Electronics > Beginners
Measuring an OCXO - or not
metrologist:
Let me know if you want the Excel sheet, but yes 1 reading per day. I set my counter to 10 sec. gate to get mHz resolution, and it is referenced from my Trimble UCCM GPSDO 10 MHz output. I get up at 4am each day and write the number down, then plug it into my Excel sheet as shown in first post.
9999999.979
9999999.987
9999999.999
10000000.011
10000000.012
10000000.010
10000000.020
10000000.023
10000000.030
10000000.035
10000000.034
10000000.040
10000000.046
10000000.055
10000000.070
10000000.073
10000000.082
10000000.091
10000000.101
10000000.109
10000000.117
10000000.123
10000000.132
10000000.146
10000000.155
10000000.162
10000000.170
10000000.182
10000000.187
10000000.199
10000000.212
10000000.224
10000000.235
10000000.243
10000000.245
10000000.251
10000000.257
10000000.262
10000000.271
10000000.280
10000000.286
10000000.282
KE5FX:
--- Quote from: metrologist on October 25, 2018, 10:51:04 pm ---Let me know if you want the Excel sheet, but yes 1 reading per day. I set my counter to 10 sec. gate to get mHz resolution, and it is referenced from my Trimble UCCM GPSDO 10 MHz output. I get up at 4am each day and write the number down, then plug it into my Excel sheet as shown in first post.
--- End quote ---
Ah, I see. I'm not sure how you obtained the result you did, though -- it's clearly scaled improperly in the Stable32 case, but the shape of the ADEV plot seems right (with the upward trend implying linear drift). In your TimeLab plot, you wouldn't expect to see the stability improving (downward ADEV trend) at such long taus.
When I import those numbers into TimeLab with these parameters:
I get this, which is similar to the Stable32 plot:
metrologist:
So, for an OCXO that is spec'd at 5E-10/day, this does not look like what I would expect, and it is getting worse with age, like an order of magnitude?
KE5FX:
--- Quote from: metrologist on October 25, 2018, 11:35:13 pm ---So, for an OCXO that is spec'd at 5E-10/day, this does not look like what I would expect
--- End quote ---
I wouldn't say that. It looks pretty much exactly like what I'd expect. Your ADEV at 1-day intervals (86400 seconds) is right at 6E-10. Given the small number of samples, the error bars would plausibly reach 5E-10.
The oscillator could easily settle in at 5E-10/day or better with a bit more aging, depending on how long it's already been running.
--- Quote --- and it is getting worse with age, like an order of magnitude?
--- End quote ---
There's definitely not enough information to draw that conclusion. What you are seeing with the upward trend to the right is just linear drift, which is occurring at a rate that's very typical for an undisciplined OCXO.
I can't seem to find any plots of OCXOs free-running for a week or more, but here's an 8-hour plot of a good-quality 10 MHz OCXO (HP 10811-60168) shown on the same graph as yours:
My measurement didn't run for anywhere near long enough to generate a stability reading at t=1 day, but you can easily see where its drift characteristics would have put it right in line with your OCXO if it had been allowed to run as long as your test did.
Conclusion is that your OCXO is probably working fine, and very close to its specs.
edpalmer42:
--- Quote from: metrologist on October 25, 2018, 11:35:13 pm ---So, for an OCXO that is spec'd at 5E-10/day, this does not look like what I would expect, and it is getting worse with age, like an order of magnitude?
--- End quote ---
Think of it this way. Over a 42 day period, your oscillator moved 0.303 Hz. So, 0.303/42 = 7.2 mHz/day. I can do that linear approximation because, so far, the aging *is* linear. Divide 7.2e-3 by the nominal frequency of 10 MHz = 7.2e-10. So the spec is 5e-10/day and yours is aging at about 7e-10 per day. It may not quite meet spec, but it's not bad at all.
Depending on its history, it might take a few months to work the kinks out after its long sleep or this could be as good as it will get. Time will tell if the aging will slow down so that it meets spec.
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