Question for my new GPSDO
The BME series requires a fair amount of code to convert data from the
sensor into human readable data like degrees. I use the BME280 in some
applications.
Personally, for just temperature sensing, I found good old fashion
thermistors to be cheaper and more accurate than most silicon sensors while
requiring a single precision resistor and one ADC channel. When used with a
12 bit ADC, they offer excellent resolution around a relatively small
temperature range, (progressively degrading resolution as you go away from
the optimum temperature) which is ideal for an oven controller. They are
also easy to use in noisy environments.
Didier KO4BB
On Tue, Oct 8, 2019, 3:01 PM Adrian Godwin artgodwin@gmail.com wrote:
Jim mentions the LM34 for sensitivity and LM34 or LM35 for cost compared
with the DS1620. But also look at the BME280 : it has digital measurement
of temperature with 0.01C resolution (50 times better than the DS1620),
costs £5 on digikey but less mounted on a breakout from ebay, and also
measures pressure. The very similar BME280 also measures humidity.
On Tue, Oct 8, 2019 at 5:01 PM Bob kb8tq kb8tq@n1k.org wrote:
Hi
What is the temperature stability of the OCXO you plan to use? In the
case
of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control
system
impacts the output. With a small enough range, you may not need a very
good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature
stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A
Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of
0.1
ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to
1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s /
references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every
component
you will find is rated for a gradual rather than fast change. The impact
of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t
open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater.
That
current flows through a ground pin. The ground pin has a resistance. On
pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return).
Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that
the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch
wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven
heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4)
show info about the OCXO temperature when connected via Lady Heather. Do
you know how they estimate the temperature? I checked almost every IC on
the STAR4 PCB and was unable to find a temperature sensor. Is it possible
to estimate the oven temperature from the current drawn? (for sure it is,
but most likely it will be strongly dependent on the particular OCXO
used,
I think).
OK, so using a PWM DAC is more a matter of cost than something else. In
this case I think I am fine with a DAC8501 or DAC8560, which are both
based
on a resistor string, and therefore are also monotonic by design.
Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature
regulator loop assumes control shouldn't require an extremely low tempco
sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost,
one tradeoff being the settling time of the low pass filter between the
PWM
output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high
resolution.
mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have
attached
the schematic of the OCXO and DAC. Because the stability of my previous
design was not yet optimal, I now chose better components; my main
criteria
was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC
having an internal 2ppm/K voltage reference. Alternatively, the DAC8501
could be used, which requires an external voltage reference for which I
selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a
0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of
the two noninverting amplifiers, both OCXOs can be fitted. The two 1k
resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO
current. The Oscilloquartz STAR4 GPSDO I have has the same design; I
assume
it measures the OCXO current to determine when the warmup time has
elapsed.
I was now consdering the tempco of the resistors involved. Should I
use there resistors having an especially low tempco, or are ordinary 1%
resistors fine?
One last question; I have further analyzed the STAR4 design and I
saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen
uses
PWM, why do they do that? what is the advantage over a DAC similar to the
one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com mailto:
To unsubscribe, go to
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Tobias: along the path suggested by TVb, might I suggest something like
a small heater/cooler for your car?
lots at https://www.amazon.com/s?k=car+cooler+6+pak&ref=nb_sb_noss
a simple controller such as
eBay item number:
192112054053
can be applied as well.
and Bob's your uncle.
Don
On 2019-10-08 06:35, Tom Van Baak wrote:
Hi Tobias,
Very interesting project you have. Not just that it's a fresh GPSDO,
but interesting because you seem to be taking so much care in the
design. A couple of comments:
- The need for ultra low tempco is more important if you expect your
GPSDO to be in holdover often; less so if it is permanently locked to
GPS.
One approach is to spend the money for low tempco devices and trust
them; another is to monitor the temperature and apply calibrated
corrections in s/w. I can't predict which is better, but don't
discount the latter approach.
Still, either method will have to be verified with actual experiment,
strip charts, correlation analysis, and ADEV plots. So I hope you have
the test equipment at hand to measure these subtle effects in a GPSDO.
- I suspect one can spend a great deal of time picking and testing
optimal parts in a GPSDO based on tempco specs. But at what point is
it cheaper to just control the environment of the GPSDO board itself?
You mention opening lab doors and windows and such. Wouldn't it be
simpler to spent time to design a box that is wind-proof, or
fan-controlled; maybe even oven controlled? That way you can relax all
your worries about exotic passive and active components and just build
a controlled enclosure.
