Mark wrote:
Here's a little info on Lady Heather's oscillator autotune function for the
Thunderbolt GPSDO
Thanks, Mark, that is very helpful.
Accordingly, for people interested in best frequency accuracy and
stability, I suggest (1) running autotune, then (2) manually setting
damping to around 10, and (3) setting the holdover recovery parameters
manually as I described in my last post (jam synch at 65-75nS and allow
a large frequency error in recovery mode). The value autotune sets for
loop time constant (500 seconds) is a good starting point for Tbolts
with the Trimble p/n 37265 OCXO. A particular Tbolt may respond to
further optimization by tweaking the TC to match the individual OCXO (by
trial and error -- Plot ADEV, adjust TC, plot ADEV, adjust TC, etc.),
but the improvement will most likely be subtle. You should also review
the elevation mask settings and adjust if necessary.
Since the unit should be locked and stable, the current DAC setting is where
the oscillator is at 10.0000000 MHz and will be set in EEPROM as the initial
DAC setting. The tbolt uses this value to speed up locking the oscillator when
powering up.
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*. Setting it to the voltage that produces 10.000000000
MHz when the oven is warm guarantees that it is set wrong when the oven
is cold, so on a cold start the loop immediately goes into saturation to
slew the DAC to the voltage that does produce 10.000000000 MHz. This
slows down locking by quite a bit. Setting the initial DAC voltage so
that the frequency is 10.000000000 MHz with a cold oven allows the loop
to slowly adjust the DAC as the oven warms up, rather than racing off at
full speed to meet the warming crystal. This speeds up locking from a
cold start very substantially.
It is not clear to me how one could automate this process -- I found the
correct DAC settings by trial and error. Ideally, this would be
determined every time the Tbolt does a cold start and the new value
would take into account any crystal drift since it was last set. But LH
has no way to tell that any given start is a cold start (AFAIK), and it
would want to determine the correct cold-oven DAC voltage very quickly
after power-up from a known cold start (say, within 10 seconds). It
would be great if Tbolts had an oven on/off command, but to my knowledge
they don't. LH could possibly give the user instructions ("OK, now
power down the Tbolt for at least 30 minutes. When the Tbolt is fully
cold, click the RESUME button below and then re-apply power to the
Tbolt...").
Best regards,
Charles
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