Question about analog oscilloscopes vs digital oscilloscopes

[nctnico]

For those who are entrenched in the belief and idea that there is zero value to repairing great classic test gear, try designing a O'scope from scratch. This would include designing the CRT, mechanicals along

This is why one of the very best things any novice or anyone interested in electronics should learn from history.

CRTs are no longer used nowadays so that technology is rather obsolete. In the front end part of a scope much remains the same but using different technology (JFET input opamp instead of a discrete JFET) and the input divider also faces the same challenges but when the ADC kicks in the worlds is entirely different. As a coincidence: the (paid ofcourse) project I'm currently working on is a 'simple' DSO for a specialistic purpose.

[tautech]

As a coincidence: the (paid of course) project I'm currently working on is a 'simple' DSO for a specialistic purpose.

Not available over the counter? 
Please tell us more. 

[joeqsmith]

I've reported posts a few times in the past, but most all of them were just for off-topic threads with a request to move it to the appropriate forum, and two instances because a certain member was specifically trying to provoke dissent.

I certainly would never report a posting just disagreeing with me.

I have never reported any posts nor have I seen a reason to.  Maybe splitting it up, it seemed like less of a direct attack.  I don't think there was anything else in there.  I think I had asked if you were in sales.   Oh well..   

No, I'm not in sales, although I happen to spend a big part of my professional time buying loads of test gear, in addition to more managerial stuff these days. As to what I do, one of my and my employers' rules are that important work stuff stays at work, and that includes details about the projects I work on, or where they are located.

I would guess that you are not alone in not disclosing work stuff.  My pile of junk I have at home is only for my hobby.  I just happen to actually use it and post from time to time things I build or work on that I think may be of interest.   These are way outside of what I do, so there is no conflict of interest.   My home business model of buying meters to stress test has not proven profitable so I can't quit my day job anytime soon!      

I assumed you have some equipment at home for a reason other than to look or make crappy videos of the update rates.  This seems like something a person in sales or management would do.  Let's see some of these skills you have honed in action!

... I did put one of the newer Transcends with the 44pin UDMA fix in the scope, and like yours it ran fine ....... I also had the acquisition board connectors fixed, I shot the shoddy video when the scope was back in fully working condition.

Unfortunately a former colleague saw the scope when visiting, and talked me into selling it to him. That bastard! 

Bastard!  I have grown to like the little scope with it's bent up case.  It gets used a fair amount and there have been no issues with it.  Good to hear you installed that drive and it worked out.   If you don't remember,  mine was basically an image and removed the swap file.  It will be interesting to see when it finally fails.   I've had brand new hard drives fail this quick. 

Kiriakos-GR was similar to Wuerstchenhund.

Hey, no reason to get offensive! 

   

I figured this topic was spent.  If not, sorry for the detour.

[rsjsouza]

That is pretty sane and sensible, with the caveat below.

From my point of view there are a few common denominators across these discussions of oscilloscopes:
- a used analog oscilloscope is nowadays not recommended for a beginner to invest a somewhat large sum of money (=their "oscilloscope money") given the risk of getting a worn out (=close to failure) or faulty equipment. Conversely, if the price is low and/or the availability is restricted =(nothing else can be had), then the principle of "something is better than nothing" prevails
- a used analog oscilloscope may be a great second equipment for hobbyists above a minimum level of experience, given the opportunity they bring to learn about the process of interpreting waveforms and improving the skills to get a "feel" for what is shown on the screen.

I don't understand what you mean there, unless you are thinking of just voltage / time / frequency measurements. Interpreting an oscillogram to understand what's happening in a circuit doesn't change too much between analogue and digital.

I shouldn't type these things on a tablet with kids running around... You are absolutely right that both types show an oscillogram that can be interpreted in a similar way. I meant to improve as in: "(...)given the opportunity they bring to improve the process of interpreting waveforms and skills to get a 'feel' for what is shown on screen."