That could not only take care of temperature, but humidity and
pressure as well if they were found to be sources of instability. One
advantage of this approach is that the same box design could then be
applied to any other T&F projects like distribution amps or phase /
frequency counters that you design in the future.
- About sensors.
many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info
about the OCXO temperature
The Trimble Thunderbolt (TBolt) uses an onboard DS1620 temperature
sensor. I don't recall it ever being called "OCXO temperature"; it's
ambient board temperature. Perhaps because the board is inside a
larger aluminum enclosure the board temperature is related to OCXO
case temperature. But as far as I know the reported temperature value
from a TBolt is definitely not the temperature of the crystal
resonator itself.
/tvb
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
--
Dr. Don Latham AJ7LL
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304
Hi,
On 2019-10-08 14:35, Tom Van Baak wrote:
Hi Tobias,
- I suspect one can spend a great deal of time picking and testing
optimal parts in a GPSDO based on tempco specs. But at what point is
it cheaper to just control the environment of the GPSDO board itself?
You mention opening lab doors and windows and such. Wouldn't it be
simpler to spent time to design a box that is wind-proof, or
fan-controlled; maybe even oven controlled? That way you can relax all
your worries about exotic passive and active components and just build
a controlled enclosure.
I had exactly this problem a long time ago, and the real problem is that
any wind-drag is coupled directly to the oscillator. This will also be
true if you have forced air convection of the box, i.e. fans. The
air-flow couples very well to the metal shield of the oscillator. For
sure you want your oscillator in a winddrag free environment, so you
want a barrier to stop direct flow onto the oscillator. You do not want
to think insulation, as the OCXO needs to dump excess heat to a cooler
environment. On the lab-bench I use a small box, but Bob's approach is
to throw a towel over it. In designs we put plastic caps over the
oscillators. In the HP5370A/B the OCXO sits in a quiet corner but not
isolated.
That could not only take care of temperature, but humidity and
pressure as well if they were found to be sources of instability. One
advantage of this approach is that the same box design could then be
applied to any other T&F projects like distribution amps or phase /
frequency counters that you design in the future.
- About sensors.
many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show
info about the OCXO temperature
The Trimble Thunderbolt (TBolt) uses an onboard DS1620 temperature
sensor. I don't recall it ever being called "OCXO temperature"; it's
ambient board temperature. Perhaps because the board is inside a
larger aluminum enclosure the board temperature is related to OCXO
case temperature. But as far as I know the reported temperature value
from a TBolt is definitely not the temperature of the crystal
resonator itself.
The ambient temperature is not perfect as measured on a distance, but
helps. What imperfections there is in the setup, the big difference will
be in how transients is handled, not the stable conditions.
Cheers,
Magnus
Hi Bob
thanks for your explanation with the Telecom Rb. I am going to make the same calculations for my OCXOs I plan to use.
The ground pin current modulating the EFC is also interesting. I never thought about that, but indeed, this is true and thus having common ground pins for the heater and the EFC makes "accurate" tuning more complicated ;-)
maybe this is the reason for the old HP 10544A and 10811A having separate ground pins for the heater and the oscillator circuit?
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Tuesday, October 08, 2019 17:18
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
What is the temperature stability of the OCXO you plan to use? In the case of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control system
impacts the output. With a small enough range, you may not need a very good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1 ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to 1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s / references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every component
you will find is rated for a gradual rather than fast change. The impact of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater. That
current flows through a ground pin. The ground pin has a resistance. On pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return). Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info about the OCXO temperature when connected via Lady Heather. Do you know how they estimate the temperature? I checked almost every IC on the STAR4 PCB and was unable to find a temperature sensor. Is it possible to estimate the oven temperature from the current drawn? (for sure it is, but most likely it will be strongly dependent on the particular OCXO used, I think).
OK, so using a PWM DAC is more a matter of cost than something else. In this case I think I am fine with a DAC8501 or DAC8560, which are both based on a resistor string, and therefore are also monotonic by design.
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high resolution.
Bruce
On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess@xwmail.ch mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have attached the schematic of the OCXO and DAC. Because the stability of my previous design was not yet optimal, I now chose better components; my main criteria was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be used, which requires an external voltage reference for which I selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a 0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume it measures the OCXO current to determine when the warmup time has elapsed.
I was now consdering the tempco of the resistors involved. Should I use there resistors having an especially low tempco, or are ordinary 1% resistors fine?
