EEVblog #878 - Oscilloscope High Res vs Average Modes

[EEVblog]

What's the difference between High Resolution (Boxcar Averaging) and traditional Average acquisition modes on a modern digital oscilloscope?
Dave explains this often misunderstood difference, traps for young players, and gives several demos to highlight the differences.

[Ivan7enych]

Artifical "noise" generated by DDS signal generator is not a good example.

To show benefits of HiRes mode, I would better connect dynamic microphone directly to oscilloscope and try to see the sound shape buried with much RF noise. HiRes can very efficiently clean in out.

[EEVblog]

Artifical "noise" generated by DDS signal generator is not a good example.

It worked just fine.

[wraper]

Artifical "noise" generated by DDS signal generator is not a good example.

To show benefits of HiRes mode, I would better connect dynamic microphone directly to oscilloscope and try to see the sound shape buried with much RF noise. HiRes can very efficiently clean in out.

For this case proper digital low pass filter is better.

[Blaffetuur]

I've used Average and High res mode on my scope but never new exactly what they did other than cleaning up my waveform

[jolshefsky]

So ... umm ... why not leave hi-res mode on all the time? When would "normal" mode be preferable, and is it really most of the time?

[integritetus]

So ... umm ... why not leave hi-res mode on all the time? When would "normal" mode be preferable, and is it really most of the time?

High-resolution acquisition trades off scope bandwidth for an effective increase in vertical resolution.

Here is a Keysight application note, somewhat technical, that explains the tradeoff.
http://literature.cdn.keysight.com/litweb/pdf/5991-1617EN.pdf?id=2318232

Here is a useful summary from the application note:

Use acquisition averaging when:
– The maximum oscilloscope bandwidth is required
– The signal is repetitive
– Large memory depth is not required
– Control of the number of averages is desired

Use high-resolution acquisition when:
– The maximum oscilloscope bandwidth is not required, or the oscilloscope has
excess sample rate relative to its bandwidth
– The signal must be captured from a single trigger
– Deep memory to capture long time ranges is required

[GlowingGhoul]

So ... umm ... why not leave hi-res mode on all the time? When would "normal" mode be preferable, and is it really most of the time?

High-resolution acquisition trades off scope bandwidth for an effective increase in vertical resolution.

Here is a Keysight application note, somewhat technical, that explains the tradeoff.
http://literature.cdn.keysight.com/litweb/pdf/5991-1617EN.pdf?id=2318232

Here is a useful summary from the application note:

Use acquisition averaging when:
– The maximum oscilloscope bandwidth is required
– The signal is repetitive
– Large memory depth is not required
– Control of the number of averages is desired

Use high-resolution acquisition when:
– The maximum oscilloscope bandwidth is not required, or the oscilloscope has
excess sample rate relative to its bandwidth
– The signal must be captured from a single trigger
– Deep memory to capture long time ranges is required

So a certain WünderSchnitzel that's been telling us the "excess" sampling rate of Keysight scopes is useless, USELESS I TELL YOU, failed to recognize the ability to use high res mode at higher useful bandwidth levels, lol!

[Smokey]

Here is a neat trick I discovered recently.  I had a ULTRA noisy signal from a high current, high voltage PWM source (40A at 300V).  Probing anything on that circuit, you lose the signal in the noise no matter what intensity the display is set to.  With normal acquisition settings, even HighRes mode didn't help.

What I ended up doing was set the scope to HighRes mode.... and.. severely restricting the sample memory to like 16k points.  With so few points, the high res mode was able to pull the signal out of the noise to an amazing clarity.  Too high a sample memory and the HighRes averaging wasn't as effective.

[Circlotron]

In HiRes mode, what is the relationship between memory depth, scan speed, and number of boxcars that are averaged per pixel. More to the point, what is the range of number of samples/boxcars per pixel?

[Galaxyrise]

If you do the same high res tests with your Rigol DS2000, you'll get very different results. 

