[time-nuts] GPSDO TC & Damping

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 9 01:18:57 EST 2009


Richard Moore wrote:
> On Jan 8, 2009, at 2:46 PM, time-nuts-request at febo.com wrote:
>
>   
>> Message: 1
>> Date: Fri, 09 Jan 2009 10:28:35 +1300
>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>> Subject: Re: [time-nuts] GPSDO TC
>> To: Discussion of precise time and frequency measurement
>> 	<time-nuts at febo.com>
>>
>> Richard Moore wrote:
>>     
>>> On Jan 8, 2009, at 2:58 AM, time-nuts-request at febo.com wrote:
>>>
>>>
>>>       
>>>> Message: 6
>>>> Date: Thu, 08 Jan 2009 11:51:50 +0100
>>>> From: Magnus Danielson <magnus at rubidium.dyndns.org>
>>>> Subject: Re: [time-nuts] GPSDO time constant
>>>> To: Tom Van Baak <tvb at leapsecond.com>, 	Discussion of precise  
>>>> time and
>>>> 	frequency measurement <time-nuts at febo.com>
>>>>
>>>> For ThunderBolt owners it is pretty straightforward to adjust the
>>>> TC and
>>>> damping, which is very nice. Use this oppertunity!
>>>>
>>>>         
>>> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?
>>> I'm running a verrry long TC now. If 1.2 is not actually critically
>>> damped, what value would be? Any guesses? BTW, I really like that
>>> plot of Tom's that tracks the oven and then gets better from the  
>>> GPS...
>>>
>>> Dick Moore
>>>
>>>
>>>       
>> Richard
>>
>> As always, the problem is how do you know that the time constant  
>> you are
>> using is anywhere near optimum?
>>
>>
>> Bruce
>>     
>
> Well, like many here, I don't actually have the equipment, especially  
> the reference std., to do these MDEV, ADEV and other analyses, so,  
> since I use the GPSDO for a frequency standard and not for UTC, I  
> thought I'd get the expert opinions. Magnus has several times  
> indicated here that a TC laying somewhere in and around 100 to 1000  
> secs is probably optimum. When I enquired some time back about  
> damping in the TBolt, the consensus seemed to be "leave it at 1.2". I  
> have, but it just seems to me that won't be optimum for a fixed- 
> position, lab-located frequency standard -- at the moment, I'm  
> leaning toward the 0.7to 1.0 area.
>
>   
Why, since it has been demonstrated that a damping factor of 1.2 is
better than one of 0.7 for a particular Thunderbolt this would tend to
indicate that adjusting the damping without good justification is
somewhat foolhardy.
If in fact the phase noise characteristics of your OCXO are similar toi
the one in the Thunderbolt that Tom measured this would degrade the
performance.

With no way of measuring the effect of such adjustments you are just
hoping that your particular Thunderbolt is similar to the one Tom measured.
Thats not engineering its more like witchcraft.

> Tom's recent chart was quite helpful, especially the 1000 sec curve.  
> Now, I hope that Tom or someone else follows up on the suggestion to  
> track performance vs. damping factor. I do understand that the  
> results for any one GPSDO don't *necessarily* translate to other  
> devices, but they don't necessarily don't, either. At least for the  
> TBolts a lot of us are playing with, one good example (like Tom's)  
> may well put mine in a better ballpark than the ballpark the factory  
> wants it to play in, given the factors that you all have described.  
> Thx everyone for the comments. Look forward to the next round!
>
> Dick Moore
>   
The probability that you will improve the performance significantly
without a means of measuring the resultant performance is fairly low.
You will never know if either an improvement or a degradation in
performance has occurred.
The one saving grace being that the factory defaults can always be restored.

Bruce




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