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DSO sampling rate at slow time bases

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jpb:
This post is prompted by some discussion on another thread where the question was asked: what is your scope's sample rate at 50msec/div timebase?

The straight forward answer in my case, where my scope has 500k memory per channel, is 1MS/sec giving a nominal bandwidth of 500kHz.

But then it struck me that my scope only displays 500 points on the screen (if persistence mode is turned off) and I realised that I don't know how it selects those 500 points from the 500k it has sampled. If, as I suspect is the case, it just selects from the samples then the effective sampling rate is a 1000 times worse at only 1kS/sec with an effective bandwidth of only 500Hz! (I'm assuming that peak detect is not being used either.)

If this is the case then if you don't use persistence mode or peak detect the fact that extensive memory allows fast sampling will not protect you from seeing aliasing effects on the screen.

Given the drive for high waveforms per second rates I would have thought that only one point per horizontal pixel is used; so are the samples sampled for display?
Or do scopes average in some way every 1000 stored samples to produce one displayed sample - I suspect that they don't because it would introduce an overhead and slowdown the display rate.


EDIT : I think my post above is based on the false premise that my scope only plots 500 points because there are only 500 pixels. :-[

KedasProbe:
Maybe you first have to ask yourself what you would ideally like to see on an 0.5sec wide screen of 10 cm. (assume infinite display pixels for a moment)

cyr:
A good scope will show you all the information it has captured, that means using all points from every waveform it has captured since it last refreshed the screen. Intensity grading will show how many samples have been captured that fall on/near a particular pixel on screen.

If your scope is a "good" scope according to that definition I have no idea :)

jpb:

--- Quote from: cyr on May 15, 2013, 10:49:10 am ---A good scope will show you all the information it has captured, that means using all points from every waveform it has captured since it last refreshed the screen. Intensity grading will show how many samples have been captured that fall on/near a particular pixel on screen.

If your scope is a "good" scope according to that definition I have no idea :)

--- End quote ---

Yes, I think that you are right and my scope plots many more than 500 points (it has a gradient display).

In fact I decided I'd been slightly silly and tried to delete my original post but as I was the OP I wasn't allowed to!

KedasProbe:
My point was that your eyes can't see that high resolution on your screen.
Assume you have a screen with an extreme high resolution 2400DPI (the 'Retina display' from apple is about 8 times lower) beyond that your eyes will just see grey.

Example: 2400DPI or about 4*2400 pixels= 9600 pixels on 10cm/0.5sec
0.5sec/9600 or about 0.05ms per pixel, for a square wave you need 2 pixels: period=0.1ms or 10kHz, so still much below your sample rate.

Or with other words your eyes are the limited factor, an infinite DPI can't help, that's why you have to stretch it out and trigger on the part you want to see.

But that doesn't really answer your question "which samples do you see".
Ideally if a spike is present you want to see it even if it is much smaller than 1 pixel. (like 200 times smaller)
Knowing that 1 pixel will be much too wide to represent the spike.
So will it?

I wasn't 100% sure what my scope would do so I did the test to see if my scope would show it.
So on the 50ms/div scale or 1 pixel/ms, 500KS/s (sample every 2µs)
I provided an 5µs wide pulse every 50ms.
As you see below, the scope is showing it without problem.
(I also tried 2 short pulses right after each other, the brightness of the spike went up, 3 spikes a little bit more bright etc.)

So no you don't see all samples and no they are not averaged and you also don't miss anything important on the screen, the 500 samples for each pixel are processes and something 'appropriate' is displayed. But obviously you need to zoom in to really see it.

On the next screen-shot you see the same but I reduced the pulse width to 1µs this is below the sample rate time and you see it's starting to show wrong measurements as expected.
Edit: the interruption in the noise line is the update position of the scope.

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