Question about analog oscilloscopes vs digital oscilloscopes

[System Error Message]

I've seen many videos about analog scopes but the one main thing that bugs me is about the sampling rate and how blind an oscilloscope is. Im not talking about digital scopes with cathode ray displays, i know they're awesome in their responsiveness.

So if i have a variable wave coming in and i want to measure in real time to see changes (i dont want to store data) and put it through an oscilloscope, will a digital oscilloscope sample the wave continously or will it sample part of the wave instead? Would an analog oscilloscope show all the wave as its form and have theoratically infinite sampling rate?

[nctnico]

Both an analog scope and digital scopes have blind time. Both will need to wait for a trigger before writing a signal on the screen. On an analogue scope with on screen cursors some time is needed to write the cursor information on the screen as well. All in all there is very little difference in blind time.

[System Error Message]

So you're saying that even an analog scope that if i set to continuously measure a wave or basically spit out everything that comes into the input it will still have blind time?

I get that there is a delay for things to get to the screen but shouldnt an analog scope work in less dependent way such that any input would get shown on the screen just the way it is + screen adjustments?

[Wuerstchenhund]

I've seen many videos about analog scopes but the one main thing that bugs me is about the sampling rate and how blind an oscilloscope is.

As nctnico said, every scope has blind time, there's no way around that. Analog scopes were pretty fast (up to 600k wfms/s) but that didn't compensate for the long list of shortcomings compared to a decent digital scope.

Im not talking about digital scopes with cathode ray displays, i know they're awesome in their responsiveness.

What makes you think this is the case? In fact, pretty much the only real high waveform rate DSO with CRT screen was the HP/Agilent 54600 Series with MegaZoom ASIC, but the majority of the DSOs back then had much lower update rates (often in the 10's or 100's per second).

So if i have a variable wave coming in and i want to measure in real time to see changes (i dont want to store data) and put it through an oscilloscope, will a digital oscilloscope sample the wave continously or will it sample part of the wave instead?

A digital scope will sample continuously until the sample memory is full, and then wait for another trigger after which the cycle starts anew.

Would an analog oscilloscope show all the wave as its form and have theoratically infinite sampling rate?

No, because even an analog has dead time (caused by the trigger circuit).

[System Error Message]

So other than a cathode ray display an analog scope has no actual benefit over a digital scope?

[Gyro]

- Potentially lower noise floor - it has no bits so 'theoretically' infinite Y resolution.
- No aliasing when the timebase is set too slow (potential trap for the unwary).
- Easier to see mixed low and hight frequency signals eg. when you have mains hum superimposed on a faster signal.
- Potentially faster to interpret IF you're experienced with them ie. Easier to twiddle a few knobs by 'feel' to work out what's going on in a strange situation.
