Scope Wars

[tom66]

To prove that modern DSOs do in fact alias on long timebases:

Attached images show Rigol DS1074Z aliasing with 50MHz input signal on long timebase.

The "Anti-Aliasing" setting does not prevent aliasing. (Not entirely sure what it does other than slow down the scope and make the render look prettier: they probably should have called it "high-quality" or something similar.) Hi-res mode also shows an aliased signal: whatever hi-res averaging Rigol is using appears to be post-decimation.  Peak detect does prevent aliasing.

[SilverSolder]

Lessons for me in this thread so far:

1) A scope can plot more information on its screen than a pure analog signal (for example, by creating histograms of points for each X value). Paradoxically, an analog scope can do this too!  Example:  100Hz modulation of 20MHz carrier, 50ms sweep.

2) The FFT function in the typical digital scope is limited by the need to display the time domain signal at the same time.  Sometimes it is not possible to get a good compromise between the two and in some cases it doesn't work at all (example:  100Hz modulation of 20MHz carrier, 50ms sweep).

3) Peak detect appears to not really have an analog scope equivalent - a very short blip during a slow sweep will be displayed on the analog scope, but is likely hard to see without cranking up brightness - whereas a digital scope will plot a "fully formed pixel" that is as easy to see as any other.   That pixel's width is going to be out of proportion to the actual signal and therefore not really a theoretically accurate representation of the duration of the signal.  That doesn't make it any less useful, of course.  Perhaps better to think of it as a marker than a part of the signal...

[bdunham7]

It's folded FIR filtering, a specific technique of removing the number of taps / implementation complexity in exchange for adders.. this might help you

https://www.embedded.com/dsp-tricks-an-odd-way-to-build-a-simplified-fir-filter-structure/

Thanks!  So the 'folding' just takes advantage of the fact that the multiplier coefficient for some of the taps is the same in some cases.  I don't see how the OP's LP filter plan differs in function from ERES:

http://cdn.teledynelecroy.com/files/appnotes/an_006a.pdf

Actually see no reason to think that manufacturers aren't already using a folded version of the filter.  I hadn't heard of 'folding' in this context because I'm wholly ignorant of the inner details of  DSP, but I'm sure the engineers at LeCroy have heard of it. 

[tv84]

Perhaps better to think of it as a marker than a part of the signal...

All good  except this last phrase. If it appears it's really part of the signal.

[tv84]

An oscilloscope is there to faithfully represent the shape of a signal.

  But doesn't this phrase contradict your tagline? ? 

BTW, I would add "...to try to faithfully.."

[SilverSolder]

Perhaps better to think of it as a marker than a part of the signal...

All good  except this last phrase. If it appears it's really part of the signal.

For example:  you have a screen that is 1,000 pixels wide, and a sweep speed of 1 second, and you are using Points mode just to avoid the complication of talking about vectors in this example.  This setting means each pixel width is "worth" one millisecond - each pixel always represents exactly 1ms of data.

Now turn on Peak Detect. Say that a 5ns pulse is detected.  The lit pixel is still 1ms wide, which makes it look like the pulse was 1ms, which is really not correct... it is a big error!  That's why I like thinking of the peak detect as a marker...   there could be a pulse behind it anywhere from 5ns (or even less, depending on your scope) to 1ms, we can't know what it actually is until we look closer at a faster sweep.

A peak detected waveform could show incorrect results in the FFT view because of this error, not sure how scope manufacturers deal with that...

[Andie]

I started this as a comparison of DSOs, not a debate about what people *think* a scope should do.  My frame of reference is a good analog scope.  So I am comparing DSOs to good analog scope signals.

The purpose of a DSO is to faithfully represent an *analog* signal.  That requires low pass filtering to prevent aliasing.

[...]

Can you show us an example of one of those scopes showing aliasing on the screen due to decimation of a signal that was within Nyquist at the native sample rate but is now aliased because of decimation?  Is that happening?  If so, I'll grant that to be an undesirable result.

