[time-nuts] Measuring oscillators against GPS

[Skip Withrow]

Hello Time-Nuts,
As has been discussed recently, GPSDOs can wander around in phase
somewhat.  They are great for making frequency
comparisons/measurements - to a point.  But if you truly want the best
measurement possible the trick is to measure against GPS directly.
Jim Lux has alluded to this (and to post-processing the data to get
even higher precision).  UTC(USNO), what GPS is steered to, in general
is very good (You can even back out its errors with BIPM Circular-T).

There are a number of GPS receivers out there that can be clocked
directly with 5MHz or 10MHz.  Feeding your oscillator under test into
the clock input allows you to measure directly against GPS.

I have attached a short PDF that summarizes my experience with using
several receivers using this technique.  It is my go-to for making
rapid, precision frequency measurements.  For 10MHz, drift (in
nanoseconds) divided by sample interval (in seconds) equals error (in
parts x 10E-xx).
Regards,
Skip Withrow
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