Scope Wars

[2N3055]

State an SA performance specification that you believe cannot be met by time domain acquisition: center frequency, span, dynamic range, RBW and sweep time.

Many modern SAs obviously do time domain acquisition (particularly real-time ones). Frequently they are still hybrids, not sampling the RF directly, but sampling the IF band after analog down-mixing, and calulating the whole IF spectrum at once via FFT which is faster than a narrow-bandwidth sweep.

RT SA are mostly hybrid design that direct convert (have real time bandwidth of) 20-100 MHz bandwidth .  In attachment SH BB60C manual with operating principle explained.

[nctnico]

I wanted to add few more observation, looking at discussion here.

But if we go with option  3. (downsampling by filtering), as suggested, we will filter out anything above 10 MHz..
On spectrum analyser, we would be looking at scale of 0-10 MHz and see nice clean spectra of that without any folding from upper bandwidth. Perfect.

What we will see on the screen of oscilloscope ?

It's all wrong. Look at your samplerates and then think about how that aliases with your 50MHz carrier. If you do these kind of tests you need to use an 'odd' frequency like 33MHz. You'll see you can only get a good picture with lower samplerates when you turn peak-detect on.

As mentioned before: if you low-pass filter a signal you'll lose the high frequency content. Think about how a basic AM demodulator works; that is not a low-pass filter but an envelope detector. Do the same test with 20MHz bandwidth on and off and you'll see.



This is an AM modulated signal using an 8.33MHz carrier and 100Hz modulation. Peak detect is on. Trace 1 (top) is unfiltered, trace 3 (bottom) is filtered using a 600kHz low-pass filter. I hope that this makes it very clear that anti-aliasing filtering based on the samplerate is a really bad idea on an oscilloscope.

[2N3055]

I wanted to add few more observation, looking at discussion here.

But if we go with option  3. (downsampling by filtering), as suggested, we will filter out anything above 10 MHz..
On spectrum analyser, we would be looking at scale of 0-10 MHz and see nice clean spectra of that without any folding from upper bandwidth. Perfect.

What we will see on the screen of oscilloscope ?

It's all wrong. Look at your samplerates and then think about how that aliases with your 50MHz carrier. If you do these kind of tests you need to use an 'odd' frequency like 33MHz. You'll see you can only get a good picture with lower samplerates when you turn peak-detect on.

As mentioned before: if you low-pass filter a signal you'll lose the high frequency content. Think about how a basic AM demodulator works; that is not a low-pass filter but an envelope detector. Do the same test with 20MHz bandwidth on and off and you'll see.

I honestly don't understand what are you sayin. I'm DEMONSTRATING the problem by deliberately provoking aliasing... To show how it looks..
Third picture IS how it SHOULD look. And Keysight seem to have some intelligent detector that senses aliasing and uses something like Peak detect mode to show proper waveform.  Funny enough it stops working if you enable FFT, probably to be able to provide proper samples for FFT engine.

And I know how it will look after filter. It is a question for a reader. You know, discussion..

[nctnico]

After reading your post 3 more times I see what you are trying to show; it is extremely hard to follow what you wrote because you didn't explain what is in your screendumps and how your oscilloscope behaves.

[2N3055]

After reading your post 3 more times I see what you are trying to show; it is extremely hard to follow what you wrote because you didn't explain what is in your screendumps and how your oscilloscope behaves.

Thank you for pointing that out. I will try to explain better next time. Sorry!

[2N3055]

(Attachment Link)

This is an AM modulated signal using an 8.33MHz carrier and 100Hz modulation. Peak detect is on. Trace 1 (top) is unfiltered, trace 3 (bottom) is filtered using a 600kHz low-pass filter. I hope that this makes it very clear that anti-aliasing filtering based on the samplerate is a really bad idea on an oscilloscope.

Correct!! If you do downsampling by filtering, in time domain plot you will see absolutely nothing once your timebase gets long enough. That is worse than aliasing. With aliasing you at least get a notion there is SOME signal. And as you correctly point out, enabling Peak detect mode (or if the scope has functioning antialiasing function), you will get correct display, similar to what you would see on the analog scope.
What is good for scope is not optimum for SA and vice versa.

