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

[Richard (Rick) Karlquist]

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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