There are of course many levels of performance in OCXOs, but in general the
ageing slows down with time, and veteran OCXOs can be very good. I recently
tested the OCXO in an old HP5245L counter (circa 1970), last calibrated in
1985, and it was still within 100ppb of 10MHz after 24 hours warmup!
One point other correspondents didn't answer is the isue of retrace. When an
OCXO is unpowered for some time, it undergoes a further ageing process when
it starts up next. This is specified in some data sheets, and can typically
require an hour of burn-in for each day left unpowered before specifications
are met.
To answer the question about whether it is best to leave an OCXO turned off
when not in use (so it doesn't 'wear out'), I'd suggest that if you don't
plan to use it again for a month, turn it off - otherwise leave it running
all the time. It should get better with time.
73,
Murray ZL1BPU
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