[time-nuts] Lady Heather's Tbolt oscillator auto-tune function
holrum at hotmail.com
Sun Sep 11 18:03:32 EDT 2016
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.
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.
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