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OCXO tuning screws not working on old equipment
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tverbeure:
I just received a GPSDO. Last night, I tried to calibrate my HP 5334A and Racal-Dana 1992 frequency counters as well as a stand-alone OCXO reference clock generator, by putting the GPSDO 10MHz and the reference output on a scope and turning the tuning screws until the clock don't drift anymore.

This worked fine with the stand-alone reference clock generator (it's an old commercial product, but really nothing more than an OCXO with power supply, 2 outputs, and a calibration screw hole.)

However, with both the 5334A and the 1992, turning the calibration screws doesn't anything. I can turn them from all the way CW or CCW yet the signal drifts just the same.

It's such an odd thing to fail on two of the 3 different devices on which I've tried this.

Is this a common failure point? I could obviously use the REF-IN to sync these devices to a calibrated clock or to the GPSDO itself, but I'd prefer them to be operating stand-alone.
Dr. Frank:
The Lissajou method is absolutely inappropriate for comparing frequencies on a 10-8 .. 10-10 level.
A pity that you mis-adjusted the OCXOs of both your counters.

Use Time Interval methods (accumulated phase shift measurement with ns resolution, between GPSDO and OCXO) instead, or simply measure the frequency of the GPSDO on these interpolating counters. They resolve 9..10 digits, when you use 10..100sec Gate Time.

Give TimeLab by John Miles: https://www.miles.io/timelab/beta.htm a try.

I assume that your counters are not supported directly, but check out if other interface options might work.

Frank
J-R:
I think this is possible especially on the HP5334A since it is easily externally accessible. Which oscillator is installed in the 1992?  Maybe pull one of the trimmers and test it.  And of course I'll be captain obvious and ask if you've double-checked your connections & setup?

Not sure what the Dr. is upset about, as using a scope in this manner is pretty common and I've done it many times.  Of course if you're going for a high-quality adjustment you need to monitor the scope for a little bit.  When I first got my GPSDO, I immediately adjusted half a dozen devices and in all cases drift or trim pots were blatantly the weak points, not the scope.  Specifically I remember on my HP5334B with option 10 having to literally tap-tap-tap the trimmer screw to get it where I wanted it.  A bit further than necessary, but a year later it's still perfection.
Dr. Frank:

--- Quote from: J-R on June 26, 2023, 09:40:19 pm ---

Not sure what the Dr. is upset about, as using a scope in this manner is pretty common and I've done it many times.  Of course if you're going for a high-quality adjustment you need to monitor the scope for a little bit.  When I first got my GPSDO, I immediately adjusted half a dozen devices and in all cases drift or trim pots were blatantly the weak points, not the scope.  Specifically I remember on my HP5334B with option 10 having to literally tap-tap-tap the trimmer screw to get it where I wanted it.  A bit further than necessary, but a year later it's still perfection.

--- End quote ---

Oscilloscope / Lissajou method might be common for trimming ordinary XTALS to 10-6..10-7, where a 360° phase shift might happen within a few seconds.
OCXOs need to be trimmed to 1000 times less deviation, i.e. that phase shift happens on a 1000 times longer time scale, so that's a quite stupid method, as that takes 15min at least, staring monotonously on the scope screen. The counters themselves offer a very comfortable method by T.I. phase measurement, and TimeLab or a simple PYTHON script would allow sampling over several hours. Stable32 then could calculate Allen Deviation and longterm drift,to be trimmed to near zero.
I really don't understand why people are fascinated by those outdated methods from way back then, when it is also much easier to measure frequency directly and trimm to 10-9 within 1 or 10 sec, respectively.

Then it would be interesting, which OCXO models are inside each counter. Is it a 10811 in the 5334A?
The 1992  had several different OCXO options, up to a high stability model.
I can't imagine, that both trimmers of both OCXO are defect. As well as there is no information, how fast the phase change appears to be, or if no change is visible.
In latter case, I would search for the error on operators side.
If the phase slips very fast, on the other side, then there must be a very high frequency deviation, that could also mean that the GPSDO does not work (yet) properly, i.e. not locked, no antenna signal, not disciplined yet, etc.

Did it really sound if I was upset? In fact, I dislike if no or rarely any information is given when asking for help, so one can only make an educated guess.

Frank
aeg:
The oscilloscope method is by adjusting for no drift when the DUT oscillator waveform is displayed with the sweep synchronized to the reference oscillator. It is, in effect, a time interval measurement. It does not involve a Lissajous display.

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