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

[Charles Steinmetz]

Mark wrote:

> I just ran a tbolt (which has been off for a couple of months) and logged the state for a couple of hours...  and then remembered something about the initial DAC value setting that I had figured out long ago... it has little to nothing to do with oscillator disciplining.    The tbolt drives the GPS from the 10 MHz ocxo.  If the ocxo is too far off freq it can't track satellites.   The initial dac setting is used to speed up acquisition of satellites and not to speed up the OCXO disciplining loop lock.

Well...  by doing the one, it also does the other.

> As soon as a satellite is acquired (after a couple of minutes), the DAC voltage jumps and the disciplining starts.  A few seconds later when more sats are tracked, it gets underway in earnest (and by then the OCXO is warm enought to be within 0.1 Hz).  After 1 hour the box temperature has stabilized and the freq is within a couple of milli Hz.  After two hours the oscillator has settled down to the point where the DAC curve goes into "wandering around" instead of following  a smooth decay compensating for the oscillator warm-up.  The attached image show the first hour of the process.

If you look carefully at the first 3-4 minutes, you'll see it does 
exactly what I described.  The DAC reference is 0.510v, and the scale is 
5000uV/division (=5mV/division).  According to the paramaters, the 
initial DAC voltage (INIT) = 0.499v.  I assume this was previously 
stored as the DAC value after the Tbolt had fully stabilized, some time 
in the past.

Sure enough, the DAC voltage starts at just about 0.499v (it looks like 
0.494v on the graph), and when the second satellite is acquired it jumps 
very quickly to 0.529v -- an overshoot of some 55% -- before settling 
back to ~0.518v, at which time it appears to be on frequency within 1e-8 
or so.  From that point disciplining continues as the crystal warms up.

If one accepted my suggestion, the initial DAC voltage would be set to 
~0.518v for this oscillator.  In that case, it should be within a few 
millivolts of the voltage required when the second satellite is acquired 
and the huge step with its 55% overshoot should be avoided.

I would be very interested to see the result of another dead cold start 
of this same Tbolt, with INIT set to 0.518v.  Of course, the time at 
which the second satellite is acquired (hence, the temperature of the 
crystal when discipline begins, and thus, the exact DAC voltage required 
for a stepless transition, will be a bit different from one start to the 
next, so it won't be perfect.  But it will be a hell of a lot better 
than starting from 0.499v).

Now -- does what happens during the first five minutes really make any 
difference, given that no time-nut is going to do serious work with a 
GPSDO for at least several hours after a cold start?  No, probably not. 
  But we are time-nuts, after all, aren't we?

Best regards,

Charles