The recent discussions about the simple digital mixer got me thinking about the performance vs. complexity trade offs when measuring accurate, high resolution, phase drift differences between two oscillators. It would seem to me, that using both the positive and negative slope edges of the high freq sinewave signal is a better way to go. Is using just one edge, acceptable for a 'state of the art' Phase drift measurements? I am not suggesting the KISS approach is the wrong solution for Simon. I am questioning if the paper posted, is the best way for CERN to make a state of the art femtosecond DDMDT? Here is an extreme example of throwing away useful data for the sake of simplicity: When measuring phase drift of a 10 MHz osc using just a 1PPS signal, 19,999,999 other possible data points are being discarded. Using all possible data points could decrease the noise floor considerably. (by ~5,000 to 1) ws >> ------------------------------------------------------------------------------- >> Tom Posted >> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >> Hi Simon, >> >> Some additional info. I first heard about the D-FF method of frequency >> comparison in the late 90's (from Rick Hambly, I think) on the old gps >> mailing list. It sounded really interesting. Since then, the subject has >> turned up every few years on this list. But each time, the topic seems to >> go away quietly with little or no data, plots or explanation. In >> addition, none of the commercial products I've taken apart appear to use >> this approach. Hmm. So that begs the question -- what's really going on, >> and why. >> >> I'm enjoying this thread because you've shown both technical competence >> and optimistic persistence. Perhaps once and for all, with your efforts, >> we can settle this matter. You will either find a working combination >> with excellent performance, or you will uncover enough uncontrolled >> variables that you never want to try it again. Either way, we all learn a >> lot. Keep the photos, data, and plots coming. >> >> Thanks, >> /tvb >> ------------------------------------------------------------------ >> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop > >> Bruce posted >> http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf >> >> among other things illustrates a modified approach to the offset >> generator by replacing the intermediate phase locked VCXO with a bandpass >> filter. >> >> ------------------------------------------------------------------ > Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop > Simon posted www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ... > The idea is based on the following article which describes creating a > digital DMTD with an FPGA for clocks @ 125mhz: >

Which is why the new style instruments sampling the waveforms with a common clock and then downsampling digitally until churning out phase data for further processing can achieve such a good measurement floor. See Sam Steins papers. For some applications the DDMTD approach is pretty amazing precision for it's simplicity. For some you can get more. Cheers, Magnus On 10/22/2014 08:09 PM, WarrenS via time-nuts wrote: > > > The recent discussions about the simple digital mixer got me thinking > about > the performance vs. complexity trade offs when measuring accurate, high > resolution, phase drift differences between two oscillators. > It would seem to me, that using both the positive and negative slope edges > of the high freq sinewave signal is a better way to go. > Is using just one edge, acceptable for a 'state of the art' Phase drift > measurements? > > I am not suggesting the KISS approach is the wrong solution for Simon. > I am questioning if the paper posted, is the best way for CERN to make a > state of the art femtosecond DDMDT? > > Here is an extreme example of throwing away useful data for the sake of > simplicity: > When measuring phase drift of a 10 MHz osc using just a 1PPS signal, > 19,999,999 other possible data points are being discarded. > Using all possible data points could decrease the noise floor considerably. > (by ~5,000 to 1) > > ws > >>> ------------------------------------------------------------------------------- >>> >>> Tom Posted >>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>> Hi Simon, >>> >>> Some additional info. I first heard about the D-FF method of >>> frequency comparison in the late 90's (from Rick Hambly, I think) on >>> the old gps mailing list. It sounded really interesting. Since then, >>> the subject has turned up every few years on this list. But each >>> time, the topic seems to go away quietly with little or no data, >>> plots or explanation. In addition, none of the commercial products >>> I've taken apart appear to use this approach. Hmm. So that begs the >>> question -- what's really going on, and why. >>> >>> I'm enjoying this thread because you've shown both technical >>> competence and optimistic persistence. Perhaps once and for all, with >>> your efforts, we can settle this matter. You will either find a >>> working combination with excellent performance, or you will uncover >>> enough uncontrolled variables that you never want to try it again. >>> Either way, we all learn a lot. Keep the photos, data, and plots coming. >>> >>> Thanks, >>> /tvb >>> ------------------------------------------------------------------ >>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop >> >>> Bruce posted >>> http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf >>> >>> among other things illustrates a modified approach to the offset >>> generator by replacing the intermediate phase locked VCXO with a >>> bandpass filter. >>> >>> ------------------------------------------------------------------ >> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop >> Simon posted www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ... >> The idea is based on the following article which describes creating a >> digital DMTD with an FPGA for clocks @ 125mhz: > > > _______________________________________________ > 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.

