EEVblog #878 - Oscilloscope High Res vs Average Modes

[Kleinstein]

The warning does not need to be a binary one. With graded display, they could superimpose a few normal and the averaged waveform, so the user could see how much variation is there.

[Someone]

The warning does not need to be a binary one. With graded display, they could superimpose a few normal and the averaged waveform, so the user could see how much variation is there.

like the color graduated display options higher end scopes use....

[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.

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

You have some filter with unknown properties which does something, but you are not sure what exactly. And there is no way to adjust it. Take for example GW Instek GDS-2000E series scope, and you can set any cutoff frequency as you want, low pass or high pass. And result is absolutely predictable.

[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.

You have some filter with unknown properties which does something, but you are not sure what exactly. And there is no way to adjust it. Take for example GW Instek GDS-2000E series scope, and you can set any cutoff frequency as you want, low pass or high pass. And result is absolutely predictable.

So you didn't mean, as you said, 'proper' you meant 'configurable or 'fully-fledged'. Assuming, from what looks to be an Austrian flag, you're not a native English speaker - the way you used 'proper' implies replacing something that is not proper, something that is pretending to be or is a substitute for something proper, perhaps even something that is 'ersatz' like wartime coffee.

[wraper]

Assuming, from what looks to be an Austrian flag, you're not a native English speaker - the way you used 'proper' implies replacing something that is not proper, something that is pretending to be or is a substitute for something proper, perhaps even something that is 'ersatz' like wartime coffee.

it's not Austria. Well, I consider high res exactly as "'ersatz' like wartime coffee". And it's not the same as digital low pass filter, it just averages nearby samples. (EDIT: I mean, it is kind of low pass filter but pretty inferior and not very predictable thing with frequency response far from perfect).

[rs20]

Take for example GW Instek GDS-2000E series scope, and you can set any cutoff frequency as you want, low pass or high pass. And result is absolutely predictable.

As a side note, this is the one feature I miss on my scope, and I wish it were more standard. Neither the Agilent Hi-res approach* ( cutoff frequency is a hard-to-figure function of record length and timebase ) nor the Rigol approach ( cutoff frequency is just a function of currently displayed timebase ) are anywhere near the utility of the fully configurable FIR filter.

* Not claiming that Agilents don't have the configuring cutoff as well...

[Cerebus]

Assuming, from what looks to be an Austrian flag, you're not a native English speaker - the way you used 'proper' implies replacing something that is not proper, something that is pretending to be or is a substitute for something proper, perhaps even something that is 'ersatz' like wartime coffee.

it's not Austria.

What flag is that then? It's really useful having some idea where people are from but I find the tiny flags on here problematic. They are too small to immediately recognise any but the most iconic and if they aren't in the top twenty or so well known flags they're damn near useless. I've heard of, say, Turkmenistan but I couldn't tell their flag from a novelty tea-towel.

Well, I consider high res exactly as "'ersatz' like wartime coffee". And it's not the same as digital low pass filter, it just averages nearby samples. (EDIT: I mean, it is kind of low pass filter but pretty inferior and not very predictable thing with frequency response far from perfect).

What is an FIR digital filter but a weighted average of nearby samples? That is exactly how (practical, non-infinite) FIR filters are implemented.

[vk6zgo]

Assuming, from what looks to be an Austrian flag, you're not a native English speaker - the way you used 'proper' implies replacing something that is not proper, something that is pretending to be or is a substitute for something proper, perhaps even something that is 'ersatz' like wartime coffee.

it's not Austria.

What flag is that then? It's really useful having some idea where people are from but I find the tiny flags on here problematic. They are too small to immediately recognise any but the most iconic and if they aren't in the top twenty or so well known flags they're damn near useless. I've heard of, say, Turkmenistan but I couldn't tell their flag from a novelty tea-towel.

Well, I consider high res exactly as "'ersatz' like wartime coffee". And it's not the same as digital low pass filter, it just averages nearby samples. (EDIT: I mean, it is kind of low pass filter but pretty inferior and not very predictable thing with frequency response far from perfect).

