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

--- Quote from: Fungus on March 06, 2018, 10:30:53 am ---
--- Quote from: Porcine Porcupine on March 06, 2018, 10:05:55 am ---I downsampled by a factor of 2,000 to go from 2.4 million raw sample points to 1,200 like the oscilloscope presumably did to get the trace data but didn't get the aliasing. Would I perhaps see similar aliasing if I tried other downsampling ratios or different sampling phases?

--- End quote ---

You just described the problem perfectly: How do you reduce 2000 points of data to a single point on screen?

I don't know what method the DS1054Z uses, sorry. I'm sure it's done in hardware though so expect it to be a compromise.  :popcorn:

(nb. The DS1054Z only displays 600 points on screen so there's a secondary 2:1 reduction somewhere...)

--- End quote ---

In the image below, taken from one of my previous posts, I plotted the points stored in the oscilloscope's 1,200-point trace buffer in red and the raw data I downsampled from 2.4 million raw points to 1,200 points in blue. So the effect happens before that final 2:1 reduction.

This is making me want an oscilloscope with open source hardware and software so I can peer inside the black box. But guessing what the black box is doing is sort of fun too in its own way. :)



Fungus:

--- Quote from: ebastler on March 06, 2018, 10:57:34 am ---Anyway, this is just guessing. But I think it likely that, within the limitations of the existing DS1000Z hardware, there would be a better way to down-sample and display the signals at slow time bases.

--- End quote ---

Just don't use dot mode, problem solved!   :-+

(why would you use dots to look at a signal like that?)

Or ... use dots but turn on averaging - also solved!


ebastler:

--- Quote from: Fungus on March 06, 2018, 06:59:53 pm ---Just don't use dot mode, problem solved!   :-+
(why would you use dots to look at a signal like that?)

Or ... use dots but turn on averaging - also solved!

--- End quote ---

Well, it's most obvious in dot mode -- but the line mode suffers from an inflated noise (trace width) if it plots only the outliers and connects them with lines.

Averaging is fine if you want to look at a stationary signal, but pretty useless when looking for fluctuations or spurious events. So you will have to live with the inflated noise when you are after that. (And at the same time have to measure at the slower time bases, to be fair.)

I'm still happy with my DS1054Z; but it would be a pity if Rigol make the noise performance look worse than it is, just by careless digital processing.
2N3055:
Sorry but i have a question:

I would like to know what you guys think an analog CRT scope would show in these circumstances...

Namely, a slow time sweep and fast pulses... For instance, 10 ms/div and feed it 1 MHz signal....

Tadaaaa...

You would get solid block, from left to right, vertically sized to P-P amplitude of signal....
Depending of waveform, parts of that "band" would be darker or brighter, depending on statistical distribution of signal amplitude over time ....
It works only in what a DSO crowd would call "Peak mode"....

I don't know who started dot mode fetish, but it is not optimal mode that shows signal best... Vector mode is your friend. Dot mode is used only sometimes, for specific reasons.

Also, unless you want to specifically filter out higher frequencies (by using BW limit, average or HiRes mode), you want a Peek mode pretty much always...
You don't want to miss super short interference bursts because you are looking at slow timebase..

I don't know what are you measuring but from my practice...

Regards,

Sinisa
ebastler:

--- Quote from: 2N3055 on March 06, 2018, 07:39:45 pm ---I would like to know what you guys think an analog CRT scope would show in these circumstances...
Namely, a slow time sweep and fast pulses... For instance, 10 ms/div and feed it 1 MHz signal....
Tadaaaa...

--- End quote ---

Read again, please, and go one page further back. The real discussion of the artefacts starts around post #237 or so. The demo signal is simply the 1 kHz square wave from the calibration output. And the problem is that the parts of the trace where the signal is stable appear either split in two (dot mode) or artificially widened, with inflated apparent noise (line mode).

The short pulses at large intervals, shown further down in the thread, only serve to show that peak mode actually works (and only works when enabled).
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