[time-nuts] Re: function generator

[Jeremy Elson]

Andy,

I was thinking in the same direction - the device can create arbitrary
waveforms so they're almost certainly using DDS. So I assumed it was some
sort of round-off error. I just can't quite convince myself of this because
it seems like round-off errors would be bigger, i.e. an error of one part
in 1E11 suggests a buffer of length 1E11 which seems unlikely.

On Mon, Nov 22, 2021 at 9:06 AM Andy Talbot <andy.g4jnt at gmail.com> wrote:

> 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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