One last question; I have further analyzed the STAR4 design and I saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why do they do that? what is the advantage over a DAC similar to the one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Hi
Indeed some OCXO’s have VCXO return pins. If you take a look at how they
get used ….hmmm…. the pin doesn’t get used properly. From trying to get
designers to use that sort of pin, indeed their reaction normally was “no can do”.
Bob
On Oct 9, 2019, at 2:28 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bob
thanks for your explanation with the Telecom Rb. I am going to make the same calculations for my OCXOs I plan to use.
The ground pin current modulating the EFC is also interesting. I never thought about that, but indeed, this is true and thus having common ground pins for the heater and the EFC makes "accurate" tuning more complicated ;-)
maybe this is the reason for the old HP 10544A and 10811A having separate ground pins for the heater and the oscillator circuit?
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Tuesday, October 08, 2019 17:18
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
What is the temperature stability of the OCXO you plan to use? In the case of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control system
impacts the output. With a small enough range, you may not need a very good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1 ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to 1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s / references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every component
you will find is rated for a gradual rather than fast change. The impact of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater. That
current flows through a ground pin. The ground pin has a resistance. On pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return). Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info about the OCXO temperature when connected via Lady Heather. Do you know how they estimate the temperature? I checked almost every IC on the STAR4 PCB and was unable to find a temperature sensor. Is it possible to estimate the oven temperature from the current drawn? (for sure it is, but most likely it will be strongly dependent on the particular OCXO used, I think).
OK, so using a PWM DAC is more a matter of cost than something else. In this case I think I am fine with a DAC8501 or DAC8560, which are both based on a resistor string, and therefore are also monotonic by design.
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high resolution.
Bruce
On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess@xwmail.ch mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have attached the schematic of the OCXO and DAC. Because the stability of my previous design was not yet optimal, I now chose better components; my main criteria was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be used, which requires an external voltage reference for which I selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a 0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume it measures the OCXO current to determine when the warmup time has elapsed.
I was now consdering the tempco of the resistors involved. Should I use there resistors having an especially low tempco, or are ordinary 1% resistors fine?
One last question; I have further analyzed the STAR4 design and I saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why do they do that? what is the advantage over a DAC similar to the one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Hello all together
thanks for your replies so far and hints for my new GPSDO design.
I have now designed the EFC circuit such that it is easily possible to use different DAC and voltage reference models. I have also reverse-engineered the circuit which is used on the Oscilloquartz GPSDO. They seem to use two cascaded Sallen-Key lowpass filters with approx. 1 Hz corner frequency to integrate the PWM signal, so I have included this circuit as well in my design. This then allows later to test different DACs as well as the PWM.
The next thing I am considering is the usage of the TDC7200 as an interpolator. I know this topic has been discussed often, but some issues still remain.
I have attached the schematic how I planned to use the TDC7200. The 1PPS pulse from the GPS module is definitely longer than 100ns, so the logic 1 will be clocked into the first flip-flop after max. 100ns. The 2nd flip-flop gives a further delay of 100ns. So, the TDC7200 is started on the rising edge of the 1PPS, and stopped with the delayed signal, such that the measurement time ranges betwenn >100ns and <200ns.
OK so the TDC7200 measures the phase difference between the 10MHz and the 1PPS. To measure the actual frequency, the 1PPS will be used on an input capture of a microcontroller (STM32F407 or something).
To trigger the input capture, should I use the same signal as for starting the TDC7200, or should I use one of its delayed versions? I think it does not really matter, but I am unsure.
Further, I also want my GPSDO to output an 1PPS pulse which is aligned to UTC. This 1PPS is generated with an ordinary timer. However, if I do that, the resulting pulse will have an arbitrary phase compared to the GPS module, so how would one deal with that? Actually, one should measure the phase difference between the two 1PPS signals, but this would be even more complicated.
I also don't know whether metastability could be an issue with my circuit, because in the unlocked state, the 1PPS could change its state any time ond so, setup or hold time of the first flip-flop is maybe violated. But I have no idea how that problem should be resolved or whether it actually is a problem.
Tobias
HB9FSX
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Wednesday, October 09, 2019 16:25
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
Indeed some OCXO’s have VCXO return pins. If you take a look at how they
get used ….hmmm…. the pin doesn’t get used properly. From trying to get
designers to use that sort of pin, indeed their reaction normally was “no can do”.
Bob
On Oct 9, 2019, at 2:28 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bob
thanks for your explanation with the Telecom Rb. I am going to make the same calculations for my OCXOs I plan to use.
