[time-nuts] Setting Rubidium to match GPS source

Pete peterawson at earthlink.net
Fri Sep 28 11:07:29 EDT 2007


Gentlemen,

Using oscilloscopes to measure relative phase has been widely
used for a long time. The setup appears in many H-P Op/Srv
manuals in the adjustment procedure section to calibrate the 
instrument timebase to a shop standard.

But, it's not a useful technique for settings more precise than
1 or 2E-10  in the best situation.

The process degrades badly as the 'scope sweep speed
increases past 10ns/div since a 10MHz sinewave slope is
approaching a rather flat line. Watching 2 rather flat lines
for precise zero crossings isn't as simple as it seems. In
addition, any noise/drift or outright shifts in vertical position
obscure the result & aren't unusual during a 16 minute window.

Delayed sweep operation just adds more uncertainty, so
you end up waiting for the phase difference to drift to the
point that both traces are displayed in the same window.
But now, they won't stay there for 16 minutes, unless they're
already VERY close to matching. The whole process demands
that you walk your way down to locking up the 2 sources.
It's more painful than it sounds!

Regards,
Pete Rawson



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