[time-nuts] OCXO Soft-Start

[Magnus Danielson]

Hi Scott,

On 04/12/2017 11:38 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.

You should be able to do that. I was just about to propose it. A short 
power off will still remain the oven temperature, but will re-set the 
oscillator.

Cheers,
Magnus