The ground pin current modulating the EFC is also interesting. I never thought about that, but indeed, this is true and thus having common ground pins for the heater and the EFC makes "accurate" tuning more complicated ;-)
maybe this is the reason for the old HP 10544A and 10811A having separate ground pins for the heater and the oscillator circuit?
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Tuesday, October 08, 2019 17:18
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
What is the temperature stability of the OCXO you plan to use? In the case of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control system
impacts the output. With a small enough range, you may not need a very good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1 ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to 1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s / references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every component
you will find is rated for a gradual rather than fast change. The impact of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater. That
current flows through a ground pin. The ground pin has a resistance. On pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return). Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info about the OCXO temperature when connected via Lady Heather. Do you know how they estimate the temperature? I checked almost every IC on the STAR4 PCB and was unable to find a temperature sensor. Is it possible to estimate the oven temperature from the current drawn? (for sure it is, but most likely it will be strongly dependent on the particular OCXO used, I think).
OK, so using a PWM DAC is more a matter of cost than something else. In this case I think I am fine with a DAC8501 or DAC8560, which are both based on a resistor string, and therefore are also monotonic by design.
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high resolution.
Bruce
On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess@xwmail.ch mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have attached the schematic of the OCXO and DAC. Because the stability of my previous design was not yet optimal, I now chose better components; my main criteria was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be used, which requires an external voltage reference for which I selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a 0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume it measures the OCXO current to determine when the warmup time has elapsed.
I was now consdering the tempco of the resistors involved. Should I use there resistors having an especially low tempco, or are ordinary 1% resistors fine?
One last question; I have further analyzed the STAR4 design and I saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why do they do that? what is the advantage over a DAC similar to the one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Hi
You probably will find that comparing the 1 pps to 10 MHz directly gives you
a rollover problem that is difficult to deal with. Comparing it to something much
lower in frequency is the more normal approach.
Bob
On Oct 14, 2019, at 6:49 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hello all together
thanks for your replies so far and hints for my new GPSDO design.
I have now designed the EFC circuit such that it is easily possible to use different DAC and voltage reference models. I have also reverse-engineered the circuit which is used on the Oscilloquartz GPSDO. They seem to use two cascaded Sallen-Key lowpass filters with approx. 1 Hz corner frequency to integrate the PWM signal, so I have included this circuit as well in my design. This then allows later to test different DACs as well as the PWM.
The next thing I am considering is the usage of the TDC7200 as an interpolator. I know this topic has been discussed often, but some issues still remain.
I have attached the schematic how I planned to use the TDC7200. The 1PPS pulse from the GPS module is definitely longer than 100ns, so the logic 1 will be clocked into the first flip-flop after max. 100ns. The 2nd flip-flop gives a further delay of 100ns. So, the TDC7200 is started on the rising edge of the 1PPS, and stopped with the delayed signal, such that the measurement time ranges betwenn >100ns and <200ns.
OK so the TDC7200 measures the phase difference between the 10MHz and the 1PPS. To measure the actual frequency, the 1PPS will be used on an input capture of a microcontroller (STM32F407 or something).
To trigger the input capture, should I use the same signal as for starting the TDC7200, or should I use one of its delayed versions? I think it does not really matter, but I am unsure.
Further, I also want my GPSDO to output an 1PPS pulse which is aligned to UTC. This 1PPS is generated with an ordinary timer. However, if I do that, the resulting pulse will have an arbitrary phase compared to the GPS module, so how would one deal with that? Actually, one should measure the phase difference between the two 1PPS signals, but this would be even more complicated.
I also don't know whether metastability could be an issue with my circuit, because in the unlocked state, the 1PPS could change its state any time ond so, setup or hold time of the first flip-flop is maybe violated. But I have no idea how that problem should be resolved or whether it actually is a problem.
Tobias
HB9FSX
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Wednesday, October 09, 2019 16:25
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
Indeed some OCXO’s have VCXO return pins. If you take a look at how they
get used ….hmmm…. the pin doesn’t get used properly. From trying to get
designers to use that sort of pin, indeed their reaction normally was “no can do”.
Bob
On Oct 9, 2019, at 2:28 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bob
thanks for your explanation with the Telecom Rb. I am going to make the same calculations for my OCXOs I plan to use.
