[time-nuts] How to measure Allan Deviation?

John Ackermann N8UR jra at febo.com
Sun Oct 22 10:44:14 EDT 2006

Hi Didier --

I have written a bunch of simple programs for GPIB data gathering, using
perl and the linux-gpib libraries under Linux.  However, the code is
pretty straightforward and it should be easy to extract the command
strings to send to the counter.  From that, you should be able to
re-write for any platform/language that can talk to a GPIB card.

Check http://www.febo.com/time-freq/tools/gpib and look for obvious
names like "hp5370-tint.pl".  I will warn you that these programs are
not examples of good programming, but most of them work.  Sort of :-)

Didier Juges said the following on 10/22/2006 01:29 AM:
> 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

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