[time-nuts] OCXO Adjustment Question

Charles P. Steinmetz charles_steinmetz at lavabit.com
Wed May 8 05:32:57 EDT 2013

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,


>  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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