The ground pin current modulating the EFC is also interesting. I never thought about that, but indeed, this is true and thus having common ground pins for the heater and the EFC makes "accurate" tuning more complicated ;-)
maybe this is the reason for the old HP 10544A and 10811A having separate ground pins for the heater and the oscillator circuit?
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Tuesday, October 08, 2019 17:18
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
What is the temperature stability of the OCXO you plan to use? In the case of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control system
impacts the output. With a small enough range, you may not need a very good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1 ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to 1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s / references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every component
you will find is rated for a gradual rather than fast change. The impact of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater. That
current flows through a ground pin. The ground pin has a resistance. On pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return). Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info about the OCXO temperature when connected via Lady Heather. Do you know how they estimate the temperature? I checked almost every IC on the STAR4 PCB and was unable to find a temperature sensor. Is it possible to estimate the oven temperature from the current drawn? (for sure it is, but most likely it will be strongly dependent on the particular OCXO used, I think).
OK, so using a PWM DAC is more a matter of cost than something else. In this case I think I am fine with a DAC8501 or DAC8560, which are both based on a resistor string, and therefore are also monotonic by design.
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high resolution.
Bruce
On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess@xwmail.ch mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have attached the schematic of the OCXO and DAC. Because the stability of my previous design was not yet optimal, I now chose better components; my main criteria was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be used, which requires an external voltage reference for which I selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a 0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume it measures the OCXO current to determine when the warmup time has elapsed.
I was now consdering the tempco of the resistors involved. Should I use there resistors having an especially low tempco, or are ordinary 1% resistors fine?
One last question; I have further analyzed the STAR4 design and I saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why do they do that? what is the advantage over a DAC similar to the one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
<tdc.png>_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Have a look at the TAPR/TICC design as an example how to use the TDC7200
as a timestamping / time interval counter. The TI chip itself is
designed only for very short-range Time-of-Flight applications so the
additional coarse counter logic that John added is the also right
approach for a GPSDO. Details on the tapr.org site.
Once you have this working, it's easy to put two of them on the board:
one is used to capture GPS/1PPS and another can be used to timestamp a
user's external pulse. That turns your plain GPSDO into a device that
also includes measurement capability -- timestamping (time-tagging),
period, period average, frequency, frequency average, ADEV, etc. Very
useful.
The Trimble TBolt GPSDO is also an informative example. When
disciplining is manually disabled by command, the internal TIC still
continues to provide time interval between GPS and OCXO. This is very
useful as it allows you to measure & characterize the OCXO against GPS
without any additional instruments. This feature comes for free when you
use the timestamping model.
If you plan to implement GPSDO holdover mode you will also want
timestamping, or a full 0 to 1 s time interval range. You get the idea
by now: a 100 ns range is not sufficient for a GPSDO; do the work for a
full 1 second range; look at how John's TICC uses the TDC7200.
For the 1PPS output, always derive that directly from the OCXO itself,
in h/w; don't use a CPU timer. Users tend to expect the 10 MHz and 1PPS
outputs to be in perfect h/w synchronization, without s/w bugs, without
tempco, without added CPU clock jitter.
I'm enjoying your thought process on your GPSDO design. Note that you'll
want to spend as much time developing a test bench so that you can test
what you come up with.
One other comment, do read the archive time nuts postings and blog
entries by Nick Sayer. He did a nice work with an affordable, not
extremely high-end but open source, GPSDO and that would be useful
background reading for you.
/tvb
On 10/14/2019 4:49 AM, Tobias Pluess wrote:
Hello all together
thanks for your replies so far and hints for my new GPSDO design.
I have now designed the EFC circuit such that it is easily possible to use different DAC and voltage reference models. I have also reverse-engineered the circuit which is used on the Oscilloquartz GPSDO. They seem to use two cascaded Sallen-Key lowpass filters with approx. 1 Hz corner frequency to integrate the PWM signal, so I have included this circuit as well in my design. This then allows later to test different DACs as well as the PWM.
The next thing I am considering is the usage of the TDC7200 as an interpolator. I know this topic has been discussed often, but some issues still remain.
I have attached the schematic how I planned to use the TDC7200. The 1PPS pulse from the GPS module is definitely longer than 100ns, so the logic 1 will be clocked into the first flip-flop after max. 100ns. The 2nd flip-flop gives a further delay of 100ns. So, the TDC7200 is started on the rising edge of the 1PPS, and stopped with the delayed signal, such that the measurement time ranges betwenn >100ns and <200ns.
OK so the TDC7200 measures the phase difference between the 10MHz and the 1PPS. To measure the actual frequency, the 1PPS will be used on an input capture of a microcontroller (STM32F407 or something).
