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

ew ewkehren at aol.com
Tue Oct 2 15:36:46 EDT 2018


Yes they are LH and the blue? is satelite
In a message dated 10/2/2018 3:30:31 PM Eastern Standard Time, k8yumdoober at gmail.com writes:

Hello Bert,

What is the cyan-colored trace seen in these screen shots?  And is
"1 ppt" the same as 1E-12 (just to be sure)?

Are these from LH?

Thanks,

Dana




On Tue, Oct 2, 2018 at 1:46 PM ew via time-nuts <time-nuts at lists.febo.com>
wrote:

> We see the glitches all the time, they exist and with proper equipment are
> very visible. These are telecom timing devices and the way they correct the
> 1 pps is by changing the frequency. Even LH shows it and it is very visible
> when you eliminate the other traces.What we call the Tbolt 2, the nicely
> packaged Trimle that also was part of the fraud listing does the correction
> once an hour. It is exact an hour function of when you powered it up.In my
> case night time spikes where mostly negative Air conditioner. Recovery is a
> function of the accumulated error. Standing next to it in front of my 19
> year old M300 it did effect the recovery  since my legs where within a
> foot. One picture shows the analog trace to.Original Tbolt does it
> constantly. We have spend a couple of years on this, hoping to optimize a
> clean up loop.so far no good results.Bert Kehren
> In a message dated 10/2/2018 1:04:50 PM Eastern Standard Time,
> tshoppa at gmail.com writes:
>
> 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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