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

[Bob Camp]

Hi


> On Jan 25, 2015, at 10:59 AM, Didier Juges <shalimr9 at gmail.com> wrote:
> 
> "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."
> 
> And the fact that the GPS's CPU clock is derived from the 10MHz and
> therefore aligned to the PPS so there is no hanging bridge and sawtooth
> correction is not required.
> 
> I am not aware of any other GPSDO implementing that scheme, which is very
> elegant in its simplicity.

It is indeed an elegant solution. Based on looking at 1 pps outputs on a group of them over a year or more,  It’s actual impact on the control loop function is pretty minor compared to a properly executed sawtooth correction process. It would have a significant advantage if compared to a GPSDO that does not use sawtooth correction. 

Bob

> 
> Didier KO4BB
> 
> 
> On Sat, Jan 24, 2015 at 8:18 AM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> 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.
>> 
>> Bob
>> 
>>> 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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