To trigger the input capture, should I use the same signal as for starting the TDC7200, or should I use one of its delayed versions? I think it does not really matter, but I am unsure.
Further, I also want my GPSDO to output an 1PPS pulse which is aligned to UTC. This 1PPS is generated with an ordinary timer. However, if I do that, the resulting pulse will have an arbitrary phase compared to the GPS module, so how would one deal with that? Actually, one should measure the phase difference between the two 1PPS signals, but this would be even more complicated.
I also don't know whether metastability could be an issue with my circuit, because in the unlocked state, the 1PPS could change its state any time ond so, setup or hold time of the first flip-flop is maybe violated. But I have no idea how that problem should be resolved or whether it actually is a problem.
Tobias
HB9FSX
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Wednesday, October 09, 2019 16:25
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
Indeed some OCXO’s have VCXO return pins. If you take a look at how they
get used ….hmmm…. the pin doesn’t get used properly. From trying to get
designers to use that sort of pin, indeed their reaction normally was “no can do”.
Bob
On Oct 9, 2019, at 2:28 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bob
thanks for your explanation with the Telecom Rb. I am going to make the same calculations for my OCXOs I plan to use.
The ground pin current modulating the EFC is also interesting. I never thought about that, but indeed, this is true and thus having common ground pins for the heater and the EFC makes "accurate" tuning more complicated ;-)
maybe this is the reason for the old HP 10544A and 10811A having separate ground pins for the heater and the oscillator circuit?
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Tuesday, October 08, 2019 17:18
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
What is the temperature stability of the OCXO you plan to use? In the case of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control system
impacts the output. With a small enough range, you may not need a very good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1 ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to 1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s / references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every component
you will find is rated for a gradual rather than fast change. The impact of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater. That
current flows through a ground pin. The ground pin has a resistance. On pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return). Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info about the OCXO temperature when connected via Lady Heather. Do you know how they estimate the temperature? I checked almost every IC on the STAR4 PCB and was unable to find a temperature sensor. Is it possible to estimate the oven temperature from the current drawn? (for sure it is, but most likely it will be strongly dependent on the particular OCXO used, I think).
OK, so using a PWM DAC is more a matter of cost than something else. In this case I think I am fine with a DAC8501 or DAC8560, which are both based on a resistor string, and therefore are also monotonic by design.
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high resolution.
Bruce
On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess@xwmail.ch mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have attached the schematic of the OCXO and DAC. Because the stability of my previous design was not yet optimal, I now chose better components; my main criteria was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be used, which requires an external voltage reference for which I selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a 0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume it measures the OCXO current to determine when the warmup time has elapsed.
I was now consdering the tempco of the resistors involved. Should I use there resistors having an especially low tempco, or are ordinary 1% resistors fine?
One last question; I have further analyzed the STAR4 design and I saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why do they do that? what is the advantage over a DAC similar to the one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Hi Tom
thanks for your inputs.
For sure the 1PPS output will be generated in hardware. I have made good experiences with using a microcontroller timer: One can use an ordinary PWM channel and this generates very precise pulses, and no interrupts or the like are involved. And since the timer can be clocked directly by the 10 MHz signal, we can even avoid PLL jitter.
Of course I know the TAPR TICC, and I was recently thinking about buying one. It seems to be a really nice device. However I didn't want to add a full TICC to my GPSDO, but it is maybe a nice idea.
"If you plan to implement GPSDO holdover mode you will also want
timestamping, or a full 0 to 1 s time interval range. You get the idea
by now: a 100 ns range is not sufficient for a GPSDO; do the work for a
full 1 second range; look at how John's TICC uses the TDC7200."
Yes, I actually did plan to use a full 1 second time interval: as you may rememver from my previous GPSDO, I clocked a timer in my microcontroller with the 10MHz signal. The 1PPS pulse from the GPS is used as input capture; and the timer runs continously. When there is a rising edge on the 1PPS, the timer value is captured into another register, and while the timer continues to run, an interrupt occurs. So every second, I get an interrupt, and when I subtract the previous captured timer value from the current one, I should always get 10000000 if the frequency is accurate. However, when the frequency has an error smaller than 1 Hz, it takes longer and longer to detect this error; so I planned to add the TDC7200 to add further digits to my captured values, e.g. 10000000.25 or so, and this would allow to detect small frequency errors faster.