Sorry for the interruption and although USB-oscilloscopes are probably off topic, but at this point I have to vent my disappointment about Agilent's U2702A (200MHz, 1GS/s, 32Mpts) in conjunction with Agilent Measurement Manager 2.2.4.0. This USB-oscilloscope can store a waveform from a single-shot-acquisition in its internal memory, then the software on the computer retrieves some amount of this data to display the waveform on the screen. You can zoom in and out, but unfortunately the decimation involved in this is done wrong, so aliasing occurs. The data is there (in the device), but the presentation is a failure. Unnecessarily. PicoScope handles this better.

The pictures show the same acquisition in different time/div settings (200µs/div and 500ns/div).

[nctnico]

An oscilloscope is there to faithfully represent the shape of a signal.

  But doesn't this phrase contradict your tagline? ? 

BTW, I would add "...to try to faithfully.."

Well... after reading the dictionary I think you could have a very long semantic discussion on this. 

[bdunham7]

To prove that modern DSOs do in fact alias on long timebases:

Attached images show Rigol DS1074Z aliasing with 50MHz input signal on long timebase.

The "Anti-Aliasing" setting does not prevent aliasing. (Not entirely sure what it does other than slow down the scope and make the render look prettier: they probably should have called it "high-quality" or something similar.) Hi-res mode also shows an aliased signal: whatever hi-res averaging Rigol is using appears to be post-decimation.  Peak detect does prevent aliasing.

OK, if you use the memory limitation to force the scope to reduce its sampling rate you should expect aliasing.  The fact that the scope reduces its sampling rate by decimation rather then slowing the ADC clock is just the way it works--the results wouldn't change except maybe to the extent that a slower ADC might have a longer sample window. 

The OP has proposed:

Data for time domain display is  downsampled to sample rate appropriate to timebase setting using a folded LP filter

which I had interpreted as meaning that the LP/downsampling operation would be applied between the memory and the screen (post acquisition) not between the ADC and memory.  However, if the folded filter can be applied in real-time, what would that look like?

Which of the three options posted here is the most 'correct' response--the one you want to see--to a signal at twice Nyquist of the posted sample rate?
If you apply the appropriate LP filter for a 20mS/div display to a 50MHz signal, what is the appropriate result?  A flat line?

In this case, an LP filter might be the appropriate default.  Aliasing has to be prevented by limiting BW before you impose the Nyquist limit by sampling or downsampling as the case may be, so if it can be done in real-time, great--especially if the user can have explicit control over the filter parameters.  However, I think it will add some complexity that will be hard to justify in an entry level product. The FFT will need its own data channel and memory.

 I cannot see how this can be claimed to eliminate the need for peak detection. I also don't have any high-end DSOs so I have no idea if any of this has already been implemented somewhere.  Anyone?

[tomato]

An oscilloscope is there to faithfully represent the shape of a signal.

  But doesn't this phrase contradict your tagline? ? 

BTW, I would add "...to try to faithfully.."

I think the correct phrase is "... to endeavor to try to faithfully represent ..."

I prefer the more formal "... to attempt to endeavor to try to faithfully represent ..."

[tom66]

OK, if you use the memory limitation to force the scope to reduce its sampling rate you should expect aliasing.  The fact that the scope reduces its sampling rate by decimation rather then slowing the ADC clock is just the way it works--the results wouldn't change except maybe to the extent that a slower ADC might have a longer sample window.

Decimation and slowing the ADC clock are functionally equivalent.  It's probably doing a combination of both as the HMCAD1511 ADC inside the Rigol only supports sample rates down to 120MSa/s or so.  In fact given the peak detect doesn't seem to fall foul of aliasing it may well just be doing the decimation on the FPGA - that's not entirely clear to me though.

The correct response would be to apply a filter once the decimation becomes necessary, but this is computationally expensive.  The Spartan-6 used in the Rigol DS1000Z only has ~90 DSP cores, which limits it to ~90 tap filter.  There may be a "cheaper" way to do it, but it is a limitation of hardware here, I believe.  I'm not sure how complex of a filter you would need or if there is a way to implement one with an IIR or moving average.

[tv84]

Now turn on Peak Detect. Say that a 5ns pulse is detected.  The lit pixel is still 1ms wide, which makes it look like the pulse was 1ms, which is really not correct... it is a big error!  That's why I like thinking of the peak detect as a marker...   there could be a pulse behind it anywhere from 5ns (or even less, depending on your scope) to 1ms, we can't know what it actually is until we look closer at a faster sweep.