What is an FIR digital filter but a weighted average of nearby samples? That is exactly how (practical, non-infinite) FIR filters are implemented.

If you"hover" the mouse over the flag,a small box comes up to show you the country.
Also,re "A Native English Speaker"---not all such hail from the UK.
In Australian English,wraper's usage of "proper" is quite proper.

[Cerebus]

Assuming, from what looks to be an Austrian flag, you're not a native English speaker - the way you used 'proper' implies replacing something that is not proper, something that is pretending to be or is a substitute for something proper, perhaps even something that is 'ersatz' like wartime coffee.

it's not Austria.

What flag is that then? It's really useful having some idea where people are from but I find the tiny flags on here problematic. They are too small to immediately recognise any but the most iconic and if they aren't in the top twenty or so well known flags they're damn near useless. I've heard of, say, Turkmenistan but I couldn't tell their flag from a novelty tea-towel.

Well, I consider high res exactly as "'ersatz' like wartime coffee". And it's not the same as digital low pass filter, it just averages nearby samples. (EDIT: I mean, it is kind of low pass filter but pretty inferior and not very predictable thing with frequency response far from perfect).

What is an FIR digital filter but a weighted average of nearby samples? That is exactly how (practical, non-infinite) FIR filters are implemented.

If you"hover" the mouse over the flag,a small box comes up to show you the country.
Also,re "A Native English Speaker"---not all such hail from the UK.
In Australian English,wraper's usage of "proper" is quite proper.

That's not English, that's Strine. 

I wasn't trying to be prescriptive over English but, given how many non-native speakers we have on here, just trying to make sure we were on the same page. i.e. I didn't want to berate him just because he might have mis-spoken.

[wraper]

What is an FIR digital filter but a weighted average of nearby samples? That is exactly how (practical, non-infinite) FIR filters are implemented.

Only pretty basic and crappy ones.
http://ptolemy.eecs.berkeley.edu/eecs20/week12/freqResponseRA.html

However, if the intent was to design a lowpass filter, then we have not done very well. Some of the higher frequencies are attenuated only by a factor of about 1/10 (for the 16 point moving average) or 1/3 (for the four point moving average). We can do much better than that.

[Cerebus]

What is an FIR digital filter but a weighted average of nearby samples? That is exactly how (practical, non-infinite) FIR filters are implemented.

Only pretty basic and crappy ones.
http://ptolemy.eecs.berkeley.edu/eecs20/week12/freqResponseRA.html

However, if the intent was to design a lowpass filter, then we have not done very well. Some of the higher frequencies are attenuated only by a factor of about 1/10 (for the 16 point moving average) or 1/3 (for the four point moving average). We can do much better than that.

I think you don't understand how an FIR filter works. They are nothing but a series of delays (or samples in time) multiplied by filter coefficients for each delay and the results all summed together. So all FIR filters are, pretty much by definition, a weighted average give or take some overall gain or attenuation. Not just the good ones, not just the 'pretty basic and crappy one' but all of them. All the link shows that is it possible to create mediocre FIR filters as well as good ones, that doesn't change how they are implemented.

Look, if you're trying to say 'I don't like boxcar filters' - fine, say that and be done, your opinion will be respected. But whatever you say won't alter the facts that the boxcar filter we're discussing is a digital low pass filter, that it's normally implemented as an FIR filter, and an FIR filter is  a weighted sum of samples taken over time. Alternatively, if you've got a design for a digital low pass filter that doesn't use an FIR filter, ls better performing and can be economically implemented at 1 Gsps then please share.

[ConKbot]

Ugh, the rigol 1074z needs to work on its high-res mode. I forget the exact details, but I was probing a 1 MHz SPI bus with more than enough samples/second to avoid aliasing, but had the time base zoomed way out because of a 18 ms pause during the spi transaction. More than enough sample memory, should be just fine.  NOPE. Some clock/data bits were only a single point long, no where near the 2.5V they should be at, etc. Decode wouldnt work until I had ~3 bytes on the screen.  Other stuff, where a fast clock waveform should have just been a fat fuzzy trace as the timebase I was at, but wasnt.  Finally figured out it was on hi-res from a previous time using the scope, put it back on normal and I can zoom in on the SPI transaction from way out like I should, and it can decode the transaction once it was wide enough to display the letters. The clock trace turned into a fuzzy line like it should.