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Tom Van Baak [tvb@LeapSecond.com]
Sent: Monday, October 14, 2019 18:08
To: time-nuts@lists.febo.com
Subject: Re: [time-nuts] Question for my new GPSDO
Have a look at the TAPR/TICC design as an example how to use the TDC7200
as a timestamping / time interval counter. The TI chip itself is
designed only for very short-range Time-of-Flight applications so the
additional coarse counter logic that John added is the also right
approach for a GPSDO. Details on the tapr.org site.
Once you have this working, it's easy to put two of them on the board:
one is used to capture GPS/1PPS and another can be used to timestamp a
user's external pulse. That turns your plain GPSDO into a device that
also includes measurement capability -- timestamping (time-tagging),
period, period average, frequency, frequency average, ADEV, etc. Very
useful.
The Trimble TBolt GPSDO is also an informative example. When
disciplining is manually disabled by command, the internal TIC still
continues to provide time interval between GPS and OCXO. This is very
useful as it allows you to measure & characterize the OCXO against GPS
without any additional instruments. This feature comes for free when you
use the timestamping model.
If you plan to implement GPSDO holdover mode you will also want
timestamping, or a full 0 to 1 s time interval range. You get the idea
by now: a 100 ns range is not sufficient for a GPSDO; do the work for a
full 1 second range; look at how John's TICC uses the TDC7200.
For the 1PPS output, always derive that directly from the OCXO itself,
in h/w; don't use a CPU timer. Users tend to expect the 10 MHz and 1PPS
outputs to be in perfect h/w synchronization, without s/w bugs, without
tempco, without added CPU clock jitter.
I'm enjoying your thought process on your GPSDO design. Note that you'll
want to spend as much time developing a test bench so that you can test
what you come up with.
One other comment, do read the archive time nuts postings and blog
entries by Nick Sayer. He did a nice work with an affordable, not
extremely high-end but open source, GPSDO and that would be useful
background reading for you.
/tvb
On 10/14/2019 4:49 AM, Tobias Pluess wrote:
Hello all together
thanks for your replies so far and hints for my new GPSDO design.
I have now designed the EFC circuit such that it is easily possible to use different DAC and voltage reference models. I have also reverse-engineered the circuit which is used on the Oscilloquartz GPSDO. They seem to use two cascaded Sallen-Key lowpass filters with approx. 1 Hz corner frequency to integrate the PWM signal, so I have included this circuit as well in my design. This then allows later to test different DACs as well as the PWM.
The next thing I am considering is the usage of the TDC7200 as an interpolator. I know this topic has been discussed often, but some issues still remain.
I have attached the schematic how I planned to use the TDC7200. The 1PPS pulse from the GPS module is definitely longer than 100ns, so the logic 1 will be clocked into the first flip-flop after max. 100ns. The 2nd flip-flop gives a further delay of 100ns. So, the TDC7200 is started on the rising edge of the 1PPS, and stopped with the delayed signal, such that the measurement time ranges betwenn >100ns and <200ns.
OK so the TDC7200 measures the phase difference between the 10MHz and the 1PPS. To measure the actual frequency, the 1PPS will be used on an input capture of a microcontroller (STM32F407 or something).
To trigger the input capture, should I use the same signal as for starting the TDC7200, or should I use one of its delayed versions? I think it does not really matter, but I am unsure.
Further, I also want my GPSDO to output an 1PPS pulse which is aligned to UTC. This 1PPS is generated with an ordinary timer. However, if I do that, the resulting pulse will have an arbitrary phase compared to the GPS module, so how would one deal with that? Actually, one should measure the phase difference between the two 1PPS signals, but this would be even more complicated.
I also don't know whether metastability could be an issue with my circuit, because in the unlocked state, the 1PPS could change its state any time ond so, setup or hold time of the first flip-flop is maybe violated. But I have no idea how that problem should be resolved or whether it actually is a problem.
Tobias
HB9FSX
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Wednesday, October 09, 2019 16:25
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
Indeed some OCXO’s have VCXO return pins. If you take a look at how they
get used ….hmmm…. the pin doesn’t get used properly. From trying to get
designers to use that sort of pin, indeed their reaction normally was “no can do”.
Bob
On Oct 9, 2019, at 2:28 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bob
thanks for your explanation with the Telecom Rb. I am going to make the same calculations for my OCXOs I plan to use.
