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What's the scope background mean?



I'm looking at a really noisy signal on my yellow channel. Annoyingly noisy as the chip's analog power has an LC filter so it's SUPPOSED to be quiet, but that's not your problem.

I notice the background is colored a faded yellow kindof following the trace.

What does the background mean? I figure you wouldn't have drawn it dark yellow if it didn't mean something. Perhaps, "this signal is horribly noisy!" It is that.

(It's also like a 10th the amplitude I was expecting.)


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By default the scope channels are acquiring averaged samples of N ADC conversion, N = 100MHz ADC rate / sample rate
The sample rate is adjusted with time base, sample rate = 8000 (default device buffer size) / time base

Beside these samples a lower number and resolution Noise samples are also stored and shown as semitransparent band.
This, to indicate glitch or noise of higher frequency than the sampling rate.

To see more details on this noise band reduce the time base to have higher resolution acquisition.

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Attila: I also noticed the semitransparent band around the oscilloscope trace, and looked in the forums to learn more about it. I found three explanations (all from you - thanks), and the one above is the most extensive. I think I almost understand it, but there are aspects of your explanation that still escape me. (Sorry -- I am sure it is my ignorance.) Could you please elaborate on your explanation above? The first sentence describing the default mode eludes me. 

You also write "a lower number and resolution Noise samples are also stored" -- How are those acquired? I don't think I'm clear on this, either.

Thank you!

Edited by RCG
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Such details probably are not too important for using the scope. It would be suffice to know that the band indicates noise and reducing the time base makes these components visible. On the other hand it is good to understand the background mechanisms of the equipment for proper interpretation.

The scope 14bit ADC is always running at 100MHz and it has a limited buffer for storing samples, by default 8k. At lower time base settings lower frequency samples are stored. These  samples are by default averaged from multiple ADC conversions, 100MHz / sample rate.
In the earlier screenshot at 1ms/div the scope stores samples at 800kHz. Which are averaged from 125 ADC conversion (100MHz/800kHz).

Beside these samples the minimum and maximum conversion values are also stored in a smaller buffer, by default 1k. These are shown as min/max band to indicate higher frequency components that could be hidden by the earlier sample averaging.
For 1ms/div setting the min/max for 100kHz period, for 1000 ADC conversion are stored and shown as "noise band".

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