I dont know if hi-res decimates the capture down to however many pixels wide the screen is or what, but it definitely wasnt being very useful. DIdnt see any options for how much decimation was performed but wasnt looking too hard.  I'll take my tools at home over my toys at work any day   

[rs20]

I dont know if hi-res decimates the capture down to however many pixels wide the screen is or what...

Yes, this is what Rigols do, as already discussed. Being able to explicitly dictate cutoff frequency is a high-end/rare feature that is unreasonable to expect in a bottom-end Rigol. Even on Dave's agilent, you only kinda get to indirectly manipulate the "cutoff frequency" by changing the record length.

I mean, to deliberately oversimplify things, the whole point of Hi-Res mode is to get rid of the fuzz. If you're complaining that Hi-Res mode is giving you a non-fuzzy line, maybe that's a PEBKAC and you shouldn't be using High-res mode!

[ConKbot]

I dont know if hi-res decimates the capture down to however many pixels wide the screen is or what...

Yes, this is what Rigols do, as already discussed. Being able to explicitly dictate cutoff frequency is a high-end/rare feature that is unreasonable to expect in a bottom-end Rigol. Even on Dave's agilent, you only kinda get to indirectly manipulate the "cutoff frequency" by changing the record length.

I mean, to deliberately oversimplify things, the whole point of Hi-Res mode is to get rid of the fuzz. If you're complaining that Hi-Res mode is giving you a non-fuzzy line, maybe that's a PEBKAC and you shouldn't be using High-res mode!

Eh, I've done data acquisition for years, when I know the signal coming in, and see a sample rate on the screen, I have expectations. If the scope is only showing 1024 or however many samples over 20ms, it should show 51.2ks/s. I don't care what the ADC is running at, I care what is being acquired. I shouldn't have to track down what hidden/non-indicated setting is on reducing the sample rate to some unknown quantity.  Cutoff freq isn't needed but setting the decimation to 2^n or displaying the decimation  really shouldn't be difficult. 

On a spec-an I use regularly, there is 16 letters in a box, and with a glimpse I can tell if the detector is normal, max hold, min hold, power rms, and which of 4 averaging modes, and a few other settings for each of the 4 traces.  An  'N' 'P' 'A' 'H' next to the sample rate wouldn't hurt anyone

[rf-loop]

If scope have what ever "High-Res" mode and if manufacturers claim that in this mode resolution is example up to 12bit or even more. Lot of talking about Hi-Res, averaging and so on. But, I have only one question.

How we get out this 12bit data from scope?

For what is 12bit resolution in 8bit ADC oscilloscope if there is not 12bit resolution acquisition data available out from scope.  Just for "nice picture" or what.  Even old Tektronix TDS can do it, up to 16bit and this data is also available out from acquisition memory.

[rs20]

rf-loop,

How to retrieve the 12-bit data out of a scope is an extremely scope-specific question. But the question of how a scope with an 8-bit ADC can output 12-bit data is very easy to answer: take 16 8 bit samples, add them up, and you now have a 12 bit number. If you average 256 samples, the noise is reduced by 4 bits. So depending on how exactly you defined "12 bits", averaging 16 or 256 8-bit samples will get you there. Further reading: http://cp.literature.agilent.com/litweb/pdf/5991-1617EN.pdf

Note that a delta-sigma ADC is the ultimate manifestation of this principle - at its core, it's basically a 1-bit ADC, but processing vast numbers of samples gives resolutions in excess of 24 bits. Disclaimer: delta-sigma ADCs are actually more advanced that just a 1-bit ADC followed by an FIR, but the general principle stands.

[rf-loop]

rf-loop,

How to retrieve the 12-bit data out of a scope is an extremely scope-specific question.

And this I wonder why there is nearly nothing about this important thing in this thread.

