[time-nuts] Calibration of Rb

Murray Greenman Murray.Greenman at rakon.com
Mon Oct 5 20:00:21 UTC 2009

Someone asked what the Time-Nuts way of calibrating a Rb source would
be. I'm no expert, but here's my 2c worth:

I don't like making adjustments, as they can lead to lowered
reliability, lowered understanding of the processes of ageing, and are
also more work. As someone else pointed out, provided you know where
your oscillator is, it is stability that is important, not accuracy, so
to me, calibration is a matter of MEASUREMENT AND DOCUMENTATION, not
twiddling of knobs.

I keep records of all my oscillators, without adjusting any, and so have
a very clear idea of their ageing rate, and can thus predict with
confidence where they will be without necessarily measuring on a daily
basis. For example, I know my Morion MV89 is currently at
10.000000271MHz, going up at 2.6e-10/day, and my HP E1938A is at
9.999998082MHz, going down at 8.7e-10/day.

As a reference I use a Samsung GCRU-D GPS Discplined Reference, and make
measurements with a Pendulum CNT-90 counter. I borrow the counter from
work when I need it, as my own HP 5315A counter lacks the resolution.

As a matter of interest, the OCXO in the Samsung (which is a C-MAC
STP2145A) has been under my eagle eye for two years now, and has an
estimated rate of -2e-9/year, incredibly low, but probably just dumb
luck. Because it is GPS locked, this is estimated from the going rate of
the EFC voltage and the known Kvco of the OCXO.

Murray ZL1BPU

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