Even more effective would be to sample the entire 10MHz waveform instead of just the zero crossing. By doing a best fit of the entire waveform, you should be able to estimate the zero crossing with much greater precision because now the noise is averaged over the entire waveform instead of a single point at the zero crossing. I wish my signal processing were better than they are and that I had some time to evaluate that. Didier KO4BB On October 22, 2014 1:09:11 PM CDT, WarrenS via time-nuts <time-nuts@febo.com> wrote: > > >The recent discussions about the simple digital mixer got me thinking >about >the performance vs. complexity trade offs when measuring accurate, high >resolution, phase drift differences between two oscillators. >It would seem to me, that using both the positive and negative slope >edges >of the high freq sinewave signal is a better way to go. >Is using just one edge, acceptable for a 'state of the art' Phase drift >measurements? > >I am not suggesting the KISS approach is the wrong solution for Simon. >I am questioning if the paper posted, is the best way for CERN to make >a >state of the art femtosecond DDMDT? > >Here is an extreme example of throwing away useful data for the sake of >simplicity: >When measuring phase drift of a 10 MHz osc using just a 1PPS signal, >19,999,999 other possible data points are being discarded. >Using all possible data points could decrease the noise floor >considerably. >(by ~5,000 to 1) > >ws > >>> >------------------------------------------------------------------------------- >>> Tom Posted >>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>> Hi Simon, >>> >>> Some additional info. I first heard about the D-FF method of >frequency >>> comparison in the late 90's (from Rick Hambly, I think) on the old >gps >>> mailing list. It sounded really interesting. Since then, the subject >has >>> turned up every few years on this list. But each time, the topic >seems to >>> go away quietly with little or no data, plots or explanation. In >>> addition, none of the commercial products I've taken apart appear to >use >>> this approach. Hmm. So that begs the question -- what's really going >on, >>> and why. >>> >>> I'm enjoying this thread because you've shown both technical >competence >>> and optimistic persistence. Perhaps once and for all, with your >efforts, >>> we can settle this matter. You will either find a working >combination >>> with excellent performance, or you will uncover enough uncontrolled >>> variables that you never want to try it again. Either way, we all >learn a >>> lot. Keep the photos, data, and plots coming. >>> >>> Thanks, >>> /tvb >>> ------------------------------------------------------------------ >>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >Flop >> >>> Bruce posted >>> >http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf >>> >>> among other things illustrates a modified approach to the offset >>> generator by replacing the intermediate phase locked VCXO with a >bandpass >>> filter. >>> >>> ------------------------------------------------------------------ >> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >Flop >> Simon posted www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ... >> The idea is based on the following article which describes creating a >> digital DMTD with an FPGA for clocks @ 125mhz: > > >_______________________________________________ >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. -- Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things.

Hi The more you “curve fit” or “average” the more you are filtering the data. Filtering does indeed impact the ADEV both at short tau’s and longer tau’s. You need to be very careful if you filter or you will mess up the data. Bob > On Oct 22, 2014, at 7:42 PM, Didier Juges <shalimr9@gmail.com> wrote: > > Even more effective would be to sample the entire 10MHz waveform instead of just the zero crossing. By doing a best fit of the entire waveform, you should be able to estimate the zero crossing with much greater precision because now the noise is averaged over the entire waveform instead of a single point at the zero crossing. > > I wish my signal processing were better than they are and that I had some time to evaluate that. > > Didier KO4BB > > > On October 22, 2014 1:09:11 PM CDT, WarrenS via time-nuts <time-nuts@febo.com> wrote: >> >> >> The recent discussions about the simple digital mixer got me thinking >> about >> the performance vs. complexity trade offs when measuring accurate, high >> resolution, phase drift differences between two oscillators. >> It would seem to me, that using both the positive and negative slope >> edges >> of the high freq sinewave signal is a better way to go. >> Is using just one edge, acceptable for a 'state of the art' Phase drift >> measurements? >> >> I am not suggesting the KISS approach is the wrong solution for Simon. >> I am questioning if the paper posted, is the best way for CERN to make >> a >> state of the art femtosecond DDMDT? >> >> Here is an extreme example of throwing away useful data for the sake of >> simplicity: >> When measuring phase drift of a 10 MHz osc using just a 1PPS signal, >> 19,999,999 other possible data points are being discarded. >> Using all possible data points could decrease the noise floor >> considerably. >> (by ~5,000 to 1) >> >> ws >> >>>> >> ------------------------------------------------------------------------------- >>>> Tom Posted >>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>>> Hi Simon, >>>> >>>> Some additional info. I first heard about the D-FF method of >> frequency >>>> comparison in the late 90's (from Rick Hambly, I think) on the old >> gps >>>> mailing list. It sounded really interesting. Since then, the subject >> has >>>> turned up every few years on this list. But each time, the topic >> seems to >>>> go away quietly with little or no data, plots or explanation. In >>>> addition, none of the commercial products I've taken apart appear to >> use >>>> this approach. Hmm. So that begs the question -- what's really going >> on, >>>> and why. >>>> >>>> I'm enjoying this thread because you've shown both technical >> competence >>>> and optimistic persistence. Perhaps once and for all, with your >> efforts, >>>> we can settle this matter. You will either find a working >> combination >>>> with excellent performance, or you will uncover enough uncontrolled >>>> variables that you never want to try it again. Either way, we all >> learn a >>>> lot. Keep the photos, data, and plots coming. >>>> >>>> Thanks, >>>> /tvb >>>> ------------------------------------------------------------------ >>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >> Flop >>> >>>> Bruce posted >>>> >> http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf >>>> >>>> among other things illustrates a modified approach to the offset >>>> generator by replacing the intermediate phase locked VCXO with a >> bandpass >>>> filter. >>>> >>>> ------------------------------------------------------------------ >>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >> Flop >>> Simon posted www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ... >>> The idea is based on the following article which describes creating a >>> digital DMTD with an FPGA for clocks @ 125mhz: > >> >> _______________________________________________ >> 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. > > -- > Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things. > _______________________________________________ > 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.

There are two ways that both positive and negative slopes could be used, that is, with the input clocks and/or with the reference clock. The PRU on the BBB is not really fast enough to identify the edge direction at a 10mhz rate, so I only collect state changes in real time and then sort it out the direction on the ARM processor afterwards. All transitions are useful for glitch identification, and this does mean I'm already capturing the negative edge of the input signals for free. Admittedly, I'm not doing anything with this data as I currently filter for rising edges fairly early on, but it is 'just a software problem' to utilise data that is already there. _How_ to use the data is the point of discussion, but it will be fairly trivial to implement any ideas and see if they stack up in practice against real data. Regarding the reference clock, utilising the negative edge could be used to double the sample rate (like DDR RAM). I've already been using a 74AC14 Schmitt inverter on the reference clock, primarily as a buffer to distribute the clock to each sampler (flip flop or shift register) and the BBB, but also to try different sample timings (e.g. clocking the samplers on the +ve edge and the BBB capture on the -ve edge for example). Clocking different samplers on alternate edges (but with the same input signal) is therefore relatively straightforward, and feeding more sampling channels to the BBB is not too much of a big deal either. It is not something I have tried though. My initial thought was that doubling the sample rate doesn't buy much, as you could get the same resolution by changing the beat frequency. However, it may help control glitching by obtaining the resolution at a higher beat frequency (greater offset between reference and DUT). Accurately knowing the duty cycle of the reference clock would be essential though so that the time of the -ve edge sample was known. Cheers Simon On 22/10/2014 19:09, WarrenS via time-nuts wrote: > > > The recent discussions about the simple digital mixer got me thinking > about > the performance vs. complexity trade offs when measuring accurate, high > resolution, phase drift differences between two oscillators. > It would seem to me, that using both the positive and negative slope > edges > of the high freq sinewave signal is a better way to go. > Is using just one edge, acceptable for a 'state of the art' Phase drift > measurements? > > I am not suggesting the KISS approach is the wrong solution for Simon. > I am questioning if the paper posted, is the best way for CERN to make a > state of the art femtosecond DDMDT? > > Here is an extreme example of throwing away useful data for the sake of > simplicity: > When measuring phase drift of a 10 MHz osc using just a 1PPS signal, > 19,999,999 other possible data points are being discarded. > Using all possible data points could decrease the noise floor > considerably. > (by ~5,000 to 1) > > ws