[Sighound36]

Afternoon Chaps

As promised the screen images of a 100Mhz square wave generated by a Tek 31252 @2G/s and 14 bits

Using T-Flex 405 18Ghz BW respectable lab cables with SS matching BW terminations, scope  calibration performed yesterday

FG generator output set to 2vpp @ 100 Mhz

Below no high res filtering used at all, basic measurements on screen followed by the FFT images from 1.5Ghz  up to 5Ghz plus colour graded version. Peak  table set at 15 results

[Sighound36]

Just as a matter of coarse I performed a full bandwidth check on the scope, via my Agilent VSG E4433B and a Bodnar 40ps rise time reference.

Measurements were started a 2Vpp @ 1Ghz and reaches -3dB (1.4  Vpp) @ 2.145Ghz plus the Bodnar rise time of 170ps. Pretty happy we have the correct bandwidth for this Rigol

[2N3055]

Thanks for that!!

I think some images got scrambled.

But on FFT you can see nicely that 9th harmonic is already 55dB lower power than 100 MHz fundamental..
So for scope time domain display, 10x more bandwidth than squarewave frequency is quite ok.

[rhb]

(Attachment Link)

This is an AM modulated signal using an 8.33MHz carrier and 100Hz modulation. Peak detect is on. Trace 1 (top) is unfiltered, trace 3 (bottom) is filtered using a 600kHz low-pass filter. I hope that this makes it very clear that anti-aliasing filtering based on the samplerate is a really bad idea on an oscilloscope.

Correct!! If you do downsampling by filtering, in time domain plot you will see absolutely nothing once your timebase gets long enough. That is worse than aliasing. With aliasing you at least get a notion there is SOME signal. And as you correctly point out, enabling Peak detect mode (or if the scope has functioning antialiasing function), you will get correct display, similar to what you would see on the analog scope.
What is good for scope is not optimum for SA and vice versa.

Attached are three Octave scripts.  Please remove the ".txt" and run them.  Then look at the time series that are output.

fig0.m creates a minimum phase Nyquist  limited impulse response
fig1.m low pass filters it with 3 different filter profiles
fig2.m downsamples the output for the 3 filter profiles

Please feel free to make the input impulse as high frequency as you like.    But please stop spouting the BS about low pass filtering making things go away.  The  scripts prove you are wrong.  The loss of amplitude after filtering is easily corrected.

This is a first week of class homework exercise.

Reg

[SilverSolder]

[...]
I think some images got scrambled.
[...]

There seems to be a bug with the forum software, it changes the order of images as compared with what the author specified when writing the post.

[nctnico]

Please feel free to make the input impulse as high frequency as you like.    But please stop spouting the BS about low pass filtering making things go away.  The  scripts prove you are wrong.  The loss of amplitude after filtering is easily corrected.

Well... it seems to me that what is lost due to limited ADC resolution AND noise can not be recovered.

[2N3055]

Attached are three Octave scripts.  Please remove the ".txt" and run them.  Then look at the time series that are output.

fig0.m creates a minimum phase Nyquist  limited impulse response
fig1.m low pass filters it with 3 different filter profiles
fig2.m downsamples the output for the 3 filter profiles

Please feel free to make the input impulse as high frequency as you like.    But please stop spouting the BS about low pass filtering making things go away.  The  scripts prove you are wrong.  The loss of amplitude after filtering is easily corrected.

This is a first week of class homework exercise.

Reg

Ok let's try this. Maybe this will help.

Create file that makes 50 MHz sinewave AM modulated  with 100 Hz. Sample it with 5 GHz, for 50 ms. That will give you 250 megasamples. Make data 8 bit signed byte. -127 to +127

Resample that to 2.5 megasamples with whatever you think is best.  We'll consider that screen buffer, it's a nice 100x decimation.

Give me that file to plot it in time domain.

In Octave ammod function in communications package can be used to create am modulated signal.

[Sighound36]

Thanks for that!!

I think some images got scrambled.

But on FFT you can see nicely that 9th harmonic is already 55dB lower power than 100 MHz fundamental..
So for scope time domain display, 10x more bandwidth than square wave frequency is quite ok.

Apologies that was down to me having a Sunday moment, all rectified now 

[SilverSolder]

Attached are three Octave scripts.  Please remove the ".txt" and run them.  Then look at the time series that are output.

fig0.m creates a minimum phase Nyquist  limited impulse response
fig1.m low pass filters it with 3 different filter profiles
fig2.m downsamples the output for the 3 filter profiles

Octave & math n00b here...  when I run fig0.m,  I get two files:  a0 containing 4096 Ones,  and d0 containing "4096" followed by 4095 Zeros.   Is that the expected output?

