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

John Franke jmfranke at cox.net
Mon Oct 1 21:30:45 EDT 2018


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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