The ground pin current modulating the EFC is also interesting. I never thought about that, but indeed, this is true and thus having common ground pins for the heater and the EFC makes "accurate" tuning more complicated ;-)
maybe this is the reason for the old HP 10544A and 10811A having separate ground pins for the heater and the oscillator circuit?
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bob kb8tq [kb8tq@n1k.org]
Sent: Tuesday, October 08, 2019 17:18
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Hi
What is the temperature stability of the OCXO you plan to use? In the case of
open windows, what is the dynamic temperature stability of the OCXO?
What is the tuning range of the EFC on the OCXO?
The wider the EFC range on the OCXO, the more the tempco of the control system
impacts the output. With a small enough range, you may not need a very good
DAC at all. With a super big EFC range then the requirements go up.
As long as the EFC stability is better than the OCXO’s temperature stability, there
really is no benefit to improving it.
One example of all this is not an OCXO, but it’s the same sort of idea. A Telecom Rb
might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1 ppb over
a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to 1,000 ppm/K
in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s / references would
be overkill in this case.
None of that gets into the issue of dynamic change. Pretty much every component
you will find is rated for a gradual rather than fast change. The impact of a fast change
can be orders of magnitude worse than a slower change. ( so indeed, don’t open the
window :) )
=======
If you want to go a bit nuts (this being time nuts):
A change in the temperature changes the current through the OCXO heater. That
current flows through a ground pin. The ground pin has a resistance. On pretty much
all OCXO’s that ground pin is also in the EFC ( = it is the EFC return). Thus ground
current “tunes” your OCXO.
However you go about measuring the tempco of your OCXO, be careful that the ground
current is handled the same in your test as in your final circuit …..
Bob
On Oct 8, 2019, at 2:21 AM, Tobias Pluess tobias.pluess@xwmail.ch wrote:
Hi Bruce,
oh yes I know, for sure no special resistor will be used for the oven heater!
By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show info about the OCXO temperature when connected via Lady Heather. Do you know how they estimate the temperature? I checked almost every IC on the STAR4 PCB and was unable to find a temperature sensor. Is it possible to estimate the oven temperature from the current drawn? (for sure it is, but most likely it will be strongly dependent on the particular OCXO used, I think).
OK, so using a PWM DAC is more a matter of cost than something else. In this case I think I am fine with a DAC8501 or DAC8560, which are both based on a resistor string, and therefore are also monotonic by design.
Tobias
From: time-nuts [time-nuts-bounces@lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths@xtra.co.nz]
Sent: Tuesday, October 08, 2019 00:19
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO
Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.
In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.
PWM DACs are inherently monotonic even when they have very high resolution.
Bruce
On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess@xwmail.ch mailto:tobias.pluess@xwmail.ch > wrote:
Hi Guys,
I am planning to make a new version of my own GPSDO. I have attached the schematic of the OCXO and DAC. Because the stability of my previous design was not yet optimal, I now chose better components; my main criteria was the lowest tempco I found.
As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be used, which requires an external voltage reference for which I selected the ADR441B (typically 1ppm/K).
I have different OCXOs which I want to test; one of them requires a 0 to 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors make the tuning range a bit smaller.
The OpAmp I chose has 2.5 uV/K tempco.
Besides that I have fitted an additional OpAmp to measure the OCXO current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume it measures the OCXO current to determine when the warmup time has elapsed.
I was now consdering the tempco of the resistors involved. Should I use there resistors having an especially low tempco, or are ordinary 1% resistors fine?
One last question; I have further analyzed the STAR4 design and I saw that they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why do they do that? what is the advantage over a DAC similar to the one I selected?
Thanks for your comments,
best
Tobias
HB9FSX
_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
On Mon, Oct 14, 2019 at 12:09 PM Tom Van Baak tvb@leapsecond.com wrote:
...
One other comment, do read the archive time nuts postings and blog
entries by Nick Sayer. He did a nice work with an affordable, not
extremely high-end but open source, GPSDO and that would be useful
background reading for you.
/tvb
Tobias,
I suggest you also review the GPSDO design by Lars Walenius described here
https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/
which is the basis for Nick Sayer's design. Lars has unfortunately passed
away, but I have worked extensively with his hardware and software and have
made some improvements which I would be happy to share with you off list if
you are interested.
Also if you are interested in pursuing a PWM based DAC, I suggest you
review this article
https://www.edn.com/design/analog/4460665/Fast-PWM-DAC-has-no-ripple
which describes a sample and hold technique for reducing the output noise
of a PWM DAC. The article also has references to other PWM noise reducing
techniques.
--Jim Harman