[time-nuts] Tbolt issues

Bob Camp kb8tq at n1k.org
Thu Sep 1 17:58:52 EDT 2016


Hi

The problem with absolute frequency is the one they ran into in the 60’s (and before):

There is no really good way to measure it. 

You certainly can take data. The data can have lots of resolution.  That part has 
always been fairly easy. The problem is that  the more carefully you look, the larger a 
number you get. The number can grow 2, 3, 4, 5X as you look in more depth. There is 
no nice simple way to limit the measurement so that does not happen. This makes it 
a very hard thing to characterize. 

Many frequency measurement systems have built in limits. They have a bandwidth (often 
narrow). They have an integration time (sometimes FIR, sometimes IIR). They have internal 
noise processes. All of that “colors” the result. This is in addition to the basics of observation 
time and number of samples. Running two differently designed devices in parallel can result
in two very different “frequency readings”. 

We do “hand waving” conversions of ADEV (or some other variance). That gives a number to 
some number of decimal places. The gotcha is still that the underlying noise processes may or 
may not be “nice enough” for the conversion to have any meaning. That issue has caught a lot
of people over the years. On things like GPSDO’s, the noise processes  rarely are “nice”. You 
have humps and bumps from control loops and multiple noise sources. It’s a messy problem. 

None of this really addresses the question of “how do I characterize absolute frequency”. It 
simply goes back to the 1960’s and the whole reason we *have* ADEV. It is a quantity that
you can measure and the measure converges as you take more data. Absolute frequency 
diverges as you take more data. Yes, the papers explain it a lot more clearly with a lot more
math.  

Bob

> On Sep 1, 2016, at 5:23 PM, Bert Kehren via time-nuts <time-nuts at febo.com> wrote:
> 
> maybe some one smarter than us can working with the parameters  that Tbolt 
> makes available better performance can be achieved but it is a fact  that 
> the frequency is being changed to compensate for time and Tom's frequency  
> data matches our's and we do not care about ADEV, we care about the actual  
> frequency at that moment it goes in to the measuring device and there is room  
> for improvement if you are a frequency nut and are looking for ways to  
> improve. Bob made a good point.
> 
> 
> 
> In a message dated 9/1/2016 4:27:02 P.M. Eastern Daylight Time,  
> csteinmetz at yandex.com writes:
> 
> Tom  wrote:
> 
>> No, again it sounds like you have a bad TBolt. Or something  is wrong 
> (antenna? reception? time constant? environment? China resoldered  parts?). I 
> appreciate that Juerg did lots of testing -- do you happen to have  his ADEV 
> plot?
>> 
>> Your claim of 1e-10 is order(s) of magnitude  worse than the TBolts that 
> I see. Something is wrong.
> 
> I second that  (not that any further evidence is necessary following 
> Tom's comprehensive  response).  Additionally, as far as I know, the 
> units Tom was testing  would have had the default tuning parameters (Tom, 
> please comment).   Most Tbolts I've seen can be tuned for much better 
> performance than this  at tau > 100 seconds, if they are equipped with a 
> 37265  OCXO.
> 
> Note that the Tbolt has tuning parameters that limit how far the  
> frequency is allowed to wander to adjust the PPS phase -- if Bert's  
> unit(s) have these parameters set to allow very fast PPS recovery, that  
> could well cause the behavior he describes.
> 
> You should also check  all of the other tuning parameters to see if there 
> are errors in the  settings.
> 
> However, rather than mucking about in the myriad tuning  settings 
> starting where they are now, I recommend doing a full factory  reset to 
> get all parameters back to the original settings.  Then  (after several 
> weeks of undisturbed running), compare ADEV performance  with Tom's graphs.
> 
> Best  regards,
> 
> Charles
> 
> 
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