Understand your thinking. Just remember, we are in peak mode so it's "more correct" (IMO) to display that 1 pixel than not displaying it at all. If you zoom out you'll still be in peak mode and it will be "even more correct".

And, if it appears (by any motive) we can be sure that there was a sample (as shorter as it could be) that had that value.

[tv84]

Well... after reading the dictionary I think you could have a very long semantic discussion on this. 

  No semantics here. I think it's a perfect image of what this thread has been showing...

Another form: 

"An oscilloscope is there to represent the shape of a signal as faithfully as the owner expects and/or is willing to pay for."

[SilverSolder]

On the subject of peak detection... I got interested in whether the scope treats peak detected pulses as "real" for the purposes of math operations probably including FFT.

It appears that at least in the case of the 54622D, that the peak detected pulses are NOT considered "bona fide signal material" for math operations.

Example:  Channel A is a 100Hz sine.   Channel B is a train of 5ns pulses coming in at 200Hz, which are not visible unless Peak Detect is turned on.  The middle, unmarked channel is the Math channel, displaying A-B (scaled down to fit the display).  As you can see, there are no signs of the peak detected pulses in A-B, meaning that Agilent appear to be using the peak detection only as a marker, not as a real signal. 

[bdunham7]

Decimation and slowing the ADC clock are functionally equivalent.  It's probably doing a combination of both as the HMCAD1511 ADC inside the Rigol only supports sample rates down to 120MSa/s or so.  In fact given the peak detect doesn't seem to fall foul of aliasing it may well just be doing the decimation on the FPGA - that's not entirely clear to me though.

Agreed as to equivalence as long as the sample window is the same in each.  In this case it seems pretty clear that it can implement the peak detect in real time, so I would assume it does it at 1GSa/S--otherwise it might not work at other speeds.  There would be no reason to slow it down.

The correct response would be to apply a filter once the decimation becomes necessary, but this is computationally expensive.  The Spartan-6 used in the Rigol DS1000Z only has ~90 DSP cores, which limits it to ~90 tap filter.  There may be a "cheaper" way to do it, but it is a limitation of hardware here, I believe.  I'm not sure how complex of a filter you would need or if there is a way to implement one with an IIR or moving average.

Now that I know about folded DSP filters, I would think that the simplest, fastest way would be to use the same number of taps as the downsampling ratio all added together into one multiplier with a coefficient of 1/downsampling ratio.  IOW, just an average over the samples.  I'm guessing this is already widely done, folding and all.

[bdunham7]

Now turn on Peak Detect. Say that a 5ns pulse is detected.  The lit pixel is still 1ms wide, which makes it look like the pulse was 1ms, which is really not correct... it is a big error! 

A peak detected waveform could show incorrect results in the FFT view because of this error, not sure how scope manufacturers deal with that...

The screen can't possibly show you everything all at once--something has to be limited, whether it is resolution vs time or resolution vs amplitude.  When you select peak detect, you are specifying  that its priority is to show you the maximum and/or minimum amplitude of any signal it can detect, to the detriment of all other factors.  It's not an error, it is an unavoidable feature.

Peak detect and FFT would seem wholly incompatible, or at least not accurate.

[SilverSolder]

Now turn on Peak Detect. Say that a 5ns pulse is detected.  The lit pixel is still 1ms wide, which makes it look like the pulse was 1ms, which is really not correct... it is a big error! 

A peak detected waveform could show incorrect results in the FFT view because of this error, not sure how scope manufacturers deal with that...

The screen can't possibly show you everything all at once--something has to be limited, whether it is resolution vs time or resolution vs amplitude.  When you select peak detect, you are specifying  that its priority is to show you the maximum and/or minimum amplitude of any signal it can detect, to the detriment of all other factors.  It's not an error, it is an unavoidable feature.

Peak detect and FFT would seem wholly incompatible, or at least not accurate.

Wonder if any scope manufacturers have fallen into that hole?

[Jay_Diddy_B]

Hi,

I have scanned this (long) thread from beginning to end.

