[time-nuts] Best counter setting for ADEV?

Adrian rfnuts at arcor.de
Mon Oct 1 20:31:21 UTC 2012


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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