[time-nuts] Re: function generator

[Andy Talbot]

Could the Rigol be using a DDS / NCO lookup approach to its frequency
generation?  In which case you'll be subject to rounding errors on the DDS
increment, whose maximum magnitude will be  dependent on the register
length.

Andy
www.g4jnt.com



On Mon, 22 Nov 2021 at 16:53, Jeremy Elson <jelson at gmail.com> wrote:

> I did not see any such setting in the Rigol, but I'll check again. in April
> I did write to Rigol to report the problem and had the following (abridged)
> conversation with support:
>
> Me:
>
> "I recently tried to use your DG1022Z signal generator to generate one
> pulse-per-second (pulse mode, frequency 1.000000hz, width 10 microseconds).
> However, it appears there is a small frequency error of one part in 1e11,
> i.e. the pulse per second gets later by about 6 nanoseconds every 1,000
> seconds." [More technical description abridged, including a link to a
> graph.]
>
> Rigol:
>
> "Please find the following datasheet for DG1000Z, the accuracy is +/-1ppm.
> If your pulse is 1second with 10us width, 6ns per 1000s is in the accuracy
> range."  [They attached an image of a page from the DG1000Z datasheet,
> showing a line that said "Accuracy: +/- 1ppm of the setting value"]
>
> Me:
>
> "Is this the specification even when the unit is provided with an accurate
> external clock?"
>
> Rigol:
>
> "I would say Yes. The internal processing circuit will effect the clock
> signal,  harmonics and phase noise will result the frequency variance."
>
> -Jeremy
>
> On Sun, Nov 21, 2021 at 11:54 PM Poul-Henning Kamp <phk at phk.freebsd.dk>
> wrote:
>
> > --------
> > Jeremy Elson writes:
> >
> > > [...] I plugged an RbXO into the DG1022Z and simply asked the
> > > DG1022Z to divide it down to a 1pps signal. That is, I configured it to
> > > create a pulse with a small duty cycle and 1hz frequency using the
> > external
> > > clock as a reference. To my surprise, it introduces a small but
> > > measurable error of one part in 1E11 in the dividing-down. You can read
> > the
> > > full story of this in my post from earlier this year:
> >
> > When you feed such instruments an external clock, you often have to
> change
> > one or more calibration constants for the internal (OC)XO's offset to
> zero.
> >
> > Another problem is that if you use the square wave output of a DDS based
> > generator, they often produce the square from the sine output of
> > the DDS chip with a schmitt-trigger, which causes lousy jitter.
> >
> > --
> > Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> > phk at FreeBSD.ORG         | TCP/IP since RFC 956
> > FreeBSD committer       | BSD since 4.3-tahoe
> > Never attribute to malice what can adequately be explained by
> incompetence.
> >
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