[time-nuts] Thunderbolt settings

Richard Moore richiem at hughes.net
Tue Dec 2 20:27:45 UTC 2008


On Dec 2, 2008, at 1:59 AM, Ulrich wrote:

> Message: 6
> Date: Tue, 2 Dec 2008 10:59:05 +0100
> From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>
> Subject: Re: [time-nuts] Thunderbolt settings
>
> Dick,
> to be honest, I did even some more things concerning the TBolt. Find
> attached the results of an measurement where the oscillator  
> disciplining
> has been disabled and the internal PPS comparison has been logged over
> some time. So it is free running OCXO vs. GPS. The falling slope in  
> the
> left is much due to GPS jitter averaging out with longer observation
> times. The ascending slope is much due to the OCXO reacting to
> environmental conditions. This measurement would indicate that  
> 1000-2000
> s is a good value for the time constant. However, I did not have the
> heart yet to use such a big value. Neveretheless I use 1200 s TC in my
> DIY GPSDO made from an M12+ and an FTS1200.
>
> I think that also an clarification is necessary what exactly different
> manufactures mean by "time constant". I am using the term "time
> constant" in the same sense as Stanford Research do in their PRS10
> manual, where they talk about the pll's "natural time constant".  
> When a
> sudden step in OCXO frequency is noticed by the pll it will react  
> to it
> by changing the EFC voltage. It is exactly the "natural time constant"
> that the pll needs to bring the OCXO frequency to its original value.
> The pll reaction as a whole lasts much longer since it wants to bring
> the phase difference back to zero.
>
> Best regards
> Ulrich

Ulrich, thanks so much. I have upped the Loop TC using the TBolt  
monitor sw to 4000.0 secs, a bit over an hour. I really have no idea  
what that actually means, but the TBolt seems to be happy. I have no  
way that I know of to do an Allan Var measurement, so I don't know if  
this is a sweet spot or not. In the plot (thanks for that) can I  
assume that disciplining will keep the right end of the slope falling  
for a while?

Dick Moore



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