[time-nuts] GPSDO with all-digital phase/time measurement?

[Bob Stewart]

Tom,

I took his 100ns figure to be simply the period of 10MHz.  He mentioned using an interrupt driven system, so the counts should not necessarily be limited to 100ns accuracy.  At least on the PIC I'm using, the CCP and timer interrupts don't seem to be synchronous with the PIC clock.  I could be mistaken.

Bob





>________________________________
> From: Tom Van Baak <tvb at LeapSecond.com>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Wednesday, February 26, 2014 11:22 PM
>Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement?
> 
>
>> At this point the time measurement is quite crude, with 100-ns resolution. 
>> But because we keep the counter running, the unknown residuals will keep
>> accumulating, and we should be able to average out this "quantization noise"
>> in the long run.  That is, we can measure any T-second period to within 100
>> ns, so the resolution on a per-second basis becomes 100 ns / T.
>
>No. The timing resolution per second is always 100 ns. You're probably thinking about average frequency, in which case dividing by T is sometimes valid, and it looks better and better as time goes by, usually.
>
>What saves you here is that your counter noise (100 ns) is likely greater than the quantization noise. So you can pretty much ignore the receiver 1PPS quantization noise. For people with much lower measurement noise (e.g., 1 ns) the quantization noise becomes a more important piece of the error pie.
>
>Try not to say average "out"; that sounds like it goes away over time or gets smaller. You're doing a timing measurement so the 100 ns measurement granularity is always there, on every measurement.
>
>> Is there any reason why this sort of processing cannot attain equivalent
>> performance to the more conventional analog phase-detection approach?
>
>All other factors equal, a GPSDO based on 100 ns measurement resolution can never attain the equivalent of a GPSDO based on 10 ns or 1 ns measurement resolution. Waiting shorter or longer doesn't change the RMS timing accuracy.
>
>/tvb
>
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