[time-nuts] OCXO Adjustment Question

Gregory Muir engineering at mt.net
Wed May 8 11:41:21 EDT 2013

A couple if items that may have already been mentioned:

1.  You've probably already seen the effects of the capacity of your adjustment tool on the oscillator frequency if you are adjusting a variable capacitor.  It sometimes becomes a "tweak, remove tool, watch and tweak again" issue.  If you are adjusting a resistive element (pot) on an oscillator with an electronic frequency control input, you might check the pot reference voltage for proper stability and/or the pot itself (may be dirty).  Tempcos of pots will not necessarily exhibit such drastic changes unless something is wrong.  Other than that, it may be a flaky oscillator.

2.  Also remember on some OCXOs that merely removing the adjustment port cover screw (if one exists) introduces cold air into the warm oven and will alter the frequency setting.  Also, introducing that cold adjustment tool will cause temperature changes and slow frequency drifts as well.

Mr. J. L. Trantham's suggestion is the most easiest approach to take for a quick field calibration.  You can attain very reasonable calibration accuracy given the accuracy of the OCXO.  Utilizing a counter in a frequency mode becomes more of a step function that a linear one and can be frustrating not to mention more time consuming and less accurate.



On Tue, 07 May 2013, Frederick Bray write:

"This might be slightly off-topic, but probably there is a time-nut who 
knows the answer.

I am trying to adjust the 10 MHz OCXO in a Cushman 5110 service 
monitor.  I am using a frequency counter driven by a GPSDO.  Perhaps 
someone can educate me about a couple problems I am encountering.

I tried making small incremental adjustments but after I am done, the 
frequency drifts several Hz and then re-stabilizes at a new value.  When 
I make further adjustments, I notice strange behavior. For example, if I 
initially turned the adjustment clockwise to increase the frequency, it 
will now decrease if I turn it clockwise and increase if I turn it 
counter-clockwise.  On the next adjustment, it will reverse again.

Is there some correct procedure to adjust an OCXO?

Many thanks for any suggestions.

Fred Bray"

More information about the time-nuts mailing list