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

Dana Whitlow k8yumdoober at gmail.com
Mon Oct 1 18:54:13 EDT 2018


I've done the Lissajous thing, but it takes an extra bit of effort to
work out the phase angle.   I've always gone back to the sliding
waveforms display for simplicity.

But I'll admit the Lissajous pattern is a lot prettier, and looks great
in Sci Fi movies.

Dana


On Mon, Oct 1, 2018 at 4:06 PM Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

> No one mentioned using Lissajous patterns for comparing
> frequencies if the scope has an XY mode.  Google Lissajous
> if interested.
>
> Rick N6RK
>
> On 10/1/2018 11:40 AM, Bryan _ wrote:
> > Interested as well
> >
> > -=Bryan=-
> >
> > ________________________________
> > From: time-nuts <time-nuts-bounces at lists.febo.com> on behalf of Chris
> Burford <cburford1 at austin.rr.com>
> > Sent: October 1, 2018 6:20 AM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] Oscilloscope-based measurements of frequency
> stability
> >
> > This sounds interesting enough and I would appreciate any notes or
> insight on doing this. I have a PRS10 and several GPSDOs that I would like
> to evaluate for performance on my scope.
> >
> > Many thanks.
> >
> >
> > ---- 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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