[time-nuts] GPSDO TC & Damping

Richard Moore richiem at hughes.net
Fri Jan 9 01:00:46 EST 2009

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.

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

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