[rhb]

Intuition is not a substitute for mathematical analysis and as most people learn very quickly in Calculus and Physics , rarely is intuition correct.  My last post was a demonstration of the obvious to anyone who knows anything about DSP.  The only part that was moderately above the fundamentals of the discrete Fourier transform was use of the Hilbert transform to compute the minimum phase time domain impulse response. 

Upon reflection since my last post I have concluded that I am wasting my time. If the simple tests I have performed so far are to be buried in such a high level of noise from the ignorant I see no hope for a useful discussion of the correct interpolation operator and more nuanced topics. So I shall not be reading or posting further.

Edit:  Just in case there are people reading this who are actually interested in in my original project of comparing low end scopes, I created scope-wars@groups.io a while back when this thread got out of control.  If you are interested in scope evaluations I invite you to join.  It will be moderated to prevent the level of sheer idiocy that has dominated this thread.  Join the group, but don't post and if enough people join I'll repost stuff from this thread and we can continue discussing what is right, wrong and a "feature" of DSOs

Have fun!
Reg

PS @SilverSolder There is one non-zero value in d0,  the first value.

PPS  @2N3055 A 50 MHz carrier modulated by a 100 Hz signal consists of 3 frequencies, 49.9999, 50.0000, and 50.0001 MHz.  This is ARRL Novice license guide knowledge.  Do your own damn DSP. I already know the answer.

[2N3055]

Intuition is not a substitute for mathematical analysis and as most people learn very quickly in Calculus and Physics , rarely is intuition correct.  My last post was a demonstration of the obvious to anyone who knows anything about DSP.  The only part that was moderately above the fundamentals of the discrete Fourier transform was use of the Hilbert transform to compute the minimum phase time domain impulse response. 

Upon reflection since my last post I have concluded that I am wasting my time. If the simple tests I have performed so far are to be buried in such a high level of noise from the ignorant I see no hope for a useful discussion of the correct interpolation operator and more nuanced topics. So I shall not be reading or posting further.

Have fun!
Reg

PS there is one non-zero value in d0,  the first value.

It wasn't a waste of time. I clearly shows that you will do anything to glorify how much smarter you are than all of us combined, but still unable to solve simplest problem put in front of you from real world...

Make that AM signal as I asked and downsample it the  way I asked  so we can see what signal will look like. If it looks like it should, kudos to you. If not than we can discuss difference between knowing how to solve math and knowing which math needs solving.

[SilverSolder]

Intuition is not a substitute for mathematical analysis [...]

Reminds me of the old Einstein quote, "Imagination is more important than knowledge."   

But he probably didn't mean that imagining our own facts is a good idea...  so yes, applying the scientific method is important once the discussion gets to a certain level of detail!   

[2N3055]

@SilverSolder,
I have no doubt Reg knows all that stuff in and out.  Since 2017 he's been on a crusade about how scopes are stupid and don't work. He started this topic with sentence "  My goal in this is to embarrass the OEMs into improving their products..."

And every time somebody asks concrete real world question like I did, he starts with ad hominem attacks how stupid is everybody and if we understood iota about DSP we wouldn't be asking stupid question.

He bought GW Instek 2000E series scope (very nice scope BTW) to improve it, and then within few weeks damaged it so it was useless afterwards. Our member here Nctnico, meanwhile, uses that same scope on daily basis (despite having more expensive scopes) in production environment, and sing praises to it all the time. 

Look, RHB obviously knows about DSP math a lot.   If he can solve some problems that we mentioned here, really solving something Keysight (or Siglent or Rigol) doesn't already know, I'm sure there would be a handsome consulting fee somewhere for him.

Problem is that all of the things he mentions are known things in DSP. He once trumpeted about sparse sampling for months and how scopes would be done that way if only manufacturers knew better. Until  they managed to explain to him it doesn't work on real time scope because it works by virtue of repetitive sampling. And is so computationally intensive that  it is simpler to just sample at high sample rate and be done with it.

So yeah he's very smart, but controversial...

