[time-nuts] Re: General query about disciplining circuitry

[Ed Marciniak]

It’s not going to be budget friendly, but the symmetricom (now Microsemi) chip scale atomic clocks might get you there or close at least. You’d probably have to ask someone about what the loop time constants are. If they are something like 128 Hz, in theory the onboard quartz would be disciplined by the atomic transition as fast as the frequency was shifting…IF the loop filter once lock is achieved has the same bandwidth as when it’s unlocked.

I’m struggling with why you’d need the equivalent of hertz per 100GHz stability on that sort of power budget. A moving phase coherent receiver suggests whatever you’re trying to transmit or receive should be designed with a more forgiving frequency tracking loop. A carrier phase GNSS can probably cover most of the other use cases I can think of, and for the ones it can’t, you’re talking about things so big a bigger power budget isn’t a problem.

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________________________________
From: Bob Camp via time-nuts <time-nuts at lists.febo.com>
Sent: Friday, February 3, 2023 12:12:17 PM
To: AC0XU (Jim) <James.Schatzman at ac0xu.com>
Cc: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>; Bob Camp <kb8tq at n1k.org>
Subject: [time-nuts] Re: General query about disciplining circuitry

Hi

With a moving platform, you get into the acceleration sensitivity of your oscillator.
That could be as high as 2x10^-9 / g. You might find a part down around 5x10^-10 / g.
Anything much past that gets you into an acceleration compensated device. Those
typically are a bit expensive and not an eBay sort of item.

If the motion is on a typical platform, you will need vibration compensation to get to your
1x10^-11 range. This is one step up from acceleration compensation. Price goes up
yet again and surplus availability goes to near zero.

One approach for time is to simply use the time pulse out of the GPS module. If you don’t
care about the PPS edge staying aligned with the (say) 10 MHz output it’s a valid solution
to the problem. Accuracy wise, it’s going to win every time. Jitter wise … not so much.

Bob

> On Feb 3, 2023, at 12:37 PM, AC0XU (Jim) <James.Schatzman at ac0xu.com> wrote:
>
> 1W is the notional power budget for the discipliner.
>
> I am interested in optimizing designs for precision time, precision frequency, and precision time and frequency (three cases). I am o.k. with the idea of  not adjusting the oscillator phase to perfectly match GNSS but to instead record and track the offset without actually tweaking the oscillator for time/phase. That might be the best, and maybe that approach reduces the distinction between precision time and precision frequency.
>
> The goal for the loosest case is 1e-11 adev over 0.1 to 1000 sec, but I am interested in doing as well as the oscillator will support. In a future design I will be using an oscillator with 1e-12 adev @ 1 sec, but the current one is good for maybe 1e-11 @ 1 sec at best. The receiver platform is moving, and no radio mfr publishes performance specs for this case.
>
> Thank you for reminding me about the sawtooth. I will need to study the sawtooth behavior of the radio. One I am looking at now is the OEM7 series.
>
> Again, given that there are many possible approaches to precision phase measurement, and being pretty sure that I want to do more than implement a frequency counter, I am looking for advice.
>
> Thanks!
>
>
>
>
>
> At 09:31 AM 2/3/2023, Bob  Camp wrote:
>> Hi
>>
>> I would back up a bit first
> .
>> What sort of oscillator is being disciplined? If the 1W is the total power budget, that rules out
>> most OCXO’s. Why ask? The stability of the oscillator does get into this pretty quickly.
>>
>> What is the end goal? Are you after precision time or precision frequency? While they are
>> connected, the approach to optimize one is not quite the same at the approach for the other.
>>
>> What are the stability goals either for time or frequency?
>>
>> Is this a mobile / portable application? If so that also impacts a lot of this and gets into a whole
>> other set of questions.
>>
>> To at least partly answer the question you asked:
>>
>> The PPS comes out of the GPS at the same time as the sawtooth correction information. In
>> most cases, you want to marry those two up immediately. That way the correction does apply
>> to the data you took. Does this matter in your case? Thus the long list of questions above.
>>
>> Bpb
>>
>>> On Feb 3, 2023, at 10:16 AM, AC0XU (Jim) via time-nuts <time-nuts at lists.febo.com> wrote:
>>>
>>> I can image all sorts of disciplining circuits for controlling a digitally-controlled oscillator against GPS. Presumably, I will measure oscillator phase at the 1 PPS times and run those values into a Kalman Filter. I think my question is about how best to measure the oscillator phase, given the constraint of low electrical power (<1W). There are zillions of commercial devices that do this but the mfrs generally do not advertize the details. I plan to use either an MCU or an FPGA, depending on which is more advantageous.
>>>
>>> I looked at
>>> <https://hackaday.io/project/6872-gps-disciplined-xcxo/details>https://hackaday.io/project/6872-gps-disciplined-xcxo/details
>>> and that seems interesting. I don't have to worry about VCO control because my oscillator has a digital control.
>>>
>>> What about measuring the phase of a large divider PLL locked to the reference? With I-Q outputs the phase can be measured with a very slow ADC applied to a sample-and-hold triggered by the 1 PPS.
>>>
>>> Thanks!
>>> Jim
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