[time-nuts] How to measure Allan Deviation?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Oct 22 20:24:21 EDT 2006

Didier Juges wrote:
> 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
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Reading between the lines on the Jupiter GPS receiver datasheet it would 
appear that the 10KHz output is phase modulated at 1Hz to realign it to 
successive PPS output pulses. As the PPS jitters about so does the 10KHz 
Most GPS receivers with higher frequency outputs than 1Hz, phase 
modulate the high frequency output in this way and the datasheets 
explicitly indicate this.

Thus there would appear to be little advantage in phase locking to the 
10KHz signal with a short loop time constant.

To be absolutely sure you will need to use an oscilloscope to observe 
the synchronous jitter in the 10KHz waveform.


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