[time-nuts] GPSDO PLL convergence question
Bob Stewart
bob at evoria.net
Fri Aug 30 23:08:38 UTC 2013
I finally (think I) understand enough about my hardware and the data that my PLL is working! Now for the hard part: When do I decide it's converged? I believe that the maximum time in seconds till convergence is on the order of:
(OCXO range / DAC range) / (error count * DAC volts per step)
where:
"OCXO range": the tuning range of the OCXO in Hz - let's use 8 Hz
"DAC range" is the voltage range of the DAC from bottom to top - let's use 6 V
"error count": the number of DAC steps till convergence
"DAC volts per step": voltage change for one step of the DAC - let's use .0003 volts per step
For example, discounting the UT+ sawtooth and OCXO stability, the last 1 step of the DAC until complete convergence could take up to: (6/8) / (1 * .0003) ~ = 2500 seconds until convergence. Two steps away would be ~1250, three steps ~833, etc.
So my dilemma is to pick a number when it's safe to say that the PLL is converged. Do I just wait until there is 2500 seconds between DAC changes or do I pick a lower number? And, conversely, when I do say it's no longer converged? Or have I just got it all wrong? (Apologies if I've gotten terminology wrong.)
The methodology is similar to a Bang-Bang PLL. I keep a running track
of the OCXO error count per 1PPS, and if it over-counts twice, the DAC is
reduced one step, and if it under-counts twice then the DAC is
increased. Any 1PPS where the count is correct is thrown out. The name
"Bang-Bang PLL" doesn't seem appropriate, so I think this should be
called a "Tick-Tock PLL". =)
Bob - AE6RV
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