One further thought: You say it drifts several Hz -- that seems like
quite a lot, if you are making small adjustments. I'd expect perhaps
several tens of mHz at most, although if it was way off when you
started, Hz might be possible at the first iteration. I suspect you
have a mechanical hysteresis and/or dirty contact problem on the
adjustment cap or pot that you need to sort out. If you can post
details about the oscillator, someone here may have experience with
that particular part and be able give you specific advice. (You are
positive it is an OCXO, not a TCXO?)
Best regards,
Charles
Fred wrote:
I tried making small incremental adjustments but after I am done,
the frequency drifts several Hz and then re-stabilizes at a new value.
That is to be expected. Adjusting an oscillator is an iterative
process. After a while, you should get a feel for how far it drifts
after adjustment, and whether or not the direction of drift depends
on the direction you were turning the adjustment when you
stopped. In future iterations, you will stop adjusting about that
far from the exact frequency and let the oscillator drift onto
frequency (instead of adjusting for dead on and watching it drift away).
It would be good to get an educated guess (or information from the
service documentation) about what you are turning (i.e., air
variable capacitor, compression trimmer, or potentiometer setting
bias on a varactor -- and if the latter, whether it is a multiturn
or single-turn pot). This information will help you understand how
to cope with the inevitable mechanical backlash. If it is a
multiturn pot, you should always adjust, then back away just enough
so that there is no further mechanical bias applied that might cause
further motion of the wiper contact (i.e., put the adjustment screw
in the middle of the backlash, biased neither one way or the
other). Also, if it is a potentiometer or air variable cap, the
wiper (or capacitor rotor contact) may be dirty at the spot where
you need to set it -- it is often helpful to exercise the pot or cap
by running it significantly farther in both directions than you will
need to go to set it on frequency, to try to clean the contact.
You should expect to see significant drift over a period of ten
minutes to several hours, then slower drift for days to weeks until
the crystal settles into its new frequency. Every crystal is
different -- some adjust right up with no fuss (a distinct minority,
IME), some you chase for several months (again, a minority
IME). Note also that oscillators exhibit some sensitivity to
gravitational orientation, so it is best to adjust it in the
orientation in which it will be used (or else characterize its
gravitational drift and set your target adjustment frequency
accordingly). Ovens aren't perfect, so if the ambient temperature
around the oscillator is different when the instrument is buttoned
up than it is when you are adjusting it, that can introduce another
small shift.
How hard it is depends on the accuracy you expect and the resolution
of your counter -- it is much easier to get it "spot on" (as far as
you can tell) if you are using a seven digit counter than if you are
using a twelve digit counter.
Best regards,
Charles
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