[time-nuts] OCXO Adjustment Question

Charles P. Steinmetz charles_steinmetz at lavabit.com
Wed May 8 03:53:20 EDT 2013

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,


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