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Rigol DS1000Z series buglist continued (latest: 00.04.04.04.03, 2019-05-30)

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Fungus:

--- Quote from: Porcine Porcupine on March 04, 2018, 01:09:02 pm ---One thing that convinced me it's stuck in peak detect mode is that I was able to produce a trace with MATLAB that looks exactly as it should by simply downsampling the raw data to the same number of points displayed on the screen.

--- End quote ---

Correct.

The problem is simply in the number of points you need to sample in your downsampling filter.

In Matlab there isn't really a problem, you can sample 100 points, no problem. It's a powerful PC.

In the Rigol ASIC there may be a much more defined limit for this downsampling. It might only be able to sample (eg.) 4 points (I don't know the exact number but it will be quite small).

This means that at some ratios of imput->output frequencies you're going to get aliasing, ie. You'll see two lines instead of one line.

In my post in the other thread you can see this happening, The aliasing on the displayed changes with zoom level.

This isn't a firmware bug, it's a hardware limitation (number of input samples in the downsampling filter).

2N3055:
@Porcine Porcupine,

With do all respect, you don't seem to really understand how peak detect mode works...

In normal mode running at let's say 100M/sec, A/D converter is discarding 9 samples and use only one out of ten..
In peak detect mode, it won't discard 9 samples, but will remember min/max value, and show those two points on screen.
That way, you can detect 10 nsec pulse on time base where one pixel would only mean 1 usec and normally you wouldn't know something happened in meantime..
So far, so good...

But if you go to time base where your scope already samples at 1GS/sec (max rate here),  Peak detect mode and Normal mode ARE THE SAME.... That is how it works... Usually, other scopes have warning in their manual that Peak detect works only on slower timebases....


And double line can also be caused by more likely reason: since A/D converter used actually works by interleaving 4x250MS/sec converters, if converters have offset relative to each other, consecutive samples wouldn't be vertically aligned..  Like what you see... If there is a bug, it is more likely it is in self cal procedure...

But it probably is not even that.. I can replicate this only on two lowest ranges, that are software zoom created. At 1mV/DIV (1X probe) vertical res of scope is about 40 pixel ... And any offset between A/D converters is multiplied by 5x, making it really visible...
Lowest real vertical range is 5mv/div...

Just something to think about...

Regards,

Sinisa

frozenfrogz:
After some fiddling I could get to show the double tracing on my scope. It is very dependent on the time base and magnification setting though.
Looks like a quantization issue to me but that is just a wild guess.
It is gone in hi-res and average mode, thus my suspicion for quantization through the ADC.

frozenfrogz:
Btw. if you are on the low end, say 10mV per unit, you are seeing all the noise.
If you zoom in per time base, the "double trace" resolves into a scattered point cloud, showing kind of a rendering issue.
The black area between the top and bottom "trace" should actually be all yellow in this case, but for some reason the individual dots add up to *null* when merged closely together.

Fungus:

--- Quote from: frozenfrogz on March 05, 2018, 10:20:19 am ---Looks like a quantization issue to me but that is just a wild guess.

--- End quote ---
I agree 100%.


--- Quote from: frozenfrogz on March 05, 2018, 10:20:19 am ---It is gone in hi-res and average mode, thus my suspicion for quantization through the ADC.

--- End quote ---

ie. If you choose sensible display settings it vanishes. Not an issue.

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