[time-nuts] OCXO Soft-Start

Scott Stobbe scott.j.stobbe at gmail.com
Wed Apr 12 17:38:12 EDT 2017


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.
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