[time-nuts] Faster/more automated Rb oscillator calibration

Brian Alsop alsopb at nc.rr.com
Fri May 31 08:24:12 EDT 2013

This is probably nothing new, but looking over the archives, I didn't 
see it posted in the last couple years

I've had an LPRO-101 Rb oscillator for several years.
It was first calibrated using the phase shift vs time method using a 
GPSDO as the reference.  Stopwatch and oscilloscope.  Ugh!

Needless to say it got really time consuming with full cycle times of 
1000's of seconds.

I've revisited this and come up with something much simpler but more 
importantly automated.

Simply input the GPSDO oscillator signal into channel A and the Rb 
oscillator into channel B of my HP 5335A counter.  Tell it to determine 
the time difference (1ns resolution).  Send these differences to a file 
via HPIB.

The neat thing is that one can quickly see trends over a fraction of a 
cycle.  When done, let it run all night and look at the output.

The output is a sawtooth or reverse sawtooth ranging from 0 to 100 ns. 
One can compute the time difference between any points of adjacent teeth 
to determine a full cycle's phase shift.  Alternately one can look at 
part of a sawtooth for a quick estimate.

It's neat to see that I'm starting to get triangles rather than a 
sawtooth-- meaning the two are essentially at the same frequency and 
drifting about each other.

BTW the Rb oscillator bias of 1 mHz present two years ago was still 
there and had changed only 0.2 mHz over the intervening time.


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