[time-nuts] 53230A noise floor

Bob Camp kb8tq at n1k.org
Fri Apr 3 16:40:38 EDT 2015


Hi

If you use the same sort of small step synthesizer approach and get very close (but not equal to) the reference, you
can find “dead spots” where the resolution is not as good as you expected it to be. You need to be able to 
step off in 1x10^-11 steps to do a good job of plotting it. Sweeping the EFC of a (very good) OCXO is one way
to do it. There are others. 

This is by no means a bash on the 53230. Properly fed, it’s a fine device. Every fancy counter I’ve seen has these
sort of issues one way or the other. The fancier they make the software enhancements (like frequency estimation) the
more odd cases they seem to have pop up….

Bob

> On Apr 3, 2015, at 11:06 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> In message <CAPnVNRUdeRCi4suLHB9dJJF7sQwiXa5oVaRjWTEYLpoht+MyUA at mail.gmail.com>
> , Anders Wallin writes:
> 
>> I ran some tests on a new 53230A counter that just arrived:
>> http://www.anderswallin.net/2015/04/keysight-53230a-noise-floor-test/
> 
> Anders,
> 
> I think that setup didn't measure what you think.
> 
> When you feed the counter EXT REF from the same source you measure,
> all the transitions will happen at a certain point relative to the
> internal clockcycles of the counter, and therefore be subject to
> only one of many possible noise-situations.
> 
> A much better way:
> 
> Use the "good" signal for your CH1/CH2 inputs, and also feed it to a
> slightly offset synthesizer generator for the EXT REF.
> 
> If you feed the EXT REF 10.0001 MHz (relative to CH1/CH2, the absolute
> value is not really important) then over a period of 10000 seconds
> your test-setup will sweep the internal clock-cycle of the counter
> at least once.
> 
> Then plot the MIN/MAX/STDDEV of the measurement versus time and you
> will discover that the noise floor is not even close to being even.
> 
> If you find a particular good spot and want to use it for measurements,
> the way to go is to tune the length of the cable for EXT REF (or use
> a tweakable delay line)
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
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