[maginnovision]

I think he's just got too much ego. You asked him to do one thing and he replies back with you're all too dumb so I'm done with it. That's just ego. I enjoyed the thread while it lasted.

[Tomorokoshi]

If the simple tests I have performed so far are to be buried in such a high level of noise from the ignorant I see no hope for a useful discussion of the correct interpolation operator and more nuanced topics. So I shall not be reading or posting further.

This topic started out as potentially being a very interesting technical review and discussion of the engineering, performance, and applicability of the FFT math function built into many oscilloscopes as compared to spectrum analyzers. If a collection of techniques and test methods had been developed and presented it would have provided the opportunity for many here to test their equipment and setups, providing data that could have been collected into a spreadsheet, similar to the one for multimeters.

Unfortunately, it quickly devolved into an appeal to your own authority by doing things like bringing up the size of your bookshelf and years of experience while belittling others. All we got from you were some screenshots without a lot of context, data analysis, consistent comparisons, deep inside the noise of a bunch of talking at people, not talking with people.

Have fun!
Reg

And what's with this sophomoric trolling in all your posts?

[SilverSolder]

@SilverSolder,
I have do doubt Reg knows all that stuff in and out.  Since 2017 he's been on a crusade about how scopes are stupid and don't work. He started this topic with sentence "  My goal in this is to embarrass the OEMs into improving their products..."

And every time somebody asks concrete real world question like I did, he starts with ad hominem attacks how stupid is everybody and if we understood iota about DSP we wouldn't be asking stupid question.

He bought GW Instek 2000E series scope (very nice scope BTW) to improve it, and then within few weeks damaged it so it was useless afterwards. Our member here Nctnico, meanwhile, uses that same scope on daily basis (despite having more expensive scopes) in production environment, and sing praises to it all the time. 

Look, RHB obviously knows about DSP math a lot.   If he can solve some problems that we mentioned here, really solving something Keysight (or Siglent or Rigol) doesn't already know, I'm sure there would be a handsome consulting fee somewhere for him.

Problem is that all of the things he mentions are known things in DSP. He once trumpeted about sparse sampling for months and how scopes would be done that way if only manufacturers knew better. Until  they managed to explain to him it doesn't work on real time scope because it works by virtue of repetitive sampling. And is so computationally intensive that  it is simpler to just sample at high sample rate and be done with it.

So yeah he's very smart, but controversial...

Smart, controversial -   and not boring!         Like many other posters in this thread as well.    The quality of many of the posts have been amazing, frankly.

Is there any doubt that scopes can, and eventually will, become even better than they are today?  - That can only happen by questioning the basic assumptions and limitations that led to the current designs, good though they may be, as we have been doing here.  Questioning long held assumptions is useful even if it leads full circle back to understanding why things are done the way they are within current practical limitations.

Tempers flare, usually we all cool off again? - this is the Internet after all...   it is just a sign that what people are saying matters to them and they take it seriously.  Makes a refreshing change from politicians etc. just saying what they think will please?

[Jay_Diddy_B]

Hi,

I think it is a good topic, that has caused us to think about the limitations of DSOs.

Here are some random thoughts on this subject.

Marking or highlighting data points


In LTspice you can mark the data points. This is a nice feature, you can easily what data the program is using to create the waveforms.

I can do the same thing on my MDO 4K scope. I would post the picture, but you can visualize sparse dots. This is a handy feature.

The forum is a melting pot with people with different backgrounds

Some members of this group are very familiar with the design decisions made in designing a successful scope, their insights are invaluable.

Anti-Aliasing Filter (or lack of one)

The LMH6518 used in the Rigol DS2072A, measured by forum member Bud:



is not suitable for anti-aliasing filter. It is only about 20dB / decade.

Sampling Rate and memory depth

In my mind there is no substitute for sampling speed and memory depth.

If we take an AM modulated RF signal, to accurately reproduce the signal you need 5 to 10 samples per period of the RF carrier.

To resolve the AM modulation in an FFT you need about a sample length equal to about 5 cycles of the modulation.
The resolution, bin width, is 1/acquisition time.

You are trying to resolve three components, RF-mod, RF and RF+ modulation.

If we take the thought experiment proposed by 2N3055

50MHz with 100Hz modulation

a sample rate of 50MHz x 5 or greater is needed

to resolve the 100Hz modulation, chose 20Hz bins, so 50ms of acquisition.

