[time-nuts] 53230A noise floor

Poul-Henning Kamp phk at phk.freebsd.dk
Fri Apr 3 11:06:56 EDT 2015

In message <CAPnVNRUdeRCi4suLHB9dJJF7sQwiXa5oVaRjWTEYLpoht+MyUA at mail.gmail.com>
, Anders Wallin writes:

>I ran some tests on a new 53230A counter that just arrived:


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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