[time-nuts] Best counter setting for ADEV?

[Magnus Danielson]

On 10/01/2012 10:31 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?

Ahem, you should be careful with certain modes of the 53131/53132 
counters, amongst others. As they do internal smoothing they do a form 
of filtering which gives you artificially lower numbers than the actual 
numbers. We have covered this several times and from many angles.

Stick to the TI mode for correct numbers.

Cheers,
Magnus