[time-nuts] Lady Heather's Tbolt oscillator auto-tune function
csteinmetz at yandex.com
Sun Sep 11 11:04:07 EDT 2016
>> 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*.
> …… 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.
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