[time-nuts] GPSDO TC

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Jan 8 16:42:58 EST 2009


Magnus Danielson wrote:
> 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
>
>   
Hej Magnus

For a second order loop, the noise bandwidth is minimised for a fixed
time constant by choosing a damping factor of 0.5.
Using a damping factor of 1.414 increases the noise bandwidth by about 60%.
Using a damping factor of 0.7071 only increases the loop noise bandwidth
by about 6%.
A damping factor of 0.3 increases the noise bandwidth by about 13%.

Bruce




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