[time-nuts] Best counter setting for ADEV?

Tom Van Baak tvb at LeapSecond.com
Tue Oct 2 12:18:35 EDT 2012


Azelio,

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.

/tvb

----- Original Message ----- 
From: "Azelio Boriani" <azelio.boriani at screen.it>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, October 02, 2012 8:18 AM
Subject: Re: [time-nuts] Best counter setting for ADEV?


> OK, got the papers. I'm still trying to figure out if a minimum resolution
> can be determined for the TI measurement OCXO_PPS/GPS_PPS. That is, if the
> GPS has a 25nS random wondering, is it worth having 25pS measurement
> resolution? At first yes, I can use the negative sawtooth but I'm not
> convinced. Then, I'm using a simple average over 100 or 1000 seconds but my
> OCXO moves menawhile: the simple average is not the ideal tool and I'm
> still investigating what magical statistic can I use.






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