[time-nuts] Lady Heather's Tbolt oscillator auto-tune function
kb8tq at n1k.org
Sun Sep 11 13:50:48 EDT 2016
> On Sep 11, 2016, at 11:04 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> I wrote:
>>> Actually, for best results from a cold start, it is best to set the initial DAC voltage to whatever voltage produces 10.000000000 MHz *when the oven is cold*.
> Bob replied:
>> …… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the target.
> No, DAC for 10.000000000 MHz with a stone cold oven is best for cold starts. This setting is still best even if the oven is partly warm, because the loop is moving the DAC in the same direction it needs to go -- it can "catch up" gracefully without racing in the wrong direction in full saturation to meet the crystal as it warms up, then banging back and forth from saturation in one direction to saturation in the other direction before finally leveling out. It is the damped oscillatory approach to the capture zone, and recovering from saturation (not just once, but multiple times), that slows things down.
> By starting the DAC to produce 10.000000000 MHz from the cold oven, all that is avoided and the loop just tracks the warming crystal (or, if it can't quite keep up, at least it is moving in the same direction and can catch up gracefully without overshoot -- still much faster than starting from the DAC voltage that produces 10.000000000 MHz with a fully warm oven).
> Trust me, I tried all of this experimentally and know whereof I speak.
To me, “stone cold oven” = OCXO has been off power for > 24 hours and the entire internal structure is at ambient. That might be 25C, it might be something else. You applied power about 2 seconds ago and you have an output. The oven is now beginning to warm up to some temperature in the 80C to 120C range.
It’s an SC cut crystal. When the oven is at 25C (or worse, even colder) the frequency is off by > 30 ppm. The tuning range of the DAC is *maybe* a couple of ppm. Effectively, with an un-heated oven, there is no DAC setting. The DAC is railed and you still are not on 10 MHz exactly. As the oven warms up, the frequency is changing at ppm’s / minute sort of rates. You are not going to be able to follow that with the TBolt loop. As a practical matter, you have to let the oven warm up a bit before there even is a DAC voltage. You have to let it warm up a bit past the "EFC in range” point to be able to begin the lock process. That is likely to be some number of minutes (5, 10, 15) past the point that you apply power to a cold OCXO.
My guess is that we have different interpretations of the term “stone cold oven”.
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