[time-nuts] OCXO Adjustment Question

J. L. Trantham jltran at att.net
Tue May 7 23:52:10 EDT 2013


What I do is to apply the GPSDO to the trigger input of a scope and the
output of the OCXO to be adjusted to the vertical input of the scope.  Set
the time base to something line 100 nSec or faster and watch the output of
the OCXO after it has warmed up for 30 minutes or so.  

This is an option only if you have an output from the OCXO you can watch.
If no output, try a x10 probe attached to a 10 MHz connection from the OCXO
inside the monitor.  However, be careful that the probe does not 'load' the
OCXO and shift the frequency.

Adjust the OCXO for a stable display, not drifting left or right.

If the sine wave (or square wave) is moving to the left, the OCXO is high in
frequency.  If it is moving to the right, the OCXO is low in frequency.  If
stable, it is matched to the GPSDO.  This is useful as long as the OCXO and
GPSDO are within a few Hz of each other.  The amount of time it takes for
the display to shift 1 cycle tells you how close the OCXO is to the GPSDO.

For instance, if it takes 10 seconds for the display to shift one cycle,
100,000,000 +/- 1 cycles went by in that 10 seconds or 1 part in 10E9 if I
have my math correct.


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Frederick Bray
Sent: Tuesday, May 07, 2013 9:25 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] OCXO Adjustment Question

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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