[time-nuts] ADEV with very short Tau
csteinmetz at yandex.com
Wed Apr 29 16:04:14 EDT 2015
>I know how to measure ADEV with frequency method (using a 53131A
>counter) or time difference method (using 1 PPS of a GPSDO for
>example) but I would like to measure ADEV in the sub-second domain
>(from 0.1s to 1s for example). Do I need a Time Interval Analyzer,
>if so, an HP 5371A is ok for that ? Or there are simpliest method ?
You may do better with a Phase Noise measurement, depending on what
instruments you have available.
Generally, we think of oscillator stability over times >= 1 second in
the time domain, as xDEV, and stability over times <= 1 second (or
so) (offsets >= 1 Hz) in the frequency domain, as Phase
Noise. Partly, this is because of the different kinds of phenomena
we are concerned about on the two different scales, and partly
because different measurement techniques are better suited to each of
the two time scales.
These limits are not absolute, particularly if you digitize signals
at a high sample rate with high resolution and do the analysis in the
digital domain. Fancy xDEV/PN analyzers, such as the Microsemi
5125A, can measure xDEV down to tau = 1 mS and PN below a 1mHz
offset. (But sit down before you ask the price.)
I usually measure xDEV down to 0.1 second, and PN at offsets >= 1
Hz. Of course, to measure xDEV at 0.1 second, you need to take at
least ten TI or frequency measurements per second with no dead time
between measurements, and with good accuracy -- so you need an
instrument with very high resolution at short gate times and fairly
fast data throughput. For that, I use a Wavecrest DTS2075.
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