[time-nuts] Question for my new GPSDO

Mon Oct 14 15:10:31 UTC 2019

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 at 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 at lists.febo.com] on behalf of Bob kb8tq [kb8tq at 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 at 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 at lists.febo.com] on behalf of Bob kb8tq [kb8tq at 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 at 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 at lists.febo.com] on behalf of Bruce Griffiths [bruce.griffiths at 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 at xwmail.ch mailto:tobias.pluess at 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
>>>> 
>>>> 
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