10.9 MHz is likely the B-mode of the SC cut. (It's a different mode, not a different overtone). This mode has a tempco of 20 ppm and is used to do thermometry.
IMHO, there is NO excuse for the oscillator designer to design an oscillator that doesn't oscillate unconditionally in the right mode. NONE! What was the actual manufacturer of the OCXO? (AFAIK, Trimble doesn't make their own). My old boss at Zeta Labs (a really great boss) used to walk up to people who were testing their latest circuit and momentarily turn down the current limit on each of the power supplies to see if the circuit recovered correctly. It often didn't, and then it was back to the drawing board... Rick On 4/12/2017 2:38 PM, Scott Stobbe wrote:
Hello, I wanted to see if I could soft-start a used OCXO (Trimble 34310) during warm-up. By default with an appropriately rated 12 VDC supply, the OCXO starts the heater at about 8 W, and eventually settles down to 2 W for 20-25 degC ambient temperature. Figure Attached. The good news is it does startup with either a current limited or power limited supply. Albeit, for the constant current case the start-up time is dramatically longer. Figures Attached. The bad news, but interesting tangent is that when this particular OCXO is soft-started, it doesn't oscillate at 10 MHz! It starts up on a spurious overtone at 10.9 MHz. There isn't even a hint of 10 MHz in the output spectrum just the 10.9 MHz tone (and some noise from the banana clip-leads loop-antenna). So this is a bit of a pain for soft-starting because, although the oven eventually comes into regulation, the oscillator is running on the wrong frequency. I'm not sure what the tempCo of the 10.9 MHz crystal mode is, but its a pretty great temperature probe of the crystal (literally). You can see the sinusoidal frequency disturbance of this tone has the same periodicity as that of the heater current (~50 s). So I don't know if the heater is oscillating at the uK level or mK level, will need to find out the tempCo of this crystal mode. Neat to see the oven dynamics. Hopefully once the oven is settled at temp, I can load switch the OCXO for 10 or 100 ms and get it to startup on the correct 10 MHz mode. With a 10+ W rated supply it starts up on 10 MHz every time. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
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