- no matter how you look at it, digital oscilloscopes are almost hopeless to be repaired (certain analog scopes are easier, but you frequently need part donors)

IMHO this a myth. Any higher end oscilloscope analog or digital will contain proprietary parts. I have run into this problem when trying to repair both analog and digital scopes.

In your opinion maybe, but all must acknowledge that for DSO's of today, schematics are generally not available whereas most scopes (CRO's and DSO's) of yesteryear they are.
For the novice with very limited understanding of the basic building blocks involved schematics are very important if one has to have any hope of repairing a scope.

IMHO a novice shouldn't get a (likely) broken scope (or any gear) other than for the purpose of fixing it while using a scope which works and is reliable. Sure I like to have schematic of my gear but the reality is that in many cases proprietary parts for older equipment can not be bought so schematics aren't helping much. Also service manuals for newer equipment usually contains a detailed block diagram which is just as usefull as a schematic. Sure a block diagram doesn't have the part values but on the other hand it clarifies how the signal flows through a piece of equipment more clearly than a schematic does.

nctnico, you probably missed the disclaimer highlighted above, which brings high end analog scopes to almost the same realm as DSOs, apart from the availability of schematics. As tautech said, a very experienced engineer can venture in doing component-level repair on anything without the schematics, but a beginner will be hopelessly lost and will require a lot of handholding from eevBlog's forums... That is the point where we agree older test gear is not suitable for beginners.

Obviously this particular point is almost irrelevant with equipment that uses discrete or simpler IC technology and has schematics available (my 20MHz Kenwood's circuitry can be read as a book). They can become a great learning opportunity for more advanced hobbyists, which intersects Bernice's point of learning from the circuit itself.

[tggzzz]

I shouldn't type these things on a tablet with kids running around... You are absolutely right that both types show an oscillogram that can be interpreted in a similar way. I meant to improve as in: "(...)given the opportunity they bring to improve the process of interpreting waveforms and skills to get a 'feel' for what is shown on screen."

You'll miss them when they're gone: regret w.r.t. the kids and relief w.r.t the tablet.

[Wuerstchenhund]

I have never reported any posts nor have I seen a reason to.  Maybe splitting it up, it seemed like less of a direct attack.

I certainly can't see any attack in what you posted.

I would guess that you are not alone in not disclosing work stuff.

Yes, probably.

My pile of junk I have at home is only for my hobby.  I just happen to actually use it and post from time to time things I build or work on that I think may be of interest.   These are way outside of what I do, so there is no conflict of interest.   My home business model of buying meters to stress test has not proven profitable so I can't quit my day job anytime soon!    

Well, you've certainly shown some interesting stuff.

I assumed you have some equipment at home for a reason other than to look or make crappy videos of the update rates.  This seems like something a person in sales or management would do.  Let's see some of these skills you have honed in action!

Yes, I indeed bought the gear for doing some real work other than the occasional demonstration, and I do have a few projects in mind. What keeps me off it is simply a lack of time. Due to my work and status there I work a lot of extended/stupid hours, with a fair amount of travel. On top of that are other duties (i.e. maintaining certain licenses, req'd for work), plus the amount of sleep my ageing body requires, and the truly pittiful rest of time has to be split up between family and friends.

The other issue is that, living in a typical British house with shoebox-size bedrooms and a few sqft of garden, I'm currently a little bit space-constrained. We had plans to buy something better but now that the Brits have knifed their economy it would probably pay out to wait as house prices will most certainly fall. If we stay in the UK, that is, as our business is likely to reduce our stake in the UK England, and move most work to somewhere else.

Oh well... 

I have grown to like the little scope with it's bent up case.  It gets used a fair amount and there have been no issues with it.  Good to hear you installed that drive and it worked out.

Yes, it worked absolutely fine, and you're right, the WRXi is a great scope. I also have an Agilent Infiniium DSO8064A (the one in the video where I used it to measure the WRXI's update rate), which was pretty much Agilent's direct competitor to the WRXi Series (both scopes are even from the same era). The DSO8064A is a nice scope (especially after hacking the options) but it really feels like two steps back compared with the WRXi.

