[time-nuts] Lady Heather's Tbolt oscillator auto-tune function

Charles Steinmetz csteinmetz at yandex.com
Sun Sep 11 11:04:07 EDT 2016

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.

Best regards,


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