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REVIEW - Rigol DS2072 - First Impressions of the DS2000 series from Rigol

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marmad:

--- Quote from: Galaxyrise on November 19, 2013, 04:39:05 pm ---Anti-aliasing is also done sample->display time, and it's almost useless since the "normal" sample->display decimation algorithm rarely introduces aliasing. And in high res, it actually tends to make aliasing worse!  What most people expect anti-aliasing to do, ie minimize sample-rate induced aliasing, Rigol's anti-aliasing cannot do.  It can make the display a little nicer looking sometimes, but I think it's generally a waste of update rate.
--- End quote ---

Yes, and Agilent's (well, it really came from HP) clever anti-aliasing technique - which could be used between the sample memory and display if oversampling - seems to be patented. I wonder if any other DSO manufacturers are using HP's technique?

Teneyes:

--- Quote from: Carrington on November 19, 2013, 02:24:31 pm ---I am more interested in the ANTI-ALIASING option, I would like to know how RIGOL implemented it, what sampling method used etc...

--- End quote ---
@Carrington
  Yes Marmad and Galaxyrise have discussed this back at:
https://www.eevblog.com/forum/testgear/first-impressions-and-review-of-the-rigol-ds2072-ds2000-series-dso/msg242861/#msg242861

marmad:

--- Quote from: Carrington on November 19, 2013, 02:24:31 pm ---By limiting the BW seems to be designed for audio applications, or something like that.
--- End quote ---

@Carrington: The bandwidth is automatically lowered by the High Res technique itself. As mentioned before, best case BW for a 2GSa/s DSO averaging to 12 bits is ~3.3MHz (0.433 * 2G / 256). In fact, calling it "High Resolution" is a bit of a marketing ploy - since it's really just a smoothing filter. The "12 bits" of resolution" are effective bits - since the displays on these cheaper 8-bit DSOs don't even have 9 bits of vertical resolution anyway. So the averaging is done to a 12-bit value - then downsampled to 8 bits again for display.

I think it would make more sense - and be more clear for people - if instead of calling the two acquisition modes "Average" and "High Res", they were called something like "Waveform Average" and "Sample Average".


--- Quote ---I am more interested in the ANTI-ALIASING option, I would like to know how RIGOL implemented it, what sampling method used etc...
--- End quote ---

It seems to be some simple technique for reducing moire patterns on the screen - but totally worthless to eliminate real DSO aliases. If you're worried about aliasing, the best bet on the DS2000 is to use the AUTO or 56MB memory depths to keep the sample rate as fast as possible at slower time base settings.

Galaxyrise:

--- Quote from: marmad on November 18, 2013, 05:44:44 pm ---This makes perfect sense since the stored frames are the waveforms constructed from the already-averaged samples (with the original samples no longer available). OTOH, when the DSO is stopped (when not in Record), the last group of captured samples still sits in sample memory - so the DSO can apply (or not apply) the averaging to the display memory by turning High Res on or off.

--- End quote ---
I had assumed that the raw samples were written to segmented memory, which would mean that the scope had to apply the high res algorithm to segmented memory in order for it to work at all in that mode, and thus there was no good reason not to enable changing between normal and high res on recorded data.

So I tested this. I generated the fastest pulse my sig gen could do, which was 32 samples at 2GSa/s.  I recorded this in high res at 5us/div.  Naturally, the display at that timebase was severely attenuated.  But zooming in to the recorded data was just like zooming in on a single waveform capture; all the data is there, confirming my assumption that the recorded data is the same in both modes.


--- Quote ---
--- Quote ---There's a practical difference between the two approaches and it affects record mode: You get to store fewer waveforms if you want the high res averaging.
--- End quote ---

I'm not sure I understand you: on my DSO I haven't noticed any difference in the maximum frames I can record when using High Res mode.
--- End quote ---
I don't mean that turning on high res changes the record length for a particular memory depth, I mean that you have to use a larger memory depth to give the algorithm enough samples to average. If I want to capture 14k points per waveform, and I want each of those points to be the result of averaging >8 samples, then I actually need to capture at 140k pts. 

But really, I would expect a difference between the algorithms any time the scope increases its time between stored samples.  Averaging 16 consecutive points that were sampled 1ns apart produces very different results from averaging 16 consecutive points that were sampled 1us apart.  I'm pretty sure the latter is what Rigol does, and so even the aforementioned 140k pts could produce different results than 14k pts of acquire-time averaging.

Svuppe:

--- Quote from: marmad on November 19, 2013, 07:36:22 am ---I'd be curious to know what the bandwidth of the High Res mode is at each time base setting between 50us/div - 10ms/div in the Agilent 2000 X-Series.

--- End quote ---
I tested that last night on my 2000X, which has the optional 1M memory option.

Time base   Bandwidth (-3db)  -   First null in stopband

10ms/div   ~34kHz   ~77kHz
5ms/div   67.87kHz   153.6kHz
2ms/div   169.6kHz   384.0kHz
1ms/div   339kHz   768.0kHz
500us/div   676.7kHz   1.536MHz
200us/div   1.689MHz   3.840MHz
100us/div   3.364MHz   7.680MHz
50us/div   6.704MHz   15.36MHz
20us/div   16.87MHz   ?
10us/div   ?      ?
5us/div   ?      ?

I used the built-in waveform generator, which only goes to 20 MHz, so I couldn't get the last few measurements.
Hmm, I can't get the table to align nicely. I hope it is readable anyway.

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