[time-nuts] OCXO Soft-Start

Richard (Rick) Karlquist richard at karlquist.com
Wed Apr 12 22:14:24 EDT 2017

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

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...


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.
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