[rs20]

So ... umm ... why not leave hi-res mode on all the time? When would "normal" mode be preferable, and is it really most of the time?

Because more often than not, the noise is the thing you're worried about. If High-Res mode destroys the little peaks of noise that, for example, causes your comparator to produce glitch pulses, that's a bad thing! An oscilloscope is a device to display truth, not a device for displaying beautifully thin lines with no respect for the truth. Leaving your scope in Hi-Res mode by default leaves you truly blind to a big part of the picture.

High-resolution acquisition trades off scope bandwidth for an effective increase in vertical resolution.

True, although keep in mind that the bandwidth that is lost does not manifest itself as the visible shape of the curve on the screen, but in the fuzziness of the curve itself.

What I ended up doing was set the scope to HighRes mode.... and.. severely restricting the sample memory to like 16k points.  With so few points, the high res mode was able to pull the signal out of the noise to an amazing clarity.  Too high a sample memory and the HighRes averaging wasn't as effective.

What kind of scope do you have? My Rigol has completely different behaviour: the ADC runs at the sampling rate as determined by the sample memory (so, exactly like normal), but at display time all the points that belong to the same column on the screen are averaged together. So if I do a single-shot capture, it looks noise-free, but as I horizontally zoom in (without doing any further captures, just zooming into the original waveform), the averaging is changed to the new size of the pixels, and more noise is revealed. I think this is rather nice, easy-to-use behaviour, because it's very explicitly de-fuzzing the signal in the way that is most appropriate to how you're currently looking at it.

In HiRes mode, what is the relationship between memory depth, scan speed, and number of boxcars that are averaged per pixel. More to the point, what is the range of number of samples/boxcars per pixel?

This is easy to figure out by playing with your scope, and you'd best do that because different scopes behave very differently (see the different between Smokey's scope and my Rigol above). Connect to the 1kHz square wave test signal on your scope, set your timebase to about 1 second per division, and mess with different memory depths, hi-res vs normal mode, and changing the horizontal timebase (both with live captures, and on stored single shot captures). You should be able to make a 5V 1kHz square wave turn in 2.5V DC, but exactly how you achieve that will depend on the scope (on Smokey's scope, changing the memory depth; on my Rigol, simply changing the timebase).

( Also, pedantically, the boxcar is the shape of the convoluting impulse. So you mean the number of ADC samples per boxcar, not the number of boxcars per pixel. )

[Dave Turner]

Hi all, I've been offline for a while and am trying to catch up.

Regarding Dave's video #878 I tried to replicate his observations on my DS1074Z-S with limited success. I really wanted to see the 'false' results as I had an idea that the 'wrong' display might be useful as a diagnostic tool if one properly understood the resulting waveform.

I used a 1MHz sinusoid with a 10Hz FM modulation, which I think is what Dave used, but couldn't get the same result that Dave did. Either my set-up is wrong or my scope does not use the same algorithms that the scope Dave used did.

Dave, you probably have more scopes immediately available to you than than most of your subscribers. Assuming that I've not screwed up my understanding somewhere perhaps it would be worthwhile seeing how your other scopes respond to the same input.

Note that I'm not contesting your warning but I am wondering how other scopes may show different 'erroneous' readings.

ciao

Dave

[Kleinstein]

The false waveform depends on the way the signal is triggered. Triggering such a signal is difficult and different scopes can fail in a different way to trigger it good.

It can also make a difference if the signal is generated from the scope itself as there the internal clocks are likely locked to each other. A separate generator is more likely to have some real jitter on top and also the time to the next trigger can be different. So I would not wonder to see a different picture.

[Cerebus]

Artifical "noise" generated by DDS signal generator is not a good example.

To show benefits of HiRes mode, I would better connect dynamic microphone directly to oscilloscope and try to see the sound shape buried with much RF noise. HiRes can very efficiently clean in out.

For this case proper digital low pass filter is better.

Erm, most 'high res' modes are exactly that, an FIR digital low pass filter.

