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| REVIEW - Rigol DS2072 - First Impressions of the DS2000 series from Rigol |
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| EV:
Thanks JDubU! This sounds good explanation. --- Quote from: JDubU on December 27, 2013, 09:36:02 pm ---There seems to be a problem with the sample-to-display interpolation algorithm (sin(x)/x) when the horizontal time base is faster than about 20ns/div. The dots display looks reasonable but the vector display introduces interpolation artifacts when the distance between sample points spans a larger number of pixels on the display. This can be seen more easily in single sweep mode. The dots reliably form a smooth curve but the vectors tend to "zig zag" between adjacent sample points with multiple 2 or 3 pixel steps in opposite directions as the trace moves from one sample point to the next. On a free running sweep, these display artifacts appear as a noisier trace. This also explains why enabling CH2 increases the apparent noise on CH1. The sampling rate is cut in half causing the distance between sample points to double (producing more display artifacts) for a given time base setting. --- End quote --- |
| marmad:
--- Quote from: JDubU on December 27, 2013, 09:36:02 pm ---There seems to be a problem with the sample-to-display interpolation algorithm (sin(x)/x) when the horizontal time base is faster than about 20ns/div. --- End quote --- I haven't noticed this. Can you post some documentation to illustrate? |
| marmad:
--- Quote from: JDubU on December 27, 2013, 10:44:47 pm ---Note that the steps are multiple display pixels high.... --- End quote --- By multiple, do you mean a multiple of 2? The Rigol maps 200 possible ADC values to a 400 pixel high display - steps are ALWAYS a minimum of 2 pixels - it doesn't matter what time base or interpolation method you use. --- Quote ---...with several examples of inappropriate direction reversals between sample points. --- End quote --- What you're talking about here is noise fluctuation between 3-4 quantization levels (which is 1-2 bits); normal on digital oscilloscopes. This has nothing to do with sin(x)/x - and can be found at every time base if you STOP the DSO and 'zoom' in on the trace with vertical scale. |
| JDubU:
--- Quote from: marmad on December 27, 2013, 11:17:29 pm --- --- Quote from: JDubU on December 27, 2013, 10:44:47 pm ---Note that the steps are multiple display pixels high.... --- End quote --- By multiple, do you mean a multiple of 2? The Rigol maps 200 possible ADC values to a 400 pixel high display - steps are ALWAYS a minimum of 2 pixels - it doesn't matter what time base or interpolation method you use. --- Quote ---...with several examples of inappropriate direction reversals between sample points. --- End quote --- What you're talking about here is noise fluctuation between 3-4 quantization levels (which is 1-2 bits); normal on digital oscilloscopes. This has nothing to do with sin(x)/x - and can be found at every time base if you STOP the DSO and 'zoom' in on the trace with vertical scale. --- End quote --- 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. |
| marmad:
--- 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. |
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