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

[Tim Shoppa]

Dana, the short term few-ns jitter of the two phases, I think in a digital
instrument is most likely data acquisition glitches.

Even on a good old analog scope, jitter in the trigger circuit or jitter in
amplitudes (with resulting changes in harmonic content and thus the shape
of the curves) can cause the apparent zoomed in zero crossing to shift very
similarly.

In days of old the telco standards for frequency stability also included
requirements for amplitude stability noise, directly related to making
repeatable measurements using scopes. I'm gonna see if I can find some of
those. I remember some crazy looking telco standard that required measuring
amplitude noise on time scales measured in weeks.

Tim N3QE

On Mon, Oct 1, 2018 at 8:47 AM Dana Whitlow <k8yumdoober at gmail.com> wrote:

> 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
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