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Dithering on 8-bit scope?

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Someone:

--- Quote from: Fungus on August 20, 2023, 05:30:42 am ---
--- Quote from: Circlotron on August 20, 2023, 12:59:00 am ---My scope has both hi res and averaging modes. Naturally there is a limit to the resolution of the signal displayed on the screen because of the pixel pitch, but if the individual samples are logged and brought out externally to be examined, in hi res or average mode have they been truncated to 8 bits or do they have greater resolution?
--- End quote ---
They'll  be truncated.
--- End quote ---
Examples?

Tek/HPAK both add more bits in their high res and averaging (acquisition) modes, plainly visible when zoomed in. Tek provide an illustrative example in:
"Tools to Boost Oscilloscope Measurement Resolution to More than 11 Bits"
page 17
https://download.tek.com/document/Tools%20to%20Boost%20Oscilloscope%20%20Measurement%20Resolution%20to%20%20More%20than%2011%20Bit_48W_27802_4.pdf

Where the 8bit quantisation is directly/side-by-side compared to the higher depth averaging mode.

2N3055:

--- Quote from: johansen on August 20, 2023, 05:36:55 am ---My 6000 count dmm produces more accurate rms values of clean sine waves than does my hacked rigol 1054.

--- End quote ---

That is only because DS1000Z has particularly bad implementation of RMS from data decimated for screen..

Even Keysight MSOX3000T (that works on decimated data too, but much larger set of 32k points) does a very decent job for a 8 bitter.
Any of scopes that do measurements on full data (like Siglents and LeCroy or some new Rigols, to mention few) will easily achieve better result for RMS than average 6000 digit meter.
Not to mention they will do that for waveforms with huge crest factor (non-sinusoidal) and for BW no normal meter will have..

Berni:
There is no need to introduce a dithering waveform because scope front end and ADC combined tend to be already noisy enough due to being so high bandwidth. So the noise is a sort of free dithering waveform.

Tho it is true that not many scopes have a good implementation of a high resolution mode. Most will just run the scope as normal and then slap filtering over it in software. Most of the Keysight scopes also change the filtering as you zoom in/out in time, so i never know how downsampled of a signal i am looking at.

But when it is implemented properly it is a very nice feature. The only scope i personally used with it is a Agilent MSO9000. That scope (and all of the modern Keysight Infiniium series aka PC windows based) has a high resolution mode where you can select 9bit 10bit 11bit 12bit and this places downsampling filter directly after the ADC, so the only side effect you see is having your max available samplerate reduced. You get full memory, no extra slowdown etc... The ADC just appears like it is spitting out samples with more bits more slowly. Since these scopes have very high sample rates (this one does 20GS/s) you still get plenty of bandwidth(i think it can do 500MHz on 12bit), while the performance looks remarkably close to a real 12bit ADC scope. Because of this you can just leave it in high res mode most of the time and enjoy the sweet low noise. More scopes should do this!

This feature also goes well with FFT since the results of that are also cleaner. However don't expect magic, it will still pick up a spurious tone here and there since this is not a spectrum analyzer (the lower noise just makes the spurious tones more apparent). Also a lot of scopes have not so great FFT implementations anyway.

switchabl:
Yes, the lower-end Keysight scopes don't really allow fine-grained control over acqusition and filtering (the automatic settings are pretty well optimized though). Depending on the timebase setting, you still get up to ~3 extra bits in high resolution mode.

Normal mode (single shot at 5V/div, zoomed to 200mV/div):


High Resolution:


Filtering seems to happen in two stages (up to some limit):
- acquisition: if the memory is too small at full sample rate, average during down-sampling
- display: if there are more samples than pixels (depending on zoom level), average again

As you can see, even at 5V/div, there is no need to add extra noise. High resolution/ERES can be quite useful in some cases but it is not a replacement for native 12bit scopes. It does nothing to improve linearity and spurious tones and it costs a lot of bandwidth. As a rule of thumb, to gain 1 bit/6dB in SNR, you need to down-sample by a factor of 4.

Someone:

--- Quote from: switchabl on August 21, 2023, 04:43:41 pm ---High resolution/ERES can be quite useful in some cases but it is not a replacement for native 12bit scopes. It does nothing to improve linearity and spurious tones and it costs a lot of bandwidth. As a rule of thumb, to gain 1 bit/6dB in SNR, you need to down-sample by a factor of 4.
--- End quote ---
in some cases it won't even consume bandwidth, as the ADC sample rate could be high enough to grab a few bits for "free" (Keysight 50x oversampling that was described as "excessive" by a particular opinionated poster).

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