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 <[email protected]> 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 <[email protected]> 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 <[email protected]> > > 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 > > > [email protected] | TCP/IP since RFC 956 > > > FreeBSD committer | BSD since 4.3-tahoe > > > Never attribute to malice what can adequately be explained by > > incompetence. > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] -- To unsubscribe > send > > an email to [email protected] > > To unsubscribe, go to and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send > an email to [email protected] > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
