[time-nuts] How to measure Allan Deviation?

Didier Juges didier at cox.net
Sun Oct 22 01:29:14 EDT 2006


OK, here is my problem. I do not think it is a unique problem, based on 
recent mail :-)

I have read about the Allan Deviation and I understand the principle, 
even though the nuances between the 3 basic Allan deviations escape me 
at the moment, but I am sure it will come once I re-read the Help file 
that comes with the the AlaVar software , and I have downloaded and 
installed AlaVar, a free software that can compute the various flavors 
of the Allan Deviation.

I have a working HP 5370A, which I believe is required (even though 
maybe other counters, such as the HP 5334 or HP 5316, both of which have 
a TI function that  might be used for that purpose) to gather the data 
that will be fed into AlaVar.

I have a working GPIB interface (actually several types) and a computer 
attached to it, and I can write a Visual Basic programs to talk to the 
counter and download data (I have already written Visual Basic/GPIB 
programs to control signal generators, power meter, spectrum analyzers 
and other instruments).

I have several HP 10811 oscillators (with EFC input), and a couple of 
Ovenair (also with EFC input for at least one of them), some are inside 
working HP instruments, and a couple are spares.

What I do not have is a procedure. What data do I need to feed the 
software and how do I actually collect the data?

I assume the 5370 should be set to measure TI between 2 oscillators. 
Should I use the built-in averaging function? What sample size and 
resolution should I use? Should I try to use the 5370 in raw mode (much 
faster, 6000 samples/sec) or in formatted mode (10-20 samples/sec)? Does 
it make a difference?
What if the oscillators are not phase locked and show frequency drift?

The 5370 has a 10811 oscillator for its time base, so it is good but no 
better than any of the oscillators I want to check. Do I use it as a 
reference, or do I compare two stand-alone oscillators?

How do I know which oscillator I am measuring when the two oscillators I 
am comparing are the same models? Should I compare 3 or more?

Regarding the GPS receiver, I thought most modern GPS receivers 
automatically switch from nav mode to survey mode when they stop moving. 
I would probably be mistaken to believe this is comparable to a true 
time-keeping GPS receiver, but how bad is it? Tom Clark wrote previously 
on Time-Nuts that his experience with the Jupiter was good, with +/- 13 
nS jitter, other than the fact the receiver will not return the timing 
error on the next pulse, which prevents from writing smart software that 
can compensate for it.

I have a Jupiter GPS receiver which I intend to use to discipline one of 
the 10811 oscillators. The Jupiter receiver has a 10kHz output, which 
would simplify the phase lock loop a little (even though it would not 
allow to speed up the loop). Is there any disadvantage in using it 
instead of the 1PPS output? It seems the 10 kHz would be easier to 
filter,  and maybe allow to speed up the loop following power up 
(assuming it is set to the normal, longer time constant once phase lock 
is achieved), but what do I know?

I also have a modified distribution amplifier to distribute the good 10 
MHz to my lab without affecting the master oscillator.

So I am anxious to use the AlaVar software and the toys I have listed 
above to do the following:

1) select the best OCXO to be the basis of my GPS disciplined frequency 
standard
2) find the best placement for the GPS antenna (the one that gives the 
most stable GPS signal)
3) fine tune the phase lock parameters and estimate the quality of the 
end product

Any further information and guidance (with practical tips) would be 
greatly appreciated.

Didier KO4BB


Dr Bruce Griffiths wrote:
> Tim Shoppa wrote:
>   
>> "John Miles" <jmiles at pop.net> wrote:
>>   
>>     
>>> Some of the concerns may not be major issues (e.g., it seems safe to say
>>> that the FLL action will correct for drift caused by the 7805's tempco,
>>> unless you plan to dump a can of R134a on it), but others are worth bringing
>>> to the magazine's attention.  If you'll forward your comments to Doug Smith
>>> at kf6dx (at) arrl.org, he'll most likely print them in the Letters column.
>>>     
>>>       
>> Having seen some bad vibes unfold through the channels of the
>> letters column and other communications with editors, I think that
>> BY FAR the most useful thing to do would be to contribute an
>> improvement (preferably with schematic) rather than a lengthy
>> textual or mathematical criticism.
>>
>> Tim.
>>
>> _______________________________________________
>> time-nuts mailing list
>> time-nuts at febo.com
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>
>>   
>>     
> Since the disciplining technique is far from optimum, this may only 
> serve to further sidetrack the less knowledgeable from the best 
> engineering solution which can achieve a far better performance for a 
> similar cost. A comparison of the performance of this method with the 
> optimum method using similar receivers and crystal oscillators would 
> have been helpful in assisting newcomers in selecting a more appropriate 
> method.
>
> Perhaps someone should set up a webpage detailing how one should go 
> about disciplining a high stability crystal oscillator.
>
> Allan deviation vs tau for various GPS receivers
> Allan deviation vs tau for various crystal oscillators.
> Various phase measurement techniques and associated tradeoffs
> Effect of various phase measurement techniques on the system Allan 
> deviation vs tau.
> Filtering phase measurements and discarding outliers.
> Choosing the appropriate loop time constant.
>
> Miscellaneous:
> Use of synchronisers to reduce probability of generating runt pulses and 
> metastability problems.
> Don't use same IC to buffer different frequencies
> Resynchronising divider chain outputs to reference frequency clock to 
> reduce jitter.
>
> DAC requirements and tradeoffs
> Monotonicity
> Resolution
> Noise
> Short term stability
> Cost etc.
>
> Monotonic high resolution DAC techniques
> Multibit Audio DACs
> PWM
> etc.
>
> Bruce
>
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
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>
>   



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