[time-nuts] Best counter setting for ADEV?

Ed Palmer ed_palmer at sasktel.net
Mon Oct 1 18:46:40 EDT 2012


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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