Electronics > Metrology

HP5386A and GPSDO - how long to wait?

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Alex Nikitin:
I was avoiding the time domain in my home lab for a very long time  ::), happy with the same level of standard accuracy as I have for resistance and voltage - about 10ppm. However now I've crossed the line and bought an old frequency counter - the HP5386A Option 004 and as an afterthought a GPSDO - the "MiniGPS Reference Clock" from Leo Bodnar. The counter has been calibrated by Keysight in May 2017 and most likely was sitting without power for a long time - some months if not years. After I've switched it on and connected it to the Mini Clock it did show a noticeable drift of the OCXO down over period of six hours since midday today. The drift is exponentially slows. So the question is - how long should I wait for the OCXO to stabilise before I attempt to correct it so it matches the GPSDO? A day? A week? A month? The error at the moment is ~1.4Hz from 10MHz or 0.14ppm. It is not that much over 4 years since the last calibration (assuming it was more accurate at that time). Should I bother at all adjusting the OCXO, providing I can use the GPSDO as a reference clock for the counter? Any suggestions on the preventive maintenance for it? I rather like the HP5386A and it is in a very good shape overall.



It depends.  If you plot the error against time it will give you a clue as to the point at which is will be stable enough for your readings to be trusted.  Add to that the aging factor which means it will never stop changing.

If you have a 10 digit readout, it will be within 100 ppb if you are within one count.

I have a rubidium standard and a counter with crystal oven and a signal generator with crystal oven.  The latter two units are alway plugged in so the crystals are always warm.  When making comparisons, the counter is almost spot on when I switch it on (because its master oscillator is always running) but the generator takes about 45 minutes to settle (because its master oscillator only runs when it's switched on).  The generator error runs a few Hz at around 500 MHz, or close enough for most work.  If I need it better, I just wait an hour or so and it's as good as it's going to get.


--- Quote ---Should I bother at all adjusting the OCXO, providing I can use the GPSDO as a reference clock for the counter?
--- End quote ---

I would say use the GPSDO, its uncertainty is going to be better. I have yet to find out what the Uncertainty is of the GPS standards but from what I have read it is good.


--- Quote from: Alex Nikitin on September 30, 2021, 05:41:07 pm ---Should I bother at all adjusting the OCXO, providing I can use the GPSDO as a reference clock for the counter?

--- End quote ---

You have two OCXOs here:
1. The OCXO inside your HP5386A, which you are wondering if you should adjust or not.

2. The OCXO inside the GPSDO, which is continually adjusted "disciplined" by the GPS signal.

If you want to keep your GPSDO continuously powered and connected to your frequency counter, you don't need to adjust the OCXO inside your HP5386A.

Otherwise, if you want to use your HP5386A for high precision frequency measurements without a GPSDO attached to it, you can adjust calibrate the OCXO inside the HP5386A once every three or six months using the GPSDO, it shouldn't take more than 15 minutes.

Alex Nikitin:
Thank you guys, I am learning this stuff as I go along. The MiniGPS Reference Clock I use doesn't have an OCXO, only a TCXO. I have, however, another, rather underused unit with an OCXO, the Marconi 2022C signal generator, which, as I've discovered now, was about 2ppm off (after at least 20 years since the last adjustment/check, possibly more as I've bought it broken and fixed it about that time). I've adjusted it and it appears to be more stable than the OCXO in the counter (at least for now). If I use the GPSDO as a reference for the counter, the drift of the Marconi is below 1ppb in an hour, around 5ppb overnight with a few degrees change in room temperature.




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