Some time ago I needed a output at 10,000000.001Hz so I tried to do that
with a SI5351.
Using pure integer math (as the PLL and divider register are integers) i
search for a combination of 3 divider/multipliers that gave the least error.
If the reference frequency is not integer related to the internal PLL
frequency and multiplier/divider registers you always will have limited
accuracy as there is a fractional error.
The amount of error will depend on the number of digits in the
multiplier/divider register lengths and the care you take to search for
the best solution.
If the DDS has a PLL driven clock this could be the cause.
Erik PD0EK
On 22-11-2021 18:14, Jeremy Elson wrote:
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.g4...@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 <jel...@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 <p...@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
p...@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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