[time-nuts] How to measure Allan Deviation?

John Ackermann N8UR jra at febo.com
Sun Oct 22 10:57:31 EDT 2006

Didier, a few comments embedded below.


Didier Juges said the following on 10/22/2006 10:34 AM:
> Hi Warner,
> Does it mean I should divide the 10 MHz down to 1 Hz output and use the 
> 5370 to measure TI compared to it's internal timebase once per second, 
> and feed that to the computer, store it to a file and feed the output to 
> AlaVar? (obviously, the divider would have to use synchronous counters)
> Or should I work directly on the 10 MHz output? The 5370 can average up 
> to 100k samples, so by averaging 100k samples and polling the GPIB 
> once/sec, would I be correct if I use the computer to average those 
> further down to 2/sec, 4/sec and so on as needed before feeding the data 
> to Alavar?

Ideally, you want to reduce both the device under test (DUT) and the
reference to 1 PPS.  The GPS can give you that directly, and you can use
some sort of divider to reduce the DUT from its nominal frequency.  Not
everyone does it the same way, but there seems to be a convention that
the reference signal is applied to the "stop" input of the counter, and
the DUT to the "start" input.  You want to bump the phase of the pulses
so that the initial time difference is relatively small -- ideally, not
more than a few hundred microseconds.  This will reduce the impact of
any noise or offset in the counter's reference.

It's possible to use 1PPS from the reference, and a higher frequency
(even 10MHz) from the DUT, but I've never had a lot of luck doing this.
 It's way too easy to slip a cycle and get artificial phase jumps.

> Now, let's assume I have a big hard drive (250 GB + 120 GB at the 
> moment, with probably close to a total of 200 GB available), other than 
> computing time (which may not be negligible), how can I determine the 
> best acquisition interval? (as the one that will give me meaningful data 
> in the shortest amount of time)

Even with the 5370's 20ps resolution, you won't be able to measure good
DUTs below about 100 seconds averaging; the trigger jitter and other
noise will get you unless you go to something sophisticated like a
dual-mixer time-difference system (which isn't as easy to implement as
it appears at first glance).

So, I normally average data for 100 seconds and then log those averages.
 You can use the counter's internal average command, or just capture
each data point and average in your software.  I output a data file with
the MJD and phase value.  That data file then feeds whatever analysis
tool I'm using at the moment.

> I understand AlaVar only works in batch mode (no real time capability), 
> so I have to collect a certain amount of data "blindly" before I can 
> find out if it is any good. If I am checking a GPS disciplined 
> oscillator, that will take several hours at a time and I am trying to 
> speed up the process, at least to make sure the procedures and 
> algorithms are OK.

This is an avocation for the patient. :-)  My current experiment logging
2 HP5061As and an HP5065A against GPS has been running for about 90 days
now, and I hope to get at least another 30 days before I have to stop to
make some equipment changes.


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