The main reason I parted with the WR64Xi was that mine had no options, and when I need a scope I mostly just use my WavePro 7300A, which is stuffed with options, so it mostly collected dust anyways. At least I know the new owner will make use of its capabilities 

If you don't remember,  mine was basically an image and removed the swap file.  It will be interesting to see when it finally fails.   I've had brand new hard drives fail this quick.

Same here, had hard drives fail after a few hours.

I also just cloned the original hard drive to the SSD, but I didn't use the SSD's full capacity (left 2GB unused for wear levelling). However, I only disabled background defrag, but I left swap alone.

I figured this topic was spent.  If not, sorry for the detour.

I concur.  Back to topic

[Wuerstchenhund]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

[coppice]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

Only a small number of 7903 and 7904 chassis were purchased with plugins faster than 100MHz, so being able to get 500MHz bandwidth out of one is not the common case. It must have worked out easy for Tek to get 500MHz bandwidth from the chassis electronics and tube, as the chassis wasn't too expensive, but 500MHz plugins were.

[GK]

Only a small number of 7903 and 7904 chassis were purchased with plugins faster than 100MHz, so being able to get 500MHz bandwidth out of one is not the common case. It must have worked out easy for Tek to get 500MHz bandwidth from the chassis electronics and tube, as the chassis wasn't too expensive, but 500MHz plugins were.

Mine is currently equipped with a 7A19 and a 7A24 plug-in. The former has a -3dB bandwidth of 600MHz, achieving 500MHz -3dB bandwidth in combination with the mainframe while the latter does 350MHz.


 

[GK]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

I can see 1GHz on my 7904, it's just that the amplitude is reduced. Furthermore a 1GHz sinewave still looks like a clean 1GHz sinewave, not some time-varying triangular approximation. Think of an equivalent real-time-sampled display that simply satisfies Nyquist for the -3dB bandwidth (1GHz sampling); that will not be the case.

To put it another way a significant amount of waveform detail displayed on the CRT of a 500MHz analogue scope can be contained in harmonic components extending significantly beyond 500MHz.

[nctnico]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

I can see 1GHz on my 7904, it's just that the amplitude is reduced. Furthermore a 1GHz sinewave still looks like a clean 1GHz sinewave, not some time-varying triangular approximation. Think of an equivalent real-time-sampled display that simply satisfies Nyquist (1GHz sampling); that will not be the case.

To put it another way a significant amount of waveform detail displayed on the CRT of a 500MHz scope can be contained in harmonic components extending significantly beyond 500MHz.

Which is why even modern high frequency oscilloscopes have an equivalent time sampling mode. My Tektronix TDS744A and TDS510A (from the early 90's) had >100Gs/s equivalent time sampling and their triggering systems worked well beyond 1GHz and due to the Gaussian roll-off it had no problem at all to work with signals over 1GHz but ofcourse these got attenuated since both are 500MHz scopes.

And don't say equivalent time sampling doesn't count because the signal must be repetitive; in order to see any signal stable on an analog scope it has to be repetitive as well.

[Wuerstchenhund]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

I can see 1GHz on my 7904, it's just that the amplitude is reduced. Furthermore a 1GHz sinewave still looks like a clean 1GHz sinewave, not some time-varying triangular approximation.

You don't need 5GSa/s to get a true sample of an 1GHz sine wave, slightly over 2GSa/s (i.e. 2.5GSa/s as offered by some entry-level scopes) is more than sufficient. A higher sample rate will also not result in an increase of details.

In addition, to capture a truly repetitive waveform, pretty much any somewhat decent DSO made in the last 20yrs or so can also use ETS/RIS, which increases the effective sample rate to somewhere between 10GSa/s and 200GSa/s.

Think of an equivalent real-time-sampled display that simply satisfies Nyquist for the -3dB bandwidth (1GHz sampling); that will not be the case.

Any DSO that satisfies Nyquist-Shannon, i.e. which samples at least at 2.2GSa/s, adequately resolves a 1GHz sine wave.

