[time-nuts] GPSDO PLL convergence question

Bob Camp lists at rtty.us
Sat Aug 31 12:36:52 UTC 2013


Hi

With OCXO A in environment B you may make it to step 6 in the time constant update table. With OCXO D in environment E you may make it to step 9 on your table of time constants. In the first case step 5 is probably a useful thing to flag, in the second case step 8 makes some sense. The problem is that if the environment changes, the final step will change as well. 

All that of course *assumes* that the logic in the loop decision making does not let it go to far down that table. If OCXO A is still in environment B and it (somehow) gets to step 9, that's probably a bad thing. It likely needs the faster time constant to keep things in order rather than the slower one. 

Bob

On Aug 30, 2013, at 10:49 PM, Bob Stewart <bob at evoria.net> wrote:

> Hi Bob,
> 
> You've given me a lot to work with.  Perhaps the best answer is to put in a user parameter for how many seconds between updates to consider a lock condition.  For warmup, I had planned to put in a 5 minute holdover period, but that could easily be user configurable, as well.
> 
> Thanks again,
> 
> Bob
> 
> 
> 
> 
> 
>> ________________________________
>> From: Bob Camp <lists at rtty.us>
>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Friday, August 30, 2013 8:55 PM
>> Subject: Re: [time-nuts] GPSDO PLL convergence question
>> 
>> 
>> Hi
>> 
>> A GPSDO is nothing more than a PLL that operates at 1Hz. If you use the same criteria as they do in some systems, as soon as the OCXO pulse is within 1/8 second (sort of 10%) of the GPS, the PLL is locked / converged / doing it's thing / functioning as it should. It's a time domain system, so time is the reasonable way to look at it. 
>> 
>> Looking at DAC voltage *assumes* that the OCXO is perfect. Consider a situation when the temperature is continuously climbing. The OCXO has a TC, the DAC will move continuously. The same thing applies to aging / warmup /  what ever. 
>> 
>> Bob
>> 
>> On Aug 30, 2013, at 9:36 PM, Bob Stewart <bob at evoria.net> wrote:
>> 
>>> Hi Bob,
>>> 
>>> I'm not sure I understand what you mean by "45 degree phase error" in a GPSDO.  This isn't a situation where I'm at the right frequency but at the wrong phase: at least not by my understanding of phase and frequency.  Maybe PLL is even the wrong term.  I dunno, but it feels right for this.  Let's say I'm counting the number of OCXO pulses in each second, and in two intervals 100 seconds apart I have a surplus of 1 count each.  I think that means my 10MHz OCXO is off by 1ppb.  Is that right: the reciprocal of 1 error count during 100 seconds times 10,000,000?  And yet my DAC still has about 100 steps to go before it reaches best solution.  So, OK, what is the "magic number" in ppb or ppt I should consider for convergence?  Maybe I should collect 24 hours of data and see what the average is for the time between DAC changes and work from there.  I suppose 5 counts from best solution would be fine.  By my math that would be 240ppt, or about 417
>>> seconds between DAC updates.  I'm out of my depth on this project.
>>> 
>>> 
>>> Thanks for your continued help.
>>> 
>>> Bob
>>> 
>>> 
>>> 
>>> 
>>>> ________________________________
>>>> From: Bob Camp <lists at rtty.us>
>>>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>>>> Sent: Friday, August 30, 2013 7:31 PM
>>>> Subject: Re: [time-nuts] GPSDO PLL convergence question
>>>> 
>>>> 
>>>> Hi
>>>> 
>>>> Convergence in a PLL has no absolute definition. In some cases 45 degree phase error = all is running fine. 
>>>> 
>>>> I would simply track GPS time vs OCXO time and declare it ok when it's inside some magic number of nanoseconds. 
>>>> 
>>>> Bob
>>>> 
>>>> 
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