[time-nuts] Best counter setting for ADEV?

[Bob Camp]

Hi

Almost any receiver that's labeled "timing receiver" will put out some sort
of saw tooth correction. The saw tooth is a result of the math, so no
there's not a cheap way to get rid of it. By the time you do a receiver that
does the phase lock thing, you have pretty much built a GPSDO.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of David
Sent: Tuesday, October 02, 2012 1:04 PM
To: Tom Van Baak; Discussion of precise time and frequency measurement
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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