[time-nuts] OCXO Soft-Start

Bob kb8tq kb8tq at n1k.org
Thu Apr 13 09:17:23 EDT 2017


It’s a good bet that there is at least one regulator IC inside any modern OCXO. 
As you slowly ramp the input voltage on a regulator, various odd things may 
or may not happen. A 1 mv / s ramp on the outside can be “who knows what”
at the oscillator level. That said, slow voltage ramps are a really good way to put
all sorts of oscillator circuits into really odd modes. Clock oscillators (XO’s) and MCU
built in clock circuits very definitely fall into this category. 

The same sort of “who knows what” problem also gets into the rest of the circuit. 
A limited supply might work fine nine times out of ten or 99 out of 100. On the
odd time out, something goes poof ! 


> On Apr 12, 2017, at 11:40 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 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
> long.
> 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
> plausible.
> 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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> <TrimbleSpur_HeaterOsc.png><OCXO_34310.JPG>_______________________________________________
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