[time-nuts] Oscilloscope-based measurements of frequency stability

[Dana Whitlow]

I cheered when I saw Dave B's "silly question", for
then I realized that I'm not the only one who likes
to measure things with an o'scope.

I had purchased a GPSDO a few weeks before and
had  been observing its behavior relative to a free-
running Rb by watching 10 MHz sinewaves drift with
respect to each other as an aid in setting the Rb's
frequency.  However, I was seeing enough fairly
rapid random drift to limit the usefulness of this kind
of observation.   It dawned on me that I was sometimes
seeing drifts of several ns over the course of just
several seconds, thus implying that sometimes the
relative frequency error between the two sources was
reaching as high as roughly 1E-9.  I wanted to be able
to capture and plot a somewhat extended run of data
so I could try to understand this behavior better.

Being TIC-less, I decided to see what I could do with
my o'scope, which is a Chinese-made 2-channel DSO
with synchronous sampling by the two channels and
with a respectable trace memory depth (28 MSA per
channel).

I began this effort  in earnest a couple of days before I
saw Dave's question, and have only now brought it to
a sufficient state of completion to feel justified in reporting
some results.

I am presently able to record about 45 minute's worth of
data as limited by the 'scope's trace memory, but my XP
computer's RAM space limits me to processing only about
35 minutes of that in a seamless run.   Over that time
span I've seen a peak relative frequency discrepancy of
about 1.4E-9, with a handful reaching or exceeding 1E-9.
I've also measured average frequency differences between
the source's a a few parts in 10E11.

Most of the effort went into developing a C program to do
the processing and then correctly scaling and displaying
the results in a form which I considered useful to me.  This
processing of course had to deal with an off-frequency and
drifting 'scope timebase, which is *horrible* compared to the
quantities under measurement (as expected from the outset).

Present indications are that at this level of GPSDO mis-
behavior, the results I'm viewing are about 20 dB higher
than the basic floor, which I am still characterizing.  I
believe that the floor is limited primarily by uncorrelated
sampling jitter between the two 'scope channels.

If there is an expression of interest in this technique, I'll
publish a detailed description of the technique and some
plots showing results, probably in the form of an attachment
in pdf format.

Dana