[time-nuts] Re: function generator

[Lux, Jim]

On 11/22/21 8:52 AM, Jeremy Elson 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:


I've noticed that a lot of modern test equipment (Keysight 33622 
function generator as a concrete example) handles an external reference 
differently than one might expect.   Historically, you'd think a signal 
generator or counter or TBD would have a 10 MHz oscillator as a time 
base, and feeding in an external reference replaces that.


Today, though, their internal time base could be almost anything at any 
frequency. What they do is compare that against the external reference, 
and implement either some sort of frequency locked loop. This might be 
analog pushing of their internal reference smoothly, or it might be a 
period stepwise correction (e.g. reprogramming a DDS), or something else.

In any case, I would NOT expect the test equipment to improve its phase 
noise or ADEV to that of the external reference (unless the manual 
explicitly says that).  I would expect that the dial reading would 
follow the external source.  If the internal source had drifted 10ppm, 
then 10.000000 MHz on the display could be 10.000,100 MHz at the output 
jack, but with an external source, it would be 10.000000 (or maybe +/- 
10 Hz, if 1ppm is the spec).

In addition, one cannot assume that there is any consistent phase 
relationship between the external source and the output of the test 
equipment.  Connecting a pair of 33622 generators to the same source, 
and expecting the outputs to be synchronized will be unsuccessful. They 
will be syntonized to within the tolerance of the device, but 
synchronization depends on using the sync inputs, with the precision of 
synchronization determined by the internal oscillators which are fairly 
high frequency, but not phase locked to the external source. So if the 
internal clocks were, say, 200 MHz, then the sync between two generators 
is no better than the period of 5 ns.