[time-nuts] Thunderbolt settings

[Bruce Griffiths]

Richard Moore wrote:
> 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
>   
You would need a statistically independent frequency standard
(preferably 2 or more) of comparable or better stability to the GPS SVs
to make meaningful ADEV measurements.
Typically a hydrogen maser is required for ADEV measurements for long
Tau, for shorter Tau a very high quality OCXO (preferably 2 or more)
will suffice.
The best you can do is repeat Ulrich's test with your own Thunderbolt,
the OCXO noise characteristics will differ for each OCXO as will the
optimum loop time constant.
All the software tools for logging, analysing and plotting the data are
readily available for free.

Bruce