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

--- Quote from: marmad on December 28, 2013, 12:13:18 am ---
--- Quote from: JDubU on December 27, 2013, 11:48:26 pm ---By multiple, I mean that the available vertical display resolution is not being used to its best advantage by the interpolation algorithm.   The individual, lower resolution ADC values may map to locations that are separated by multiple display pixels, but the interpolated trace that connects them is not limited to the resolution of the ADC.  The interpolation algorithm should produce smooth traces that fully utilize the available vertical resolution of the display.

--- End quote ---

All of the interpolation is done on the original 8-bit values - with scaling for the display done as the last stage of the process (which I believe most lower-prices DSOs do - see attached image from Agilent X-3000 showing a minimum of 2 pixel steps - and even some quantization error at the top of the second sine wave). This method also guarantees that the display memory produces 8-bit values when read by another device.

In any case, what you've pointed out is not a 'problem with the sample-to-display interpolation algorithm' - it's the way it's normally done - and while your suggestion would lead to a nicer looking waveform when the DSO was stopped, it would definitely be slower.

--- End quote ---

The problem is that the interpolation algorithm is adding the appearance of higher frequency content than what the ADC is actually measuring.  The ADC is only producing the "dots" display.  Everything in between the dots in the vector display should be as close to an estimate of what the waveform looks like as possible.  This is the role of the sin(x)/x convolution window which attempts to use Nyquist criteria to reasonably interpolate what the actual waveform is doing between measured sample points.  Since these are simply mathematically derived estimates and not measured, there is no reason that there should be higher frequency content in the interpolation than there is in the dots display itself.   It seems like what is happening (and what you describe) is that the interpolation is rounding off to the nearest of the 256 quantization levels of the ADC rather than to the 400 level vertical resolution of the display and then linearly scaling (with more roundoff error) to the vertical display resolution.
marmad:

--- Quote from: JDubU on December 28, 2013, 01:07:33 am ---The problem is that the interpolation algorithm is adding the appearance of higher frequency content than what the ADC is actually measuring.  The ADC is only producing the "dots" display.  Everything in between the dots in the vector display should be as close to an estimate of what the waveform looks like as possible.  This is the role of the sin(x)/x convolution window which attempts to use Nyquist criteria to reasonably interpolate what the actual waveform is doing between measured sample points.  Since these are simply mathematically derived estimates and not measured, there is no reason that there should be higher frequency content in the interpolation than there is in the dots display itself.   It seems like what is happening (and what you describe) is that the interpolation is rounding off to the nearest of the 256 quantization levels of the ADC rather than to the 400 level vertical resolution of the display and then linearly scaling (with more roundoff error) to the vertical display resolution.

--- End quote ---

I don't agree that there is higher frequency content added: there is quantization and rounding error - again, visible in the Agilent's screen capture as well - and within the error bounds of the DSO. But it seems to me you have a very simplified idea about how the captured samples end up as interpolated display. If vectors are ON, the display memory ALWAYS contains 1400 bytes (700 vectors) - it doesn't matter if the ADC captures 14M - or - 28 samples during the acquisition window. There are a WHOLE bunch of steps involved in arriving at those 1400 bytes - just one of which is the actual sin(x)/x or linear interpolation algorithm.

But anyway, it's late and I'm done here - if you want to believe there is a problem with the Rigol, fine by me.  :)
JDubU:

--- Quote from: marmad on December 28, 2013, 01:26:36 am ---
--- Quote from: JDubU on December 28, 2013, 01:07:33 am ---The problem is that the interpolation algorithm is adding the appearance of higher frequency content than what the ADC is actually measuring.  The ADC is only producing the "dots" display.  Everything in between the dots in the vector display should be as close to an estimate of what the waveform looks like as possible.  This is the role of the sin(x)/x convolution window which attempts to use Nyquist criteria to reasonably interpolate what the actual waveform is doing between measured sample points.  Since these are simply mathematically derived estimates and not measured, there is no reason that there should be higher frequency content in the interpolation than there is in the dots display itself.   It seems like what is happening (and what you describe) is that the interpolation is rounding off to the nearest of the 256 quantization levels of the ADC rather than to the 400 level vertical resolution of the display and then linearly scaling (with more roundoff error) to the vertical display resolution.

--- End quote ---

I don't agree that there is higher frequency content added: there is quantization and rounding error - again, visible in the Agilent's screen capture as well - and within the error bounds of the DSO. But it seems to me you have a very simplified idea about how the captured samples end up as interpolated display. If vectors are ON, the display memory ALWAYS contains 1400 bytes (700 vectors) - it doesn't matter if the ADC captures 14M - or - 28 samples during the acquisition window. There are a WHOLE bunch of steps involved in arriving at those 1400 bytes - just one of which is the actual sin(x)/x or linear interpolation algorithm.

But anyway, it's late and I'm done here - if you want to believe there is a problem with the Rigol, fine by me.  :)

--- End quote ---

Thanks for the discussion Marmad.
Have a good night.   :)
Teneyes:
Here are displays showing how variations in the trigger timing result in sample dots occuring at different points along the sin wave curve. With the noise the dots are shown as streaks and clusters
The displays
1. Shows input as a display in vectors
2. Shows input as a display of dots
3. Shows input as a display of dots SINGLE scan
4. Shows input as a display of dots with trigger level higher and there is a streak of dots
5. Shows input as a display of 1 small cluster of dots with trigger level higher
6. Shows input as a display of 3 small cluster of dots at some conditions.
7. Shows input as a display of 3 small cluster of dots under some conditions. With 'Trigger out' fed into chan 2

I hope this helps explain how the DS2000 operates.

Is the variations of the trigger point by design, or an effect of noise & clock variations??
cyr:
Not quite sure I understand your question, but of course the sample points will be at different points on the signal unless it happens to be perfectly synchronized with the sampling clock of the scope... 
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