To put it another way a significant amount of waveform detail displayed on the CRT of a 500MHz analogue scope can be contained in harmonic components extending significantly beyond 500MHz.

That's also true for DSOs, which often have an analog bandwidth much larger than what is specified. For example, the elsewhere mentioned 600MHz WR64Xi had a measured bandwidth of slightly over 1GHz, and my 600MHz Agilent DSO8064A has been measured with a BW of 1.25GHz. The latter one also has a max sample rate of 4GSa/s, and even that is enough to appropriately resolve the true analog bandwidth.

Asking for a 5GSa/s sample rate for 1GHz BW is nonsense.

[GK]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

I can see 1GHz on my 7904, it's just that the amplitude is reduced. Furthermore a 1GHz sinewave still looks like a clean 1GHz sinewave, not some time-varying triangular approximation.

You don't need 5GSa/s to get a true sample of an 1GHz sine wave, slightly over 2GSa/s (i.e. 2.5GSa/s as offered by some entry-level scopes) is more than sufficient. A higher sample rate will also not result in an increase of details.

In addition, to capture a truly repetitive waveform, pretty much any somewhat decent DSO made in the last 20yrs or so can also use ETS/RIS, which increases the effective sample rate to somewhere between 10GSa/s and 200GSa/s.

Think of an equivalent real-time-sampled display that simply satisfies Nyquist for the -3dB bandwidth (1GHz sampling); that will not be the case.

Any DSO that satisfies Nyquist-Shannon, i.e. which samples at least at 2.2GSa/s, adequately resolves a 1GHz sine wave.

To put it another way a significant amount of waveform detail displayed on the CRT of a 500MHz analogue scope can be contained in harmonic components extending significantly beyond 500MHz.

That's also true for DSOs, which often have an analog bandwidth much larger than what is specified. For example, the elsewhere mentioned 600MHz WR64Xi had a measured bandwidth of slightly over 1GHz, and my 600MHz Agilent DSO8064A has been measured with a BW of 1.25GHz. The latter one also has a max sample rate of 4GSa/s, and even that is enough to appropriately resolve the true analog bandwidth.

Asking for a 5GSa/s sample rate for 1GHz BW is nonsense.

Sorry, but you don't know what you are talking about. Why do you think that reputable manufacturers like Lecroy make DSOs with real-time sampling rates ten times the -3dB (analogue) bandwidth if it is "nonsense" even to sample at five times the BW?

A "2.2 GHz" sampling rate will not accurately reproduce a 1GHz sinewave unless there is a brick-wall anti-aliasing filter/re-construction filter, which there isn't. This is not analogous to digital audio.

 

[GK]

To match the 7904, 500MHz/5GSa/s would be the minimum required spec.

Why?

I can see 1GHz on my 7904, it's just that the amplitude is reduced. Furthermore a 1GHz sinewave still looks like a clean 1GHz sinewave, not some time-varying triangular approximation. Think of an equivalent real-time-sampled display that simply satisfies Nyquist (1GHz sampling); that will not be the case.

To put it another way a significant amount of waveform detail displayed on the CRT of a 500MHz scope can be contained in harmonic components extending significantly beyond 500MHz.

Which is why even modern high frequency oscilloscopes have an equivalent time sampling mode. My Tektronix TDS744A and TDS510A (from the early 90's) had >100Gs/s equivalent time sampling and their triggering systems worked well beyond 1GHz and due to the Gaussian roll-off it had no problem at all to work with signals over 1GHz but ofcourse these got attenuated since both are 500MHz scopes.

And don't say equivalent time sampling doesn't count because the signal must be repetitive; in order to see any signal stable on an analog scope it has to be repetitive as well.

I won't say that ETS doesn't count, but it still comes with limitations. I regularly view non-periodic but repetitive waveform bursts at 1nS/div on my 7904 that would potentially play havoc with ETS; there are also triggering limitations.