But the question of how a scope with an 8-bit ADC can output 12-bit data is very easy to answer: take 16 8 bit samples, add them up, and you now have a 12 bit number. If you average 256 samples, the noise is reduced by 4 bits. So depending on how exactly you defined "12 bits", averaging 16 or 256 8-bit samples will get you there. Further reading: http://cp.literature.agilent.com/litweb/pdf/5991-1617EN.pdf

Of course but ...
16 give two extra bit for resolution. After 16 we get, yes 12 bit number but only 10bit have valuable data, there need do two times right shift because these two lowest bits do not carry valuable data. After then we have 10 bit resolution.

Perhaps bit better than Keysight papers what are maany times bit colored "truth" for marketing.
http://www.atmel.com/images/doc8003.pdf
and there 3.3


Also in attached image from Texas Instruments.
This table is also equal with Siglent ERES mode. (disadvantage (or bug) there is that final result is then truncated to 8bit in memory so it only reduce noise but do not give out for use this enchanged resolution)

[rs20]

How to retrieve the 12-bit data out of a scope is an extremely scope-specific question.

And this I wonder why there is nearly nothing about this important thing in this thread.

Because it's an extremely scope-specific question.

But the question of how a scope with an 8-bit ADC can output 12-bit data is very easy to answer: take 16 8 bit samples, add them up, and you now have a 12 bit number. If you average 256 samples, the noise is reduced by 4 bits. So depending on how exactly you defined "12 bits", averaging 16 or 256 8-bit samples will get you there. Further reading: http://cp.literature.agilent.com/litweb/pdf/5991-1617EN.pdf

Of course but excuse me, not.  This is (partially) wrong.
16 give two extra bit. After 16 we get, yes 12 bit but only 10bit have valuable data.

Only if AWGN noise dominates over quantization noise. You seem to be confusing ENOB with resolution (think accuracy vs resolution). If you take 8-bit samples of a practically noiseless signal with appropriate offsets, you can get a genuine 12-bit output with only 16 samples. Ultimate extension of this principle is an  8-bit flash ADC is just 256 1-bit ADCs after all. Fair enough, this isn't relevant to an oscilloscope ADC, but it's not "wrong"!

In any case, I clearly stated 256 samples are required for matching noise reduction. You just clearly prefer my second definition of "12 bits".

[rf-loop]

Yes now it is ok. Also as you see I modify my msg before I see this your answer. (because also I find this problem in my original answer)

[Cerebus]

If you take 8-bit samples of a practically noiseless signal with appropriate offsets, you can get a genuine 12-bit output with only 16 samples. Ultimate extension of this principle is an  8-bit flash ADC is just 256 1-bit ADCs after all. Fair enough, this isn't relevant to an oscilloscope ADC, but it's not "wrong"!

In any case, I clearly stated 256 samples are required for matching noise reduction. You just clearly prefer my second definition of "12 bits".

It's probably worth pointing out, 'for those following along at home',  that in a real-world signal with noise the noise floor is improved by multiple sampling and improves roughly as the square root of the number of samples that are averaged whereas the resolution improves with the number of samples (e.g. Your example, where 16 samples gives you 4 more bits resolution but only 2 more bits accuracy). That obviously only improves things in the presence of uncorrelated Gaussian noise (i.e. 'white' noise) not systematic or correlated noise. In the ultimate case it's possible to drag a signal out from below the noise floor if you can take enough samples and the signal is sufficiently strongly correlated from sample to sample.

[thm_w]

If scope have what ever "High-Res" mode and if manufacturers claim that in this mode resolution is example up to 12bit or even more. Lot of talking about Hi-Res, averaging and so on. But, I have only one question.

How we get out this 12bit data from scope?

For the rigol at least, I had to download the full sample memory in 8-bit and perform external FIR filtering via matlab.
To add this export option in firmware, depending on the output format, is probably not much work. But I rarely see complaints about it.

[generic_username]

Hi, I was a long time absent in the forum and while I was just searching for the high res mode on the internet I found out that dave made a vid about that 
It is a very nice vid explaining the differences between those modes but I'm still wondering why does the ampplitude drop at about 8:40 when increasing the timebase in high res mode?
cheers.