As you've found, time interval counters by themselves can't measure
Allan Deviation to the levels required for today's precision oscillators.
Here are three google searches - either a general search or search the
Time Nuts archive at www.febo.com. They will get you started in the
topic of Allan Deviation measurement.
"single mixer" -"dual mixer" "allan deviation"
"Tight PLL"
DMTD oscillator
These describe the three main methods of measuring Allan Deviation. To
oversimplify, they increase the time-domain noise in a way that allows
measurements that don't require such small time interval resolutions
(i.e.. less than 1 picosecond). They each have strong and weak points.
The single mixer method is probably the easiest, but requires a
reference oscillator that is comparable in quality to the device under
test and is offset in frequency by typically 1-100 Hz.
The Tight PLL method was discussed a few years ago on this list but
hasn't really caught on much even though it appears to be as good as the
other methods.
The DMTD (Dual Mixer Time Difference) method is the standard for
comparing two oscillators that are running at the same frequency. It
also requires an offset frequency oscillator, but it doesn't have to be
as good as the devices under test because it's noise is cancelled out.
It has the most complicated hardware setup of the three methods.
All of these methods require extreme attention to detail to get top
performance.
Note: Once you start down this rabbit hole, there is no turning back.
You have been warned! :-)
Ed
On 10/1/2012 2:31 PM, Adrian wrote:
Hi All,
thanks to John's superb free Timelab software, I tried ADEV for the
first time, after having been mostly interested in phase noise yet.
I learned that ADEV sensitivity is limited by the 500 ps resolution of
the counter that leads to a noise floor of 5E-10 at 1 sec, going down
to 5E-11 at 100 sec etc.
Actually, the dynamic range allows only for meaningful results beyond
a few hundred seconds. Anything below 100 sec is simply below the
system noise floor.
Note that the measurement noise floor appears to be determined
exclusively by the resolution of the counter time interval mode,
which, for a HP 53131A is 500 ps.
A 53132A with 200 ps resolution should produce a noise floor of 2E-10
at 1 sec. Likewise, a 20 ps counter (5370A, SR620) should get you to
2E-11 at 1 sec.
Here is an example that shows what I mean: http://www.ke5fx.com/rb.htm.
Add limit lines to the diagram as discussed above to see what I mean.
Even a 5370A or SR620 would not be good enough to measure the 5065A
below 100 sec in that example, since anything below a straight line
through the points 2E-11 / 1 sec and 2E-14 / 1000 sec would be below
the measurement limit.
I was wondering why the resolution in TI mode is so much limited,
since I never had any problems measuring 5 or 10 MHz frequencies with
up to 12 figures on that counter.
As a quic 'n dirty test, I tried to measure ADEV with my 53131A in
frequency mode using a gate time of 5.1 s for max resolution, and
found the noise floor had shifted down to 3...4E-12 at 10 sec, and
going further down to 5E-13 at 1000 sec from where it was equal to the
TI mode noise floor.
So, what is the best method to use for ADEV? What instruments and
setups are you using, and what works best for you? How to get the max
out of the given instruments?
Adrian
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