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OCXO tuning screws not working on old equipment

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james_s:

--- 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:

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

--- End quote ---

I knew immediately what he was talking about, you display the two signals side by side and when they are not exactly the same frequency one of them will appear to drift relative to the signal that the scope is triggering from. Yes he could have been a little more specific but it's a common technique that is the only sensible way to use a scope to adjust something to a precision reference.

BillyO:
There is nothing wrong with the drift method and with a modern DSO it's a snap to tune a 10MHz reference to 10^-10 in a few seconds.

I'd also be willing to bet that 99.999% of folks on this forum don't need any better than one part in 100 million.  You can do that without even having zoom on your scope in just a few seconds.  You can do that on an old Tek 465.

rhb:
The voltage required to set an OXCO to frequency varies as it ages.  As a consequence it can drift outside the range of the divider.   I bought 10 CTI OSC5A2B02 OXCOs for $30.  There's on the order  of a volt between the highest and lowest voltage required  to set them to 10 MHz.

When the divider is in the can it's difficult to fix.  The $20 PCBs that use the CTI OXCOs are remarkably good.  Better than the OXCO in my 5386A which is very hard to shift a small amount.  I have 2 that I was able to trim to ~1e-10 out of 4.  The other 2 will have to have the divider resistors changed.

If you put them in a temperature controlled enclosure you should be able to limit temperature effects.  The best precision, high resolution trimmer I know of is a $25 Vishay part with a 50 ppm/C tempco.  That means  that the trimmer can only be a small part of the divider with the rest being lower tempco fixed parts and should be paired with a resistor with the opposite.  The stability of the reference voltage is critical. Hence the value of a double oven.

Reg

tverbeure:
Here's what the drift looks like for one 10MHz and one 9,999,999.97MHz signal. Deviation: 3x10-9. The scope is set to a pedestrian 10ns/div. The counter is set to 1s gate time.



The drift is obviously trivial to see and you see the changes of turning the trim screw instantly. Very quick to calibrate!

And here's what it looks like for one 10MHz and one 9,999,999.999MHz signal. Deviation: 1x10-10. (Whether or not the GPSDO is that accurate is a different story!)



The scope is now set to 1ns/div. The counter is set to 10s gate time. The drift is now much harder to see, but they still drive 1 div in 20 to 25 seconds. If all you need to see is whether or not the signal is where it needs to be, you don't need to wait a full diff: it's obvious after a few seconds that there's drift.

If you'd calibrate with the counter alone, it would have to wait up to 20 seconds after each trim.

Calibrating with a scope is just more convenient and faster.


tverbeure:

--- Quote from: rhb on June 28, 2023, 03:33:10 am ---The voltage required to set an OXCO to frequency varies as it ages. As a consequence it can drift outside the range of the divider.
--- End quote ---
Oh, definitely. Of course, even if it's out of range, you'd still expect the screw to at least do something!


--- Quote ---I bought 10 CTI OSC5A2B02 OXCOs for $30.  There's on the order  of a volt between the highest and lowest voltage required  to set them to 10 MHz.
--- End quote ---
I see that I'm not the only one with a bunch of OSC5A2B02 and other OCXOs in my part box.  :)


--- Quote ---If you put them in a temperature controlled enclosure you should be able to limit temperature effects.  The best precision, high resolution trimmer I know of is a $25 Vishay part with a 50 ppm/C tempco.  That means  that the trimmer can only be a small part of the divider with the rest being lower tempco fixed parts and should be paired with a resistor with the opposite.  The stability of the reference voltage is critical. Hence the value of a double oven.
--- End quote ---
I've seen others add temperature control to their enclosures. DIY, it's probably cheaper than a DOCXO.

I've been wondering why people don't use a precision voltage reference with PWM and 5 cent microcontroller or a DAC on their stand-alone OCXOs to avoid the hassle with trimpots. It's too costly for a OSC5A2B02, but it starts to make sense for more expensive ones like my eBay 8663. Maybe GPSDOs are just to cheap to justify it.

I also have an HP 5384A that was suspiciously stable for not having the high stability option. Here's what I found when I opened it up:



Note the AD780 voltage reference!

I bought that 5384A from the lead designer of the HP 5334A.

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