[time-nuts] GPSDO TC & Damping

[Steve Rooke]

Maybe this should be the subject of a separate thread but to enable
ordinary time-nuts to be able to test their ocxo's and gpsdo's for
phase stability at "home", what would it take as a minimum to be able
to perform something like an ADEV test? This would enable us (the
other half) to see the results of our experiments and tuning of the
gear we have otherwise it is a lot like working blind. I appreciate
that what is normally used is a counter which can continually
timestamp a dut as opposed to a gated counter but what would be the
cheapest way we could achieve this sort of setup?

Thanks and 73, Steve

2009/1/9 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
> 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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-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet