Unlike the 1992, the trimmer of the 5334A seems to be outside of the OCXO itself. So that seems to be first step: open it up and check the potmeter itself.
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Hello Tom,
I have to admit that the scope method is usable for trimming the OCXOs.
The T.I. and direct freq. methods are much easier, anyway, also from point of cabling.
Please check, what happens if you use 10..100sec Gate Time on your 5334A, whether the display always rounds to 9 digits, or displays the rightmost digits with OF.. That could give additional resolution.
Identical procedure like your scope method applies, if you connect GPSDO signal to channel A of your 5334A, and the 1992 output to channel B and then choose T.I. A->B on the 5334A (mode which allows +/- T.I.?).. it should resolve 1..2ns, maybe additional averaging is possible, which you can either track visually over several minutes, or by a PYTHON script.. The effort is the same, but to my experience much more comfortable. TimeLab would directly plot this curve and freq. deviation.
I last used the scope method (XY and 2 channel) when I was a teen, building my first TTL grave counter with an OCXO, comparing to 200 kHz Droitwich, so I don't have to redo this experiment again.
Btw.: you have a really nice frequency blog, so you could now try and add these different phase measurement methods, which were regularly used by those 'time-nuts', but also for the Allen Deviation measurements. Maybe you could reveal, which GPSDO you have acquired. Some of them, especially when there is no disciplined OCXO inside, show a very bad short term jitter on the order of 10
-8.
Concerning your counters, it's getting really interesting. The 5334A/B uses the external EFC trimming, if a 10811 is assembled inside.
That's not the case with its bigger brother HP5335A, which I own. I have to open the lid and trim the capacitor directly inside the 10811.
For my 5370B, I drilled a hole into the lid, to access this said capacitor trimmer in the 10811.
It might turn out that in your 5334A, a different OCXO is assembled, or that this feature had been disabled for some reason.
I'm not so familiar with the 1992, I repaired one with that high stability option 27 years ago .. didn't that have two trimmers, coarse and fine?
Maybe you only have access to the fine trimmer from outside, so you won't see any difference, as the scope method is not sensitive enough for very small changes?
Same might occur for the 5334A.
It would be great, if you would provide some Photographs from the interior of the 5334A, please.
Frank