Depends on what dominant noises you try to measure. Phase white and phase flicker noise depends on bandwidth, and averaging provides filtering effects that effect those. Filtering will also effect systematic signals, but you should never use ADEV for such noises, it's a bad estimator for them. Cheers, Magnus On 10/23/2014 02:42 AM, Bob Camp wrote: > Hi > > The more you “curve fit” or “average” the more you are filtering the data. Filtering does indeed impact the ADEV both at short tau’s and longer tau’s. You need to be very careful if you filter or you will mess up the data. > > Bob > >> On Oct 22, 2014, at 7:42 PM, Didier Juges <shalimr9@gmail.com> wrote: >> >> Even more effective would be to sample the entire 10MHz waveform instead of just the zero crossing. By doing a best fit of the entire waveform, you should be able to estimate the zero crossing with much greater precision because now the noise is averaged over the entire waveform instead of a single point at the zero crossing. >> >> I wish my signal processing were better than they are and that I had some time to evaluate that. >> >> Didier KO4BB >> >> >> On October 22, 2014 1:09:11 PM CDT, WarrenS via time-nuts <time-nuts@febo.com> wrote: >>> >>> >>> The recent discussions about the simple digital mixer got me thinking >>> about >>> the performance vs. complexity trade offs when measuring accurate, high >>> resolution, phase drift differences between two oscillators. >>> It would seem to me, that using both the positive and negative slope >>> edges >>> of the high freq sinewave signal is a better way to go. >>> Is using just one edge, acceptable for a 'state of the art' Phase drift >>> measurements? >>> >>> I am not suggesting the KISS approach is the wrong solution for Simon. >>> I am questioning if the paper posted, is the best way for CERN to make >>> a >>> state of the art femtosecond DDMDT? >>> >>> Here is an extreme example of throwing away useful data for the sake of >>> simplicity: >>> When measuring phase drift of a 10 MHz osc using just a 1PPS signal, >>> 19,999,999 other possible data points are being discarded. >>> Using all possible data points could decrease the noise floor >>> considerably. >>> (by ~5,000 to 1) >>> >>> ws >>> >>>>> >>> ------------------------------------------------------------------------------- >>>>> Tom Posted >>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>>>> Hi Simon, >>>>> >>>>> Some additional info. I first heard about the D-FF method of >>> frequency >>>>> comparison in the late 90's (from Rick Hambly, I think) on the old >>> gps >>>>> mailing list. It sounded really interesting. Since then, the subject >>> has >>>>> turned up every few years on this list. But each time, the topic >>> seems to >>>>> go away quietly with little or no data, plots or explanation. In >>>>> addition, none of the commercial products I've taken apart appear to >>> use >>>>> this approach. Hmm. So that begs the question -- what's really going >>> on, >>>>> and why. >>>>> >>>>> I'm enjoying this thread because you've shown both technical >>> competence >>>>> and optimistic persistence. Perhaps once and for all, with your >>> efforts, >>>>> we can settle this matter. You will either find a working >>> combination >>>>> with excellent performance, or you will uncover enough uncontrolled >>>>> variables that you never want to try it again. Either way, we all >>> learn a >>>>> lot. Keep the photos, data, and plots coming. >>>>> >>>>> Thanks, >>>>> /tvb >>>>> ------------------------------------------------------------------ >>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>> Flop >>>> >>>>> Bruce posted >>>>> >>> http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf >>>>> >>>>> among other things illustrates a modified approach to the offset >>>>> generator by replacing the intermediate phase locked VCXO with a >>> bandpass >>>>> filter. >>>>> >>>>> ------------------------------------------------------------------ >>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>> Flop >>>> Simon posted www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ... >>>> The idea is based on the following article which describes creating a >>>> digital DMTD with an FPGA for clocks @ 125mhz: > >>> >>> _______________________________________________ >>> 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. >> >> -- >> Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things. >> _______________________________________________ >> 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 Back in the early days of ADEV, the standard HP gear had a 60 KHz bandwidth. The question that came up *every* FCS and PTTI was “why does it change with bandwidth / should we spec the bandwidth?”. This went on for at least 15 years before anybody really came up with a “use a narrow bandwidth” answer. Bob > On Oct 23, 2014, at 5:40 PM, Magnus Danielson <magnus@rubidium.dyndns.org> wrote: > > Depends on what dominant noises you try to measure. Phase white and phase flicker noise depends on bandwidth, and averaging provides filtering effects that effect those. > > Filtering will also effect systematic signals, but you should never use ADEV for such noises, it's a bad estimator for them. > > Cheers, > Magnus > > On 10/23/2014 02:42 AM, Bob Camp wrote: >> Hi >> >> The more you “curve fit” or “average” the more you are filtering the data. Filtering does indeed impact the ADEV both at short tau’s and longer tau’s. You need to be very careful if you filter or you will mess up the data. >> >> Bob >> >>> On Oct 22, 2014, at 7:42 PM, Didier Juges <shalimr9@gmail.com> wrote: >>> >>> Even more effective would be to sample the entire 10MHz waveform instead of just the zero crossing. By doing a best fit of the entire waveform, you should be able to estimate the zero crossing with much greater precision because now the noise is averaged over the