[time-nuts] Tbolt issues

[Bob Camp]

Hi

> On Sep 10, 2016, at 6:40 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Bert wrote:
> 
>> would you please share your settings, this is exactly what we are  looking
>> for. We are doing it by trial and error but your expertise will help greatly.
> 
> Well, I spent the holiday weekend looking for the "safe place" where I recorded my final Tbolt tuning parameters -- without success.  After I tuned them and qualified them as fit for long-term duty, I disconnected the com ports and stashed them away in a very quiet and hard to access location with just the 10MHz coax connected, so I can't use the comms to extract the parameters without disturbing the Tbolts (which have now settled nicely, undisturbed for ~10 years).
> 
> But what I did wasn't rocket science -- I just read up on the tuning parameters, determined which ones would likely affect the stability of the 10MHz output, made some educated guesses about the likely best settings, and started playing.  It took me several weeks of experimentation (on and off), and the parameters I settled on were somewhat different between the two units I kept (primarily, the loop time constant and damping, which ideally should be set to complement the particular OCXO in each unit).
> 
> I recommend extreme caution when you hear suggestions to use low loop damping, or to monkey very much with the oscillator scale factor.  I found that high damping (far above the 1.2 default value) worked best for my units.  (Like you, I care most about the stability and accuracy of the 10MHz output.  I don't even have the PPS turned on.)
> 
> Before you do anything else, I strongly suggest a full factory reset to put everything into a known state, and work from there.
> 
> You have received some advice to use the "autotune" routine in Lady Heather.  I seem to recall several people reporting that it worked well for them, and there is nothing to lose by trying it.  However, in my case it screwed up the tuning of both units so badly that I had to do factory resets and then re-enter my custom parameters.  I might have been using a version of LH that didn't have the latest autotune code, or perhaps the autotune function needs to start from factory default settings, or maybe the phase of the moon was wrong -- but I was sure glad I had recorded the tuning parameters I worked out by experiment, so I didn't have to start over again!  Just be prepared to do another factory reset and start experimenting if autotune doesn't work to your liking.

The one thing that autotune seems to do well is to come up with the actual sensitivity of the OCXO you have. It depends on a few things to do this so it might go wrong. I’ve never seen it come up with the wrong number. It then appears to drop in a gain and damping that make more sense than the original numbers. Since it is a pre-defined pair of numbers, it is indeed a “one size fits all” solution. 

Bob

> 
> So, I would suggest: (1) do a full factory reset, let it run for a few weeks, and take data; (2) use the autotune routine, let it run for a few weeks, and take data.  If you are not satisfied with the results at this point, (3) do another full factory reset and begin experimenting manually.
> 
> Best regards,
> 
> Charles
> 
> 
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