[nctnico]

Sorry, but you don't know what you are talking about. Why do you think that reputable manufacturers like Lecroy make DSOs with real-time sampling rates ten times the -3dB (analogue) bandwidth if it is "complete nonsense" even to sample at five times the BW?

Because they use the same sampling hardware for various bandwidth models. Every DSO manufacturer does that!
Anyway discussing signal components far beyond the bandwidth is moot because there will also be phase shifts due to the various components doing the filtering (either on purpose or by functional limit).

[GK]

Because they use the same sampling hardware for various bandwidth models. Every DSO manufacturer does that!

Yeah right, that's all there is to it.

Anyway discussing signal components far beyond the bandwidth is moot because there will also be phase shifts due to the various components doing the filtering (either on purpose or by functional limit).

This response was entirely predictable and quite wrong. I've identified/found 1GHz and slightly higher instabilities on my 7904 that would have been lost on an equivalent analogue-bandwidth DSO without the sampling rate to properly resolve these signals.

[nctnico]

Anyway discussing signal components far beyond the bandwidth is moot because there will also be phase shifts due to the various components doing the filtering (either on purpose or by functional limit).

This response was entirely predictable and quite wrong. I've identified/found 1GHz and slightly higher instabilities on my 7904 that would have been lost on an equivalent analogue-bandwidth DSO without the sampling rate to properly resolve these signals.

I've read that before but for once I'd like to see the proof why such signals would be lost on a DSO. If the signal is visible then it will be picked up just the same. You'll see an irregulatity in the signal (part of the signal won't 'converge' into a waveform in ET mode) just like on an analog scope.

[GK]

Anyway discussing signal components far beyond the bandwidth is moot because there will also be phase shifts due to the various components doing the filtering (either on purpose or by functional limit).

This response was entirely predictable and quite wrong. I've identified/found 1GHz and slightly higher instabilities on my 7904 that would have been lost on an equivalent analogue-bandwidth DSO without the sampling rate to properly resolve these signals.

I've read that before but for once I'd like to see the proof why such signals would be lost on a DSO. If the signal is visible then it will be picked up just the same. You'll see an irregulatity in the signal (part of the signal won't 'converge' into a waveform in ET mode) just like on an analog scope.

A 1GHz oscillation burst is more faithfully transcribed and much more readily discerned if it is sampled at 5GHz rather than 1GHz. I don't think that this should be a contentious fact.
 

[Rupunzell]

CRTs are still used in the Photonics industry as image converters, micro channel plate image intensifiers and more. Other CRT variants include photo multiplier tubes and IR imager tubes. Much like speciality RF and Microwave tubes, these vacuum tubes have no solid state equivalent. CRTs are no longer used in mass market displays does not mean CRTs are an obsolete technology, that technology has migrated to where it is most effectively used and applied.

For those who labor under the mis-guided belief Op-Amps can replace all discrete device amplifiers, this is another mis-guided idea and belief. We have had this discussion once before on eve-blog under a different heading. Rather than drag out those previous barbs, it would be far better to allow discovery of what limitations of Op-Amps are and can be. What is fact, there are no Op-Amps that can ever replace speciality discrete device amplifiers in high performance analog systems..... This is quickly encountered and realized once any higher performance data acquisition system is attempted.

Simplest form of a DSO could be a single analog to digital converter (signal conditioning & filtering included) streaming data to a host computer with display. Even more rudimentary would be a Digital Volt Meter or any similar conversion device. Given the large number of devices easily available on the market today, is there retinal justification to design a, "simple DSO" even if it is a specialty item?

What is clear based on what has been written, these are ideas based on cook book systems designs that are essentially picking building blocks much like Lego blocks then slamming them together using simulation software to predict what and how the completed system might work, As the system is put together as a real word device, problems are encountered limiting the potential system performance or numerous other problems occur due to design myopia.

Design excellence rarely happens using this methodology.


Bernice

CRTs are no longer used nowadays so that technology is rather obsolete. In the front end part of a scope much remains the same but using different technology (JFET input opamp instead of a discrete JFET) and the input divider also faces the same challenges but when the ADC kicks in the worlds is entirely different. As a coincidence: the (paid ofcourse) project I'm currently working on is a 'simple' DSO for a specialistic purpose.

