[time-nuts] ADEV with very short Tau

Charles Steinmetz csteinmetz at yandex.com
Wed Apr 29 16:04:14 EDT 2015


Claude wrote:

>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.

Best regards,

Charles






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