[time-nuts] OCXO Adjustment Question

Bob Camp lists at rtty.us
Wed May 8 07:39:33 EDT 2013


Full blown, troublesome OCXO set procedure:

1) Touch the adjustment gizmo with your tuning tool. Don't adjust it. Watch the frequency, does it bounce? (change over several minutes). If so, your tuning tool is cooling off the oven. Find a lower thermal conductivity tool.

2) Take a toothpick, Push on the trimmer. Does the frequency move? If so, you likely have a broken / worn / cheap trimmer. Adjustment will take a long time. 

3) Mark your tuning tool, mark the adjustment hole, call that point zero. Track your adjustments relative to that point. 

4) Let the 5110 warm up for at least a couple of days. Record the frequency. Wait a day or two and check it again. If it's not settled wait a bit more. The normal approach with this box when new was to leave it on all the time. 

Now you're ready to adjust the OCXO. Record the frequency, rotate the trimmer 1/8 turn clockwise. Watch what happens for at least a couple of minutes. Maybe it goes from +3 to +3.5 Hz relative to 10 MHz. What matters here is the start and finish frequencies, not what it does in-between. Rotate it back 1/8 turn. I'm guessing it goes from 3.5 to 3.2 Hz. That's backlash, if it's present the answer is to only adjust (slowly) in one direction. 

Next adjust it another 1/8 turn CCW. One would hope that the frequency goes from 3.2 to 2. 7 Hz. If it goes from 3.2 to 3.3 Hz, that's non-linearity. If it's that significant, you will need to spend some quality time doing the adjustment. The trick here is to do a search for one of the many possible zeros.

Next step is to move in the appropriate direction, 1/8 turn at a time, until you go from x.x Hz high to x.x Hz low. Once that happens, take a break. Come back in a half hour and check it again. 

Finally, bump the trimmer small fractions of a turn back towards zero. The idea is to get it closer with each bump, but not past zero. There's always some backlash ….

Lots of fun. All this is why cheap trimmer caps went out of favor a while back.


On May 7, 2013, at 10:24 PM, Frederick Bray <fwbray at mminternet.com> wrote:

> 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
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