[nctnico]

For those who labor under the mis-guided belief Op-Amps can replace all discrete device amplifiers, this is another mis-guided idea and belief. We have had this discussion once before on eve-blog under a different heading. Rather than drag out those previous barbs, it would be far better to allow discovery of what limitations of Op-Amps are and can be. What is fact, there are no Op-Amps that can ever replace speciality discrete device amplifiers in high performance analog systems.

I agree; sometimes I take the discrete device route myself.

Simplest form of a DSO could be a single analog to digital converter (signal conditioning & filtering included) streaming data to a host computer with display. Even more rudimentary would be a Digital Volt Meter or any similar conversion device. Given the large number of devices easily available on the market today, is there retinal justification to design a, "simple DSO" even if it is a specialty item?

Yes. The DSO I'm working on is 'just' a signal monitoring feature in a large data acquisition system to be used in (electrically) very harsh environments. I'm not designing the entire system (only doing FPGA and wireless power) but there are very few manufacturers of this particular kind of systems.

[joeqsmith]

I think Duddell had it right.  Forget the CRT. 

[Wuerstchenhund]

Sorry, but you don't know what you are talking about.

Really?

Here's a 1GHz sine wave sampled @ 20GSa/s:




Here at 10GSa/s:




Here at 5GSa/s




Here at 2.5GSa/s




Here at 1GSa/s




That's a 1GHz signal from an Anritsu MG3700A Vector Signal Generator fed into a 3GHz 20GSa/s LeCroy WavePro 7300A scope (the BNC cable I used had a flaky connector which resulted in slight amplitude variations but I couldn't be bothered to unpack a new cable).

I guess we agree that it's obvious why the 1GSa/s screenshot looks like shit, so please go ahead and point out to me the additional details you believe you see on the shots sampled at 5GSa/s and higher vs the 2.5GSa/s shot. 

Yeah, there aren't any.

Next time, before you tell others they don't know what they're talking about, I recommend you get a at least a basic understanding of waveform math and signal physics, because the simple fact is that a continuous-time sine wave that is sampled at >2xf₀ can truly reproduced using sin(x)/x interpolation, and any increase in sample rate will not result in additional details. The math behind it doesn't lie.

A 1GHz oscillation burst is more faithfully transcribed and much more readily discerned if it is sampled at 5GHz rather than 1GHz. I don't think that this should be a contentious fact.

(emphasis mine)

Yes, a 1GHz burst requires a higher sample rate than 2.5GSa/s (or 5GSa/s, for that matter), as due to its non-continuous properties it will contain frequency components that are much higher than f₀. But it also requires an analog bandwidth of a lot more than 1GHz, so the 7904 is utterly useless for such signals. You might see something, but it won't have much to do with how the input signal really looks like.

Why do you think that reputable manufacturers like Lecroy make DSOs with real-time sampling rates ten times the -3dB (analogue) bandwidth if it is "nonsense" even to sample at five times the BW?

Simple, really:

- As nctnico said, most scope series contain higher bandwidth models, and a 1GHz scope requires a faster sampling rate than the 350MHz model. You'll also find that for some scope series, the lower BW models come with a much lower sample rate (i.e. the LeCroy WaveRunner2 LT, WR6k(A), WR6zi, WP7Zi(-A))

- Higher than required sample rates allow the use of oversampling techniques to increase vertical resolution to up to 16bit (i.e. R&S RTE, RTO Series)

- While low-end scopes usually use standard components for ADCs, mid-range and high-end scopes usually use ADC hybrids that are inhouse developments of the scope manufacturers, which makes them pretty expensive. To keep costs down, they are often re-used across scope ranges, with as few as possible variants

- A high sample rate looks impressive on the spec sheet

[GK]

author=Wuerstchenhund link=topic=69974.msg971729#msg971729 date=1467092531]
****snip***
Next time, before you tell others they don't know what they're talking about, I recommend you get a at least a basic understanding of waveform math and signal physics, because the simple fact is that a continuous-time sine wave that is sampled at >2xf₀ can truly reproduced using sin(x)/x interpolation, and any increase in sample rate will not result in additional details. The math behind it doesn't lie.

