make the tight loop approach so simple. >> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> "then a part in 10^14 is going to be at the 100 of >>>>>>>>>>> >> nanovolts level." >> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> For that example, just need to put a simple discrete 100 >>>>>>>>>> >> to 1 resistor divider >> >>>>>>>>>> in-between the control voltage and the EFC and now you >>>>>>>>>> >> have a nice workable 10uv. >> >>>>>>>>>> BUT the bigger point is, probable not needed, cause you >>>>>>>>>> >> are NOT going to do any better than the stability of the OSC with >> a grounded shorted EFC input. >> >>>>>>>>>> as you said and I agree is so true: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> "There is no perfect way to do any of this, only a lot >>>>>>>>>>> >> of compromises ... you need to watch out for". >> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> But you did not offer any easier way to do it, which is >>>>>>>>>> >> what the original request was for and my answer addressed. >> >>>>>>>>>> This is the cheapest easiest way BY FAR to get high >>>>>>>>>> >> performance, at low tau, ADEV numbers that I've seen. >> >>>>>>>>>> ws >>>>>>>>>> *************** >>>>>>>>>> >>>>>>>>>> ----- Original Message ----- From: "Bob Camp"<lists@cq.nu> >>>>>>>>>> To: "Discussion of precise time and frequency >>>>>>>>>> >> measurement"<time-nuts@febo.com> >> >>>>>>>>>> Sent: Saturday, February 06, 2010 12:09 PM >>>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> Hi >>>>>>>>>>> >>>>>>>>>>> It's possible / likely to injection lock with the tight >>>>>>>>>>> >> loop approach and get data that's much better than reality. A lot >> depends on the specific oscillators under test and the buffers >> (if any) between the oscillators and mixer. >> >>>>>>>>>>> If your OCVCXO has a tuning slope of 0.1 ppm / volt >>>>>>>>>>> >> then a part in 10^14 is going to be at the 100 of nanovolts >> level. Certainly not impossible, but it does present it's own set >> of issues. Lab gear to do it is available, but not all that >> common. DC offsets and their temperature coefficients along with >> thermocouple effects could make things exciting. >> >>>>>>>>>>> There is no perfect way to do any of this, only a lot >>>>>>>>>>> >> of compromises here or there. Each approach has stuff you need to >> watch out for. >> >>>>>>>>>>> Bob >>>>>>>>>>> >>>>>>>>>>> -------------------------------------------------- >>>>>>>>>>> From: "WarrenS"<warrensjmail-one@yahoo.com> >>>>>>>>>>> Sent: Saturday, February 06, 2010 2:19 PM >>>>>>>>>>> To: "Discussion of precise time and frequency >>>>>>>>>>> >> measurement"<time-nuts@febo.com> >> >>>>>>>>>>> Subject: Re: [time-nuts] ADEV vs MDEV >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>> Peat said: >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>> I would appreciate any comments or observations on >>>>>>>>>>>>> >> the topic of apparatus with demonstrated stability measurements. >> >>>>>>>>>>>>> My motivation is to discover the SIMPLEST scheme for >>>>>>>>>>>>> >> making stability measurements at the 1E-13 in 1s performance level. >> >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>> If you accept that the measurement is going to limited >>>>>>>>>>>> >> by the Reference Osc, >> >>>>>>>>>>>> for Low COST and SIMPLE, with the ability to measure >>>>>>>>>>>> >> ADEVs at that level, >> >>>>>>>>>>>> Can't beat a simple analog version of NIST's "Tight >>>>>>>>>>>> >> Phase-Lock Loop Method of measuring Freq stability". >> >>>>>>>>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7 >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> By replacing the "Voltage to freq converter, Freq >>>>>>>>>>>> >> counter& Printer with a Radio shack type PC data logging DVM, >> >>>>>>>>>>>> It can be up and running from scratch in under an Hr, >>>>>>>>>>>> >> with no high end test equipment needed. >> >>>>>>>>>>>> If you want performance that exceeds the best of most >>>>>>>>>>>> >> DMTD at low Tau it takes a little more work >> >>>>>>>>>>>> and a higher speed oversampling ADC data logger and a >>>>>>>>>>>> >> good offset voltage. >> >>>>>>>>>>>> I must add this is not a popular solution (Or a >>>>>>>>>>>> >> general Purpose one) but >> >>>>>>>>>>>> IF you know analog and have a GOOD osc with EFC to >>>>>>>>>>>> >> use for the reference, >> >>>>>>>>>>>> as far as I've been able to determine it is the BEST >>>>>>>>>>>> >> SIMPLE answer that allows High performance. >> >>>>>>>>>>>> Limited by My HP10811 Ref OSC, I'm getting better than >>>>>>>>>>>> >> 1e-12 in 0.1 sec (at 30 Hz Bandwidth) >> >>>>>>>>>>>> Basic modified NIST Block Diag attached: >>>>>>>>>>>> The NIST paper sums it up quite nicely: >>>>>>>>>>>> 'It is not difficult to achieve a sensitivity of a >>>>>>>>>>>> >> part in e14 per Hz resolution >> >>>>>>>>>>>> so one has excellent precision capabilities with this system.' >>>>>>>>>>>> >>>>>>>>>>>> This does not address your other question of ADEV vs MDEV, >>>>>>>>>>>> What I've described is just a simple way to get the >>>>>>>>>>>> >> Low cost, GOOD Raw data. >> >>>>>>>>>>>> What you then do with that Data is a different subject. >>>>>>>>>>>> >>>>>>>>>>>> You can run the raw data thru one of the many ADEV >>>>>>>>>>>> >> programs out there, 'Plotter' being my choice. >> >>>>>>>>>>>> >>>>>>>>>>>> Have fun >>>>>>>>>>>> ws >>>>>>>>>>>> >>>>>>>>>>>> ************* >>>>>>>>>>>> >>>>>>>>>>>> [time-nuts] ADEV vs MDEV >>>>>>>>>>>> Pete Rawson peterawson at earthlink.net >>>>>>>>>>>> Sat Feb 6 03:59:18 UTC 2010 >>>>>>>>>>>> >>>>>>>>>>>> Efforts are underway to develop a low cost DMTD apparatus with >>>>>>>>>>>> demonstrated stability measurements of 1E-13 in 1s. It >>>>>>>>>>>> >> seems that >> >>>>>>>>>>>> existing TI counters can reach this goal in 10s. >>>>>>>>>>>> >> (using MDEV estimate >> >>>>>>>>>>>> or 100+s. using ADEV estimate). The question is; does >>>>>>>>>>>> >> the MDEV tool >> >>>>>>>>>>>> provide an appropriate measure of stability in this >>>>>>>>>>>> >> time range, or is >> >>>>>>>>>>>> the ADEV estimate a more correct answer? >>>>>>>>>>>> >>>>>>>>>>>> The TI performance I'm referring to is the 20-25 ps, >>>>>>>>>>>> >> single shot TI, >> >>>>>>>>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I >>>>>>>>>>>> >> have data >> >>>>>>>>>>>> from my CNT81showing MDEV< 1E-13 in 10s. and I believe the >>>>>>>>>>>> other counters behave similarly. >>>>>>>>>>>> >>>>>>>>>>>> I would appreciate any comments or observations on this topic. >>>>>>>>>>>> My motivation is to discover the simplest scheme for making >>>>>>>>>>>> stability measurements at this performance level; this is NOT >>>>>>>>>>>> even close to the state-of-the-art, but can still be useful. >>>>>>>>>>>> >>>>>>>>>>>> Pete Rawson >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>