It may be better to use a spectrum analyzer?

A spectrum analyzer still needs the acquisition time, but it can reduce the number of sampling rate by mixing with an LO.

Design Decisions

A scope manufacturer could invest time and effort and design software to extract every last drop of performance out of a signal chain.

or they could invest in the acquisition hardware, increase the sample rate, interleave ADCs etc.

And get similar improvements in performance per dollar.

Given the choice of

1GHz bandwidth 2.5Gsps and advanced DSP

or

1GHz bandwidth 5Gsps and modest signal processing.

I would chose the later. I might make the wrong choice.

I have a 1GHz scope, 5Gsps on my bench, not because I am looking at 500MHz signals, because I don't have to be concerned about the scope performance when looking at 100 MHz (and often much less) signals.


Regards,
Jay_Diddy_B

[SilverSolder]

[...]

Sampling Rate and memory depth

In my mind there is no substitute for sampling speed and memory depth.

[...]

That reminds me of old school hot rodders saying "There's no substitute for cubic inches"...   until a smaller, lighter car with a four cylinder engine and a high pressure turbo blows them off the track! 

Basically you are right, of course, but making the best use of whatever sampling rate the hardware can provide is still going to make for a better scope.

For example, the dithering of samples that HP/Agilent introduced.  It seems to me that by dithering the samples slightly, you end up with more fine grained results that would otherwise have to come from using a much higher sample rate - but the "punishment" is probably a longer acquisition time, as the dithered samples slowly build up the complete picture?

[2N3055]

@SilverSolder,
I have do doubt Reg knows all that stuff in and out.  Since 2017 he's been on a crusade about how scopes are stupid and don't work. He started this topic with sentence "  My goal in this is to embarrass the OEMs into improving their products..."

And every time somebody asks concrete real world question like I did, he starts with ad hominem attacks how stupid is everybody and if we understood iota about DSP we wouldn't be asking stupid question.

He bought GW Instek 2000E series scope (very nice scope BTW) to improve it, and then within few weeks damaged it so it was useless afterwards. Our member here Nctnico, meanwhile, uses that same scope on daily basis (despite having more expensive scopes) in production environment, and sing praises to it all the time. 

Look, RHB obviously knows about DSP math a lot.   If he can solve some problems that we mentioned here, really solving something Keysight (or Siglent or Rigol) doesn't already know, I'm sure there would be a handsome consulting fee somewhere for him.

Problem is that all of the things he mentions are known things in DSP. He once trumpeted about sparse sampling for months and how scopes would be done that way if only manufacturers knew better. Until  they managed to explain to him it doesn't work on real time scope because it works by virtue of repetitive sampling. And is so computationally intensive that  it is simpler to just sample at high sample rate and be done with it.

So yeah he's very smart, but controversial...

Smart, controversial -   and not boring!         Like many other posters in this thread as well.    The quality of many of the posts have been amazing, frankly.

Is there any doubt that scopes can, and eventually will, become even better than they are today?  - That can only happen by questioning the basic assumptions and limitations that led to the current designs, good though they may be, as we have been doing here.  Questioning long held assumptions is useful even if it leads full circle back to understanding why things are done the way they are within current practical limitations.

Tempers flare, usually we all cool off again? - this is the Internet after all...   it is just a sign that what people are saying matters to them and they take it seriously.  Makes a refreshing change from politicians etc. just saying what they think will please?

RHB is right in theory. If all scopes start using 12 bit converters with very high sample rate (like using 10 GS/s for a 1 GHz scope) a scope could be made that would have digital corrections making it have perfect pulse response. If data pumps in scopes where doubled so we could independently feed data to time domain scope segment and to FFT based realtime SA segment, we could have both being perfect without compromises..

But all that stuff is nothing new, it's been done for years by Lecroy, Tektronix, Keysight. Even modest Rigol has some form of pulse response equalization in their new ASIC.
But most of that stuff means massive (20-50 fold) increase in sampled and processed data. And that is expensive.
Will Rigol come out with new version of chipset that will quadruple amount of data it can process? Will manufacturers eventually come out with something like Zynq UltraScale+ RFSoC but 10 times more powerful, so you can squeeze whole scope inside it, and start selling it for 20 USD a chip? They might. But I wouldn't hold my breath.