[time-nuts] GPSDO TC & Damping

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 9 23:11:50 EST 2009


Richard Moore wrote:
> On Jan 9, 2009, at 12:10 AM, time-nuts-request at febo.com wrote:
>
>   
>> Message: 4
>> Date: Fri, 09 Jan 2009 19:18:57 +1300
>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>> Subject: Re: [time-nuts] GPSDO TC & Damping
>>     
>
>   
>>> 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
>>     
>
> Bruce, thx for the reminder -- my friend and mentor Paul W. Klipsch  
> was fond of saying that "You can't make what you can't measure 'cause  
> you don't know when you've got it made!" At the same time, all sorts  
> of wonderful things have come about thru just fooling around. Again,  
> I remark that for all the reasons Tom enumerated -- er, listed -- the  
> manufacturer's choice of settings may not be the best choice for a  
> particular use. When in a strange country, local enquiry is usually  
> recommended. For GPSDOs, a strange country to me, what better place  
> to enquire than here?
>
> Dick Moore
>
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>
>   
Dick

What does the plot of ADEV vs tau look like when the Thunderbolt OCXO is
unlocked and one just logs the Thunderbolt's own measurements of the
phase and frequency errors.
It should exhibit a minimum at a value of certain value of tau.

Setting the loop TC to somewhere near this vlue of Tau is perhaps a good
start in the absence of any other data.
No other equipment other than the Thunderbolt and a PC is required to do
this.

Bruce




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