[time-nuts] 10811A settling time

[alan bain]

I have several of these HP ovenised 10MHz oscillator modules which are
not able to be run 24/24  although I leave the ovens operating (they
are generally in power-hungry HP test equipment which has a standby
mode which leaves power to the oven).

I've been noticing that they take a decent number of hours (minimum
8h) of running as oscillators from standby before the short term
frequency is adequately stable (in terms of not drifting) for some
purposes.  It's very obvious when trying to phase lock multiples of two of them
together using EFC to tune one and a narrow bandwidth PLL. In trying
to find zero beat the beat frequency can be seen drifting on a scope /
counter fluctuations usually around 0.5-1Hz over 20-30min period.

During this time they compare less well to a boring modern non-ovenised
oscillator for short term stability. No actual quantitative
information because there isn't a better standard in this location to
use as a timebase, although I do have an HP53132A counter (but with
the rubbish timebase option) which is what I use to measure the
beatnote frequency.

I'm wondering what the physical process might be for this settling? Is
heat dissipation in the oscillating crystal needing to find thermal
equilibrium?  Or do they need repair in some way? I've never taken one
apart but I do have the service manual.

The manual for the 10811A suggests running for 10days before checking
drift, but to do so requires a Caesium frequency reference which -
alas - I lack!

I'm aware that there's a lot of knowledge of these HP oscillators
around here and would be grateful for any pointers.

Alan