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OCXO tuning screws not working on old equipment

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james_s:

--- Quote from: aeg on June 26, 2023, 11:13:42 pm ---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.



--- End quote ---

I don't know why people keep talking about Lissajou, the OP said nothing about it.

I have used this method of triggering on a reference oscillator while displaying the signal I want to check and it is extremely sensitive, gently blowing on the outside casing of a double oven OCXO is enough to cause the waveform to start moving for a while before settling down.

Dr. Frank:

--- Quote from: james_s on June 27, 2023, 06:51:10 am ---

I don't know why people keep talking about Lissajou, the OP said nothing about it.

I have used this method of triggering on a reference oscillator while displaying the signal I want to check and it is extremely sensitive, gently blowing on the outside casing of a double oven OCXO is enough to cause the waveform to start moving for a while before settling down.

--- End quote ---

Very easy: Because the OP initially also said nothing specific about the setup:

'...by putting the GPSDO 10MHz and the reference output on a scope and turning the tuning screws until the clock don't drift anymore.'

From such scarce information, you can only guess, how it's done, either Lissajou / XY mode, or you could display both channels and trigger on first channel, but you could as well only display one signal and only use the 2nd on the trigger channel. Depends on your scope.
And it was not evident at all, how fast the scope / timebase used was.. 10ns? 1ns? Less than that? If you have a slow scope, the method described in the hp manual
gets really tedious, especially when you measure even more stable atomic standards, like a Rb clock vs. GPSDO, latter having a lot of jitter.

That's always what annoys me, that no details about the methods are given.
So if I made a wrong guess, it's now even more annoying if some know-it-all try to tell me, that I could have known it correctly upfront.  :-//


Frank

Dr. Frank:

--- Quote from: tverbeure on June 27, 2023, 06:42:39 am ---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.

(Attachment Link)

--- End quote ---

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

Wrenches of Death:

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

I'm guessing that you are using a standard insulated trimmer tool. I've found that a simple mod will make them much easier to use for precise adjustments. I slip a couple of inches of snug fitting clear plastic hose over them to increase the diameter. Now the tool diameter is doubled and it is MUCH easier to make tiny adjustments.

WoD


tverbeure:

--- Quote from: Dr. Frank on June 27, 2023, 08:36:59 am ---Very easy: Because the OP initially also said nothing specific about the setup:
--- End quote ---
To be honest, I have never heard about the Lissajou method for trimming an OCXO and none of the OCXO tutorials that I ran into use it. To borrow your terminology, it seems like quite a stupid method, more like a party trick to show during physics class, because it's much a harder to zoom into an XY signal and get the required precision.

But also, my question wasn't about the trimming method to begin with, but about how common it is for these trimming screws to fail...


--- Quote ---So if I made a wrong guess, it's now even more annoying if some know-it-all try to tell me, that I could have known it correctly upfront.  :-//
--- End quote ---
Alright, so far I let it slide, but since you're doubling down, here's some well-meaning advice: others commented about you sounding upset. It wasn't that, it came over as condescending, the way some doctors talk down to a nurse. "a pity you misadjusted both of your counters", "quite a stupid method", incorrectly assuming that observing Lissajou is normal way of observing drift, claiming you need see 360 degrees shifts, and then calling *others* annoying know-it-alls when they don't make that assumption. It may be a cultural or language thing, and totally acceptable where you live, but you'll just have to accept that this is an international forum where that kind of stuff will rub people the wrong way.

Anyway, let's move on...


--- Quote ---It might turn out that in your 5334A, a different OCXO is assembled, or that this feature had been disabled for some reason.
--- End quote ---
I'll open it up tonight and report back.


--- Quote ---didn't that have two trimmers, coarse and fine?
--- End quote ---
It does. Both screws are accessible from the outside. Both do nothing.


--- Quote ---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?
--- End quote ---
Leaving aside that the scope method is actually sensitive enough to observe 10-10 changes (seriously, try it!), I've done all the regular measurements with the 1992 using a 10s gate time and 10 digits precision. The last digit of the 1992 doesn't move no matter which position I put the fine trimmer in.



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