[time-nuts] OCXO Soft-Start

Bob kb8tq kb8tq at n1k.org
Wed Apr 12 21:30:39 EDT 2017


A SC cut crystal has multiple resonance “modes” in can operate on. This is in addition to the 
normal set of fundamental, third overtone, fifth overtone and so on. For various interesting reasons
the SC modes are called A, B and C.  On a normal 10 MHz crystal, the “main mode” is the C mode. The 
A mode is well below this mode both in frequency and amplitude. The B mode is more problematic, It is
8 to 12% above the C mode in frequency and may be higher in amplitude (lower resistance) than the 
desired mode. 

The C mode has a “useful” third order temperature characteristic. The B mode has a fairly steep linear
temp co. One of the classic design experiments on a SC based design is to force the oscillator onto the 
B mode. This lets you investigate the oven gain while running a temperature run. What you have likely 
done is to put the unit onto the B mode.

Oscillators tend to be fractal when switching between modes. Minor startup issues can drive the circuit
one way or the other. Once it gets onto a mode, it may be quite difficult to get it off of that mode ….


> On Apr 12, 2017, at 5:38 PM, Scott Stobbe <scott.j.stobbe at gmail.com> 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.
> <SoftStart2p5W.png><SoftStart300mA.png><Trimble10p9MHz_SpurMode.png><TrimbleSpur_Spectrum.png><TrimbleWarmupPower.png>_______________________________________________
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