[time-nuts] How to measure Allan Deviation?

Tom Van Baak tvb at leapsecond.com
Mon Oct 23 13:46:52 EDT 2006


I've been out of town and I see a flurry of postings to
your original query about Allan deviation. It sounds
like your goal is to measure the stability of various
oscillators that you have lying around?

First, your 5370 or any other TIC (Time Interval Counter)
will be adequate for this. I'd suggest using a 'scope on
the input channel(s) to make sure your signal and trigger
levels are what you think they are. Outliers or high 5370
STDEV is a hint of poor triggering.

It sounds like you have the computer data logging part
solved. Most ADEV tools take accumulated phase
difference data; just the data that a 5370 in TI mode
will generate.

Second, your Jupiter GPS board, or any other sub-100 ns
GPS timing board, will work fine. Use the 1PPS output.

You don't have to build a GPSDO in order to make the
measurements that you want! In fact, it may only get
in the way. Instead, consider these two points.

1) For short-term stability tests, just make a set of
quick pair-wise measurements between stand-alone
oscillators at short gate-times. If they are close in
frequency, or can be made close in frequency, then
the 5370 built-in STDEV statistic is useful in real-time
(use an N or 10 or 100). If not, collect raw phase data
and run it through your favorite ADEV tool.

This method should work fine for fractions of a second
to 10s or 100s of seconds.

The good and bad pairs will be obvious; in a matter of
minutes you can tell which one or two oscillators are
the best, short-term.

Remember that your measurements are the RMS sum
of both oscillators and the counter itself so there will be
limits to the resolution, especially visible at short times.

2) For long-term stability measurements, one at a time,
compare your oscillators against the raw GPS 1PPS,
collect phase data, and run it through your ADEV tool.

To avoid phase wrap-around, depending on the frequency
offsets, you may need to divide the 5/10 MHz oscillator
down to at least the kHz range, or the obvious 1PPS, to
get clean phase measurements.

Let the GPS 1PPS be the start channel. For this sort
of long-term measurement any TTL/CMOS homebrew
divider will work (the jitter and tempco are in the noise).

It's OK to average the samples down by 100 to 1000
to reduce the volume of data you collect or process.
I often use something around 300 (5 minutes).

This will give you frequency offset, mid- to long-term
stability, and frequency drift information. A couple of
hours or days of data per oscillator should be sufficient.

Again, you don't need to build a GPSDO for any of this.


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