[time-nuts] Best counter setting for ADEV?

shalimr9 at gmail.com shalimr9 at gmail.com
Wed Oct 3 02:00:30 UTC 2012


The Thunderbolt is a special case that does not provide sawtooth correction because it does not need it. 

It uses the OCXO as the clock for the processor while disciplining it to GPS so there is no nominal timing error between where the 1PPS is versus where it should be. 

The processor is able to bring the PPS edge exactly where it wants it, instead of the typical 25 to 40 ns granularity of most other GPS receivers that operate on a separate clock.

Pretty simple and elegant solution.

Didier KO4BB


Sent from my Droid Razr 4G LTE wireless tracker.



-----Original Message-----
From: David <davidwhess at gmail.com>
To: Tom Van Baak <tvb at leapsecond.com>, Discussion of precise time and
 frequency measurement <time-nuts at febo.com>
Sent: Tue, 02 Oct 2012 12:04 PM
Subject: Re: [time-nuts] Best counter setting for ADEV?

Is there a list of GPS timing receivers that provide the sawtooth
correction message or implement sawtooth correction internally?

I assume there is a design compromise that prevents economically phase
locking the GPS receiver clock to the GPS signal to remove that
contribution to timing error.

On Tue, 2 Oct 2012 09:18:35 -0700, "Tom Van Baak" <tvb at LeapSecond.com>
wrote:

>Correct, all GPS timing receiver boards have jitter, sawtooth, or random wandering of some sort, on the order of tens of nanoseconds. This is normal. And so if you use a counter to compare the OCXO 1 Hz with the GPS 1PPS, a TIC resolution of 1 ns or 500ps is sufficient. I would say 25 ps is overkill.
>
>In many cases (e.g., Motorola Oncore series) the sawtooth correction message itself has a granularity of 1 ns. So again, a 25 ps measurement is especially overkill given a correction granularity of 1 ns. Depending on the receiver applying the correction will improve the average timing performance by, say, a factor of 3.
>
>See: http://www.leapsecond.com/pages/m12-adev/
>
>As for your averaging question, yes, the OCXO will move during the average. This is normal. That's why too long an averaging interval is problematic. Depends on the quality of the OCXO. And if the averaging interval is too short, you pick up too much GPS jitter. Depends on the quality of the GPS receiver. There is no perfect answer; instead you choose something between too short and too long.

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