entire waveform instead of a single point at the zero crossing. >>> >>> I wish my signal processing were better than they are and that I had some time to evaluate that. >>> >>> Didier KO4BB >>> >>> >>> On October 22, 2014 1:09:11 PM CDT, WarrenS via time-nuts <time-nuts@febo.com> wrote: >>>> >>>> >>>> The recent discussions about the simple digital mixer got me thinking >>>> about >>>> the performance vs. complexity trade offs when measuring accurate, high >>>> resolution, phase drift differences between two oscillators. >>>> It would seem to me, that using both the positive and negative slope >>>> edges >>>> of the high freq sinewave signal is a better way to go. >>>> Is using just one edge, acceptable for a 'state of the art' Phase drift >>>> measurements? >>>> >>>> I am not suggesting the KISS approach is the wrong solution for Simon. >>>> I am questioning if the paper posted, is the best way for CERN to make >>>> a >>>> state of the art femtosecond DDMDT? >>>> >>>> Here is an extreme example of throwing away useful data for the sake of >>>> simplicity: >>>> When measuring phase drift of a 10 MHz osc using just a 1PPS signal, >>>> 19,999,999 other possible data points are being discarded. >>>> Using all possible data points could decrease the noise floor >>>> considerably. >>>> (by ~5,000 to 1) >>>> >>>> ws >>>> >>>>>> >>>> ------------------------------------------------------------------------------- >>>>>> Tom Posted >>>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>>>>> Hi Simon, >>>>>> >>>>>> Some additional info. I first heard about the D-FF method of >>>> frequency >>>>>> comparison in the late 90's (from Rick Hambly, I think) on the old >>>> gps >>>>>> mailing list. It sounded really interesting. Since then, the subject >>>> has >>>>>> turned up every few years on this list. But each time, the topic >>>> seems to >>>>>> go away quietly with little or no data, plots or explanation. In >>>>>> addition, none of the commercial products I've taken apart appear to >>>> use >>>>>> this approach. Hmm. So that begs the question -- what's really going >>>> on, >>>>>> and why. >>>>>> >>>>>> I'm enjoying this thread because you've shown both technical >>>> competence >>>>>> and optimistic persistence. Perhaps once and for all, with your >>>> efforts, >>>>>> we can settle this matter. You will either find a working >>>> combination >>>>>> with excellent performance, or you will uncover enough uncontrolled >>>>>> variables that you never want to try it again. Either way, we all >>>> learn a >>>>>> lot. Keep the photos, data, and plots coming. >>>>>> >>>>>> Thanks, >>>>>> /tvb >>>>>> ------------------------------------------------------------------ >>>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>>> Flop >>>>> >>>>>> Bruce posted >>>>>> >>>> http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/36903/1/01-2617.pdf >>>>>> >>>>>> among other things illustrates a modified approach to the offset >>>>>> generator by replacing the intermediate phase locked VCXO with a >>>> bandpass >>>>>> filter. >>>>>> >>>>>> ------------------------------------------------------------------ >>>>> Re: [time-nuts] Digital Mixing with a BeagleBone Black and D Flip >>>> Flop >>>>> Simon posted www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf ... >>>>> The idea is based on the following article which describes creating a >>>>> digital DMTD with an FPGA for clocks @ 125mhz: > >>>> >>>> _______________________________________________ >>>> 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. >>> >>> -- >>> Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things. >>> _______________________________________________ >>> 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. >> > _______________________________________________ > 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 Warren, I arrive a bit late to this discussion, but I hope I can help. I guess the reason for using only one edge is based on the fact that WR is originally designed to measure the phase between a decoded data clock and a system clock. The problem is that this decoded data clock is locked to the incoming data by means of a PFD in the Spartan6/Virtex6 GTP. The PFD normaly only looks at rising edges, so any change in the clock duty cycle will translate in a phase change in the falling edge and not in the rising edge. I am not sure this is really the case, but we certainly had this discussion at the time, but I don't remember if there was any real measurement made. Cheers, Pablo

> ...The problem is that this decoded data clock is locked to the incoming > data by means of a PFD in the Spartan6/Virtex6 GTP. The PFD normaly only > looks at rising edges, so any change in the clock duty cycle will translate > in a phase change in the falling edge and not in the rising edge. I am not > sure this is really the case, but we certainly had this discussion at the > time, but I don't remember if there was any real measurement made. > Well, I don't like correcting myself but this is not right. The PFD is only used in the TX path. In the RX path the clock is decoded using CDR using a VCO which operates at the 1.25Gb/s and then is divided down to 125, so the duty cycle problem is not really a big issue here. In any case it is the typical bug one tries to avoid when doing precise timing. Notice that in this case it would not have been a good idea to sample with the system clock falling edge to increase the performance. pablo >