[time-nuts] Recommended parameters for Timelab ADEV analysis

Adrian rfnuts at arcor.de
Sun Nov 18 11:25:01 EST 2012


Hi Magnus,

when I noticed the difference between frequency and time interval mode 
on my 53131A, I compared some sources in both modes.
There was little difference between both modes where the signal was 
above the TI mode noise floor.
Below that t-range (that is, below the point where in TI mode the signal 
is buried by the 1/f slope noise floor), the frequency mode showed 
significantly lower values, but I had nothing at hands to compare them 
with. At least they looked plausible.

In the meantime I could replace the 53131A with a Quartzlock A7 / HP 
5370B combination.
You might understand that I was no longer motivated to investigate 
further, especially because the '131 showed much less 'improvement' as a 
'132 which I saw a plot of but don't have access to.

Cheers,
Adrian


Magnus Danielson schrieb:
> Adrian,
>
> On 15/11/12 03:59, Adrian wrote:
>> Hi Edgardo,
>>
>> however, the 53132A is considerably more sensitive in frequency mode as
>> we have recently discovered.
>> I saw an astoundingly low ADEV noise floor of some 6E-13 at t=10 sec as
>> opposed to only 2.5E-11 in TI mode.
>> I don't have a 53132A, only a 53131A that I don't use anymore for ADEV,
>> so I can't be more specific.
>
> No. The filtering that the 53132A does on frequency data does not make 
> it more sensitive in the ADEV context, it pre-filters the data to 
> improve frequency reading, yes, but that pre filtering cause bias in 
> the ADEV measurement and when you compensate for that bias you are 
> back where you started. We have been over this many times in the 
> history of the list, there is papers explaining it, so let's not again 
> spread this misconception. The 53132A is still just a 150 ps 
> resolution counter, which forms the 1/f limit slope on the ADEV plot 
> (1/f^2 power slope).
>
> Doing a ADEV with the 53132A frequency mode gives you the stability 
> measure of that measurement mode, true, the trouble is that the 
> filtering will also applied to the source, so we do not get closer to 
> the source while we see an lower value in the plot and fools ourselves 
> that we got closer. The only thing we have achieved is a lower value, 
> but the relative distance between source and counter noise limit 
> remains the same.
>
> The improvement you may do is to combat trigger jitter, so squaring 
> the signal up could get you a bit closer to the counters abilities.
>
> If you want to break the trigger jitter and resolution noise limit of 
> the pure counter, besides squaring up you got to look into mixer 
> enhancements, and you end up doing the Dual Mixer Time Difference 
> (DMTD) game, but filterings such as that in the 53132A isn't it.
>
> This filtering, which also applies to the later Pendulum counters such 
> as CNT-90, isn't a bad thing when you want to provide a higher 
> frequency resolution while still maintaining relatively high reporting 
> rate. It's actually a very good method. It's just that you don't get 
> "pure" ADEV that way, and we already have a systematic method of doing 
> something similar called the modified Allan Deviation (MDEV) which 
> actually builds on such filtering, but applied in a more systematic 
> way. The MDEV has known different behaviour to the noises compared to 
> ADEV, and this fact is used to separate noise-variants better. Dr. 
> Allan even is eager to point out that MDEV is actually fixing what is 
> broken with ADEV, and we should be using MDEV. I tend to agree.
>
> This type of prefiltering of 53132A frequency readings will not 
> improve MDEV measures either.
>
> So, that is the wrong tool for improving our measures.
>
> The only thing is that it for most of the quality stuff we measure 
> won't do much harm, since it only dominates the part of the curve 
> where we are we are usually counter limited, so we just got another 
> shape of that, but it adds nothing and we gain nothing. Hence, it's 
> not helping us one bit. Only subtle benefit would be that auto-scaling 
> could work a little better, but I doubt it's a strong enough reason.
>
> Please enlight me if I missed something important.
>
> Cheers,
> Magnus
>
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