[time-nuts] Questions regarding tuning Thunderbolt with Lady Heather --> GPSDO's

Bob Camp kb8tq at n1k.org
Sat Jan 24 09:18:15 EST 2015


Maybe a bit more information, much of it applies to all GPSDO’s :

The TBolt first goes through a process to get the OCXO roughly on frequency and to get the PPS approximately aligned. That process is not impacted by the time constant and damping. The OCXO goes a bit crazy during this process. 

It then starts the phase lock process with a short time constant. As the OCXO settles in, it will step out to a progressively longer time constant. It does this based on it’s internal estimates of lock quality. Unless you are already at maximum time constant and have a good internal estimate, changing the time constant has no immediate impact. On most GPSDO’s and with most OCXO’s under most conditions, the step out process takes days or weeks. 

The damping number does impact the performance in the maximum time constant mode. It may be scaled as the time constant is changed. 

There does appear to be a D in the TBolt loop. For what ever reason, that’s not a changeable value. The D does scale with the time constant. 

When in lock mode, the TBolt is a PLL and not a FLL. As the “phase in” (the pps from the gps) moves, the frequency of the OCXO will change to keep the “phase out” (PPS output) aligned. As the unit is running, it keeps track of the average DAC value that puts the OCXO on frequency. Since it’s a PLL, that number may or may not be the last instantaneous value of the DAC when it goes into holdover. Since it’s running a PLL, the PPS output will indeed be the best value, so no correction is needed there when it goes into holdover (not quite true, but that is the assumption made). 

This operation is very typical of all of the cell site GPSDO’s. The only part that is unique to the TBolt is the ability to fiddle the loop characteristics a bit. 


> On Jan 23, 2015, at 10:58 PM, Skip Withrow <skip.withrow at gmail.com> wrote:
> Hello Nuts,
> I have been playing a bit tuning a Thunderbolt with Lady Heather and now
> have more questions than answers.  The collective time-nut brain would be
> appreciated.
> 1. Using the '&' command I can change the damping and time constant in LH.
> Are these values immediately transferred to the TB?
> 2. Do I have to use the LH 'e' command to permenantly save new damping and
> time constant values to the TB?
> 3. After using the '&' and 'e' command the lock-in behaviour of the TB does
> not seem to change.  Is this normal behaviour?  Is one set of values used
> when locking and the adjusted values used once it is phase locked?
> 4.Is there some way to read out the values stored in the TB?  When I use
> the 'e' command on the TB, change values in LH, then restart LH and the TB
> I see the last values given to LH, and not what I thought was saved with
> the 'e' command.
> 5. If the TB is placed in hold mode and the DAC set to 0.0 volts it
> actually goes to 0.2 volts (min is at -5, max is at +5, and iv is 0.0 which
> it actually starts at on power up).  Anyone know why?
> Thanks in advance for any guidance!
> Regards,
> Skip Withrow
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