How important is Vertical Resolution on Rigol and other scopes?

[Wim_L]

The question is what 'scopes use the raw data for cursors and how can one tell the difference? Is it normally given in the specification?

It doesn't make sense, as the cursors are moved by the scope user, who only can see their position on the pixel grid on the screen.

Now, for effective resolution of an analog scope, I have a Hameg combiscope (analog scope plus digital storage mode). In analog mode, speaking of a display resolution doesn't make much sense. However, cursor resolutions are based on a 2000 horizontal by 1000 vertical grid. So the manufacturer does consider that a useful resolution, even though in digital mode, the 8-bit converters limit vertical waveform resolution to 256.

[alm]

The question is what 'scopes use the raw data for cursors and how can one tell the difference? Is it normally given in the specification?

It doesn't make sense, as the cursors are moved by the scope user, who only can see their position on the pixel grid on the screen.

But many scopes do indicate the intercept of the current cursor with the signal (eg. voltage of the signal at the current time cursor), which could use the same resolution as the raw signal. Same for automated measurements.

[Mechatrommer]

i think it should be like this... whats digitized (ie saved in memory) thats supposed to be displayed on proper pixel location (or double pixelated if necessary depending on screen size). when the cursor moved, it should be reading that pixel location and hence the same value in the memory. otherwise, the manufacturer should find another business. even so there is discrepancy between screen pixel position and memory value, it should be somewhere just between ±1 pixel. so far i cannot tell the difference in any reading i've done, it doesnt matter, there's alot of noise there if you really peek into a single particular pixel. and as someone said, dso got more error % in reading accuracy, then i will simply admit that with salt and grain.

[Zero999]

I don't even see why a low X resolution should cripple the measurement capability. Suppose the 'scope is set to 1ms per screen and the X resolution is 500 which is 2us per pixel but the memory buffer holds a total of 2000 samples, one every 500ns. All that needs to be done is have a fine tune mode which causes the x measurement to be incremented sample by sample, even if the cursor is only moved every four samples.

[seattle]

The question is what 'scopes use the raw data for cursors and how can one tell the difference? Is it normally given in the specification?

The raw data is all you have.

Say a scope is at 2V/div, and there are 10 gridlines. In order to not the show the user a clipped signal, you'd want the input range to be +20V to -20V (not including any offset voltage the scope might be able to apply). That means 40V covered by an 8-bit ADC. That gives you 156 mV resolution.

On a good scope such as Rigol and beyond, you'd have a VGA after the input and as you reduced the input sensitivity, you'd increase the gain, so again your entire 8 bit could be used to digitize a +/- 10V signal, which would give 20/256=78mV resolution.

As you move down to 5mV/div, you'd again adjust the gain so that 8 bits was digitizing +/-50mV or 100/256 = 390uV resolution.

And of course, the analog offset voltage allows you still get that full 8 bit resolution on a signal riding atop a large DC component. For example, if I have a 30V DC signal, I can offset that with -30V and then set to 5mV/div and get 390uV resolution. Or switch to AC coupling.

Now, on a cheap scope, like a Link Instruments USB scope (MSO19), there isn't a variable gain amp after the input. So, as you get to lower voltage levels you can actually see the ADC quantiziation. But these are fortunately rare.

Thus, given a proper scope with a VGA following the input stage, you can practically measure just about any resolution signal you might want to know with great accuracy, regardless of scope ADC.

THE PRIMARY CASE where the an 8-bit ADC might bite you is if you are looking at noise and using your scopes FFT function.

An 8-bit scope will BEST CASE give you a noise floor of around -56 dB full scale at the highest sample rate. A 10-bit converter will do about 12 dB better. A 12-bit converter will be about 24 dB better than an 8 bit scope.

Note that fast sample rates can be traded off for resolution. A 10-bit scope running at 100MSPS that is sampling a signal at an effective 100KSPS can pick up almost 6 more bits of effective resolution, meaning a 10-bit converter can look (almost) like a 16-bit converter at the lower sample rates. 

[chscholz]

In my world cursors are estimates and measurements are, well measurements.

Here is an admittedly extreme example of what happens when not using the full voltage swing. We are measuring risetime, falltime, width and period of a 1-0 pattern with 6.4 Vpp, once using 5 V/div the other time using 0.65 V/div.
Measurement uncertainty (as measured by standard deviation over 1,000 edges) for the risetime at 5 V/div is 41 ps, for 5 V/div it is 214 ps, etc.




Chris

The question is what 'scopes use the raw data for cursors and how can one tell the difference? Is it normally given in the specification?

The raw data is all you have.

Say a scope is at 2V/div, and there are 10 gridlines. In order to not the show the user a clipped signal, you'd want the input range to be +20V to -20V (not including any offset voltage the scope might be able to apply). That means 40V covered by an 8-bit ADC. That gives you 156 mV resolution.

[...]

[vk6zgo]

Of course,there is seldom any reason not to use as large a display height as you can with a non-CRT display.
With Analog CRT 'scopes,you can get some deflection distortion at the extremes of vertical display size,so for best accuracy,the full range of the screen is not usually used.

The error from trying to read rise time,etc with insufficient display height happens with analog too,but here it is due to
the fact that we humans have a hard time discerning the 10% to 90% interval on the waveform edge,with a very small displayed height on the screen.

VK6ZGO

[Mechatrommer]

@chscholz: thats normal, thats the world of digital (binary) quantisation, there is only integers, not real numbers, in this case of screen pixel mapping and limited bits domain. if you can amplify the pixels into large visible blocks and analyze the x,y distance and dx/dy, then you'll know why using max swing will give you better result. for properly design scope esp the analog front end, full swing will translate to max resolution.

[Wim_L]

Of course,there is seldom any reason not to use as large a display height as you can with a non-CRT display.
With Analog CRT 'scopes,you can get some deflection distortion at the extremes of vertical display size,so for best accuracy,the full range of the screen is not usually used.

The error from trying to read rise time,etc with insufficient display height happens with analog too,but here it is due to
the fact that we humans have a hard time discerning the 10% to 90% interval on the waveform edge,with a very small displayed height on the screen.

VK6ZGO

To help with this, many analog oscilloscopes have markers etched on the inside of the tube to align the waveform on for these measurements, so you don't really need to guess that much. Some even have double cursors designed specifically for it (horizontal cursors that align to the starting and ending voltages, followed by crosshairs at the 10 and 90% positions relative to the horizontal cursors that you can then place on the waveform).