[time-nuts] GPSDO PLL convergence question

[Bob Stewart]

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
>
>