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

Tue Oct 2 16:37:48 UTC 2018

I barely remember that article but recall it seemed like a very nice
approach.
Regards
Paul
WB8TSL

On Mon, Oct 1, 2018 at 9:32 PM John Franke <jmfranke at cox.net> wrote:

> I like the  sliding waveforms and a variation used by General Radio with
> their circular sweep 1109A comparison oscilloscope. I built a slightly
> different model:
>
> Franke, John M.: “A Circular Sweep Frequency Calibrator,” The AMSAT
> Journal, Volume 31, No. 4, July/August 2008, pp. 4-7.
>
> Reprinted in Proceedings of Microwave Update 2008, Bloomington, Minnesota,
> October 17-18, pp. 167-170, published by the American Radio Relay League
> (ARRL), Inc.
>
>
>
> > On October 1, 2018 at 6:54 PM Dana Whitlow <k8yumdoober at gmail.com>
> wrote:
> >
> >
> > 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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