[Smokey]

What I ended up doing was set the scope to HighRes mode.... and.. severely restricting the sample memory to like 16k points.  With so few points, the high res mode was able to pull the signal out of the noise to an amazing clarity.  Too high a sample memory and the HighRes averaging wasn't as effective.

What kind of scope do you have?

This actually was with a Rigol DS1054Z.  Next time I have more of those PWM units going through I'll take some screen shots of before and after memory restriction in highRes mode. 

[vk6zgo]

Hi all, I've been offline for a while and am trying to catch up.

Regarding Dave's video #878 I tried to replicate his observations on my DS1074Z-S with limited success. I really wanted to see the 'false' results as I had an idea that the 'wrong' display might be useful as a diagnostic tool if one properly understood the resulting waveform.

I used a 1MHz sinusoid with a 10Hz FM modulation, which I think is what Dave used, but couldn't get the same result that Dave did. Either my set-up is wrong or my scope does not use the same algorithms that the scope Dave used did.

Dave, you probably have more scopes immediately available to you than than most of your subscribers. Assuming that I've not screwed up my understanding somewhere perhaps it would be worthwhile seeing how your other scopes respond to the same input.

Note that I'm not contesting your warning but I am wondering how other scopes may show different 'erroneous' readings.

ciao

Dave

I was thinking along those lines,too,Dave----if the crossovers,say, were uniquely spaced for a particular frequency deviation,it would be very useful.
I don't have a DSO,& am unlikely to get one,so couldn't check it myself.

[EEVblog]

Regarding Dave's video #878 I tried to replicate his observations on my DS1074Z-S with limited success. I really wanted to see the 'false' results as I had an idea that the 'wrong' display might be useful as a diagnostic tool if one properly understood the resulting waveform.
I used a 1MHz sinusoid with a 10Hz FM modulation, which I think is what Dave used, but couldn't get the same result that Dave did. Either my set-up is wrong or my scope does not use the same algorithms that the scope Dave used did.

Here you go.
Result depends upon sample rate etc.

[AF6LJ]

This was a good video, one more complication I am missing with my Tek analogue scopes.
Good Stuff Dave   

[rsjsouza]

Good video, Dave. Coincidentally this weekend I was playing with modulation on my func gens and today I captured this short clip on my Rigol DS4014. 

[alank2]

Would it be fair to say that:

Average mode = averaging multiple waveform displays together (requires a steady properly triggered display or it goes badly)

High resolution mode = averaging multiple samples from the DAC into a single sample that will be used to make one waveform.

I like both modes, they are both very useful...

[Wuerstchenhund]

What's the difference between High Resolution (Boxcar Averaging) and traditional Average acquisition modes on a modern digital oscilloscope?
Dave explains this often misunderstood difference, traps for young players, and gives several demos to highlight the differences.

Nicely done, as always!    And I do agree, it's annoying when the current sample mode is not shown on the screen but hidden in the settings.

The only thing that I felt was missing was ERES (or maybe I just missed the part), especially now when Siglent uses it (or at least claims that they do). ERES works slightly different, and there are various advantages and disadvantages of each mode.

So a certain WünderSchnitzel that's been telling us the "excess" sampling rate of Keysight scopes is useless,

The only occasions where I said something like that was when comparing scopes, and what I was said was that Keysight can't turn it's slightly higher sample rate into an advantage.

Don't worry, I'm not expecting you to understand the context.

[NiHaoMike]

Why couldn't they check the difference between captures and display a warning if it's unusually large?

[rs20]

Why couldn't they check the difference between captures and display a warning if it's unusually large?

Because depending on where they set the threshold on that heuristic, you'll either end up with a warning that appears all the time and therefore gets ignored, or you'll end up with a crutch that people would assume to be infallible, while giving false negatives. Or worse, the heuristic would end up doing both of the above things.

It'd be like the "Auto" button all over again.

[nctnico]

Why couldn't they check the difference between captures and display a warning if it's unusually large?

That would be like a hammer with a finger avoidance system