The impression that I get is that we are ridiculing  comparing some 100MHz (class) scopes based only on their ability to perform FFTs without making (rookie) mistakes.

or we are expecting the FFT of a broadband source, impulse function, to reveal the filter shapes.
Given that a lot of these scopes have SW options to increase the BW to a maximum, that alone would imply the BW is determined by SW.



I tested my Tektronix MDO4104 ($30k) with an HP 8406A comb generator. The scope was behaving very nicely and not show FFT information above 1/2 the sampling frequency. The BW limiters worked as expected.

Did I miss something?

Regards,
Jay_Diddy_B

[2N3055]

This is 50 MHz AM modulated with 100 Hz. It has very well functioning antialiasing filter.



But, if you enable FFT, it starts aliasing.



That means that it is mindful of the fact that FFT needs clean signal sampled under certain rules, so it disables mumbo jumbo and feeds FFT clean data. Antialiased "envelope display mode" is gone.

Also, on cursory glance, it doesn't seem Peak detect is fed to FFT. Averaging and Hires are, but for Peak detect and Normal FFT look pretty much the same.

So scope manufacturers know a bit about these things.

[2N3055]

Hi,

I have scanned this (long) thread from beginning to end.

The impression that I get is that we are ridiculing  comparing some 100MHz (class) scopes based only on their ability to perform FFTs without making (rookie) mistakes.

or we are expecting the FFT of a broadband source, impulse function, to reveal the filter shapes.
Given that a lot of these scopes have SW options to increase the BW to a maximum, that alone would imply the BW is determined by SW.



I tested my Tektronix MDO4104 ($30k) with an HP 8406A comb generator. The scope was behaving very nicely and not show FFT information above 1/2 the sampling frequency. The BW limiters worked as expected.

Did I miss something?

Regards,
Jay_Diddy_B

No, pretty much you're right.

[rhb]

Here are attempts at an eye diagram on the DS1102E, DS1202Z-E, MSO-2204EA and ZDS-2102A.  Source is Keysight 33622A at 20 MHz 200 mVpp.

The Owon doesn't have an option to trigger on a rising or falling edge. So it's just here to be complete.   The DS1202Z-E does, but it doesn't work.

Have Fun!
Reg

[SilverSolder]

This is 50 MHz AM modulated with 100 Hz. It has very well functioning antialiasing filter.

(Attachment Link)

But, if you enable FFT, it starts aliasing.

(Attachment Link)

That means that it is mindful of the fact that FFT needs clean signal sampled under certain rules, so it disables mumbo jumbo and feeds FFT clean data. Antialiased "envelope display mode" is gone.

Also, on cursory glance, it doesn't seem Peak detect is fed to FFT. Averaging and Hires are, but for Peak detect and Normal FFT look pretty much the same.

So scope manufacturers know a bit about these things.

But the FFT is not showing anything at either 50MHz or 100Hz?

[tomato]

I have scanned this (long) thread from beginning to end.

The impression that I get is that we are ridiculing  comparing some 100MHz (class) scopes based only on their ability to perform FFTs without making (rookie) mistakes.
Did I miss something?

No, you didn't miss anything.  (Cheap) oscilloscopes make poor dynamic signal analyzers.  Shocking.

[rhb]

Here is the DS1202Z-E and the MSO-2204EA being fed a 200 MHz and 300 MHz signal from an HP 8648C.

It's as good a demonstration of aliasing as you can get.  I've got the scopes in dot mode with infinite persistence. Both scopes have all channels enabled to force 500 MSa/s so Nyquist is at 250 MHz.

I didn't think the Rigol was turning the anti-alias filter on and off when I was looking at the step response.  The two photos of the Rigol screen prove that it does not work. I reset the persistence between photos and rest the frequency.

So, do you want a scope that shows you the truth or one that lies? These both lie.  They indicate that the 300 MHz signal is 200 MHz.

Have Fun!
Reg

[Bud]

Given that a lot of these scopes have SW options to increase the BW to a maximum, that alone would imply the BW is determined by SW.

No, the BW in these scopes is determined by the band limiting amplifier IC in front of the ADC. All software does is sends commands to switch the band limits. Therefore in order to figure out a scope's front end filter shape  all one needs to do is refer to that IC's datasheet.