[time-nuts] GPSDO won't lock: OCXO aged out?

[Bob kb8tq]

Hi

Ok, what do you know / have a pretty good guess at?

1) There’s an SC (or similar)  cut crystal in there 

2) That crystal has a curve that looks something like the “0” or “1” curve in figure 7 of:

https://coloradocrystal.com/applications/ <https://coloradocrystal.com/applications/>

3) It probably is *not* a true SC so those curves are only approximately correct. 

Since the frequency of the oscillator is low, there are a few possibilities. 

1) It was originally on the lower turn and now is on the upper turn. ( = it got hotter)

2) It was in the vicinity of the turns and now is colder. 

Without knowing where it started, there isn’t much way to work out which
one is correct. Also without being able to repair the circuit / adjust the
temperature the information isn’t of a lot of use. 

Simple answer it so watch eBay for the next couple months and grab a 
few replacement OCXO’s when they next come up for sale. 

Bob


> On Jan 16, 2021, at 12:00 PM, Oz-in-DFW <lists at ozindfw.net> wrote:
> 
> On 1/15/2021 8:30 PM, paul swed wrote:
>> Not sure how hard it might be to get a thermocouple into the oven. That
>> would give you a hint.
>> Regards
>> Paul
> Seems to me you already have a temperature sensor in the OCXO.
> 
> If you temperature soak it overnight and monitor the frequency as power is applied, you should see the starting freq of the oscillator.  The heater will come on immediately and start it warming up, but the oscillator should stabilize in less than a 100 ms or so, and the heater should not be a major factor.  Do this at several temps and you should get a pretty decent picture.
> 
> I suspect the fly in the ointment in this approach is that the oven temp is usually picked to be around the flattest spot in the temp/freq curve. You may not see much resolution.
> 
> You can also monitor oven current which I guess should translate to a temperature error term, though I'm not sure off hand how you'd calibrate that.
> 
> Oz (in DFW)
> 
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