YAWN. I was specifically NOT talking about interpolation processing (that ruins the fluidity and real-time display of a complex, varying waveform).

A 1GHz oscillation burst is more faithfully transcribed and much more readily discerned if it is sampled at 5GHz rather than 1GHz. I don't think that this should be a contentious fact.

(emphasis mine)

Yes, a 1GHz burst requires a higher sample rate than 2.5GSa/s (or 5GSa/s, for that matter), as due to its non-continuous properties it will contain frequency components that are much higher than f₀. But it also requires an analog bandwidth of a lot more than 1GHz, so the 7904 is utterly useless for such signals. You might see something, but it won't have much to do with how the input signal really looks like.

Bollocks to the nth degree. Even in an ideal case it is only the first and last cycles of a 1GHz burst of several cycles is significantly corrupted in a 500MHz bandwidth (the bandwidth for the transition in and out of DC is theoretically infinite, but the rest of the cycles pass for all sakes an purposes unscaved).
 
I recently had one very such HF burst - occurring right at the crest of a ~200MHz fundamental-f arbitrary but periodic and repetitive waveform. It was due to op-amp oscillation. Only a handful of cycles. This instability was clearly visible on the 7904 and when I dialled the timebase setting down to 1ns/div and fiddling a bit with the trigger I could easily make a ~+/-10% measurement of the frequency of instability right off graduated graticule. How, pray tell, oh wise and all-knowing one, does that make my 7904 "utterly useless for such signals"?

[GK]

Why do you think that reputable manufacturers like Lecroy make DSOs with real-time sampling rates ten times the -3dB (analogue) bandwidth if it is "nonsense" even to sample at five times the BW?

Simple, really:

- As nctnico said, most scope series contain higher bandwidth models, and a 1GHz scope requires a faster sampling rate than the 350MHz model. You'll also find that for some scope series, the lower BW models come with a much lower sample rate (i.e. the LeCroy WaveRunner2 LT, WR6k(A), WR6zi, WP7Zi(-A))

- Higher than required sample rates allow the use of oversampling techniques to increase vertical resolution to up to 16bit (i.e. R&S RTE, RTO Series)

- While low-end scopes usually use standard components for ADCs, mid-range and high-end scopes usually use ADC hybrids that are inhouse developments of the scope manufacturers, which makes them pretty expensive. To keep costs down, they are often re-used across scope ranges, with as few as possible variants

- A high sample rate looks impressive on the spec sheet

ETS, RIS, linear and sin(x) interpolation are not substitutes for greater than Nyquist sampling, they are workarounds for when you need to compromise. They all come with limitations and potentially severe shortcomings (if not used wisely) which simply do not exist when you can sample sufficiently far above Nyquist.

It is not mere marketing, “specsmanship” and hardware rationalization that DSOs with sampling rates well above Nyquist exist. All else being equal, a 500MHzBW-1GSa/s DSO is not an equally versatile and capable high frequency measuring instrument as a 500MHzBW-5GSa/s DSO.

[tggzzz]

Being the owner-user of an hp 8505A vector voltmeter and numerous other vintage test gear that required understanding and ability from the user, not just knowing how to access each and every feature on the display or writing code to drive each and every measurement.

Unfortunately understanding is not highly prized.

As for DSOs being easier to interpret, easier to make measurements, and easier for a beginner to understand, have a look at the screen dump in the link below. I would hope and expect that even a beginner might question the measurement. But what about a less obviously "suboptimal" measurement that might be lurking?

https://www.eevblog.com/forum/beginners/my-new-(i-suspect-faulty)-ds1054z/msg963322/#msg963322

Please don't let this degenerate into a "what do you expect for $400" discussion. My point is about the need for understanding.