[time-nuts] OCXO Soft-Start

Scott Stobbe scott.j.stobbe at gmail.com
Wed Apr 12 23:40:52 EDT 2017

Bob, Rick, my use of modes vs overtones was loose, you guys are spot on.

That's a fun challenge, suppressing a mode 10% higher in frequency. That's
a bit of a fussy LC filter/trap.

So if I assume the tempCo of the B-mode to be 20 ppm/degC, that would mean
the heater servo has a sustained oscillation of about 250 udegC all day

Power cycling for 100 ms isn't long enough for the crystal motion to die
out, 200 ms seems to do it, which ~2 MCycles. At which point it runs on the
10MHz C-Mode as desired.

I couldn't say who Trimble went with, to me its an ebay special :), but
photo attached anyways.

When the supply is current or power limited the effective supply slew rate
is something like 1 mV/s, and under these conditions, at least this
particular unit starts up on B-mode every time.

Running one of these on a USB supply (5V, 500mA = 2.5W) is looking

On Wed, Apr 12, 2017 at 9:30 PM, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
> 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 ….
> Bob
> > 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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