[time-nuts] GPSDO TC

[Magnus Danielson]

Dick,

Richard Moore skrev:
> 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...

Assuming that damping factors match classical analysis of damping, then 
the square root of 2 is the answer... 1.414 or there abouts.

I would be more conservative than that. I would consider damping factors 
such as 3-4 or so. I have no support from measurements on GPSDOs but it 
is reasonable that the rise of gain at and near the PLL frequency we see 
for other systems will occur and result in similar effects even here.
This gain raises the noise floor and amount of gain is directly coupled 
to the damping factor. It's just standard PLL stuff all over again. The 
only difference is that we view the result in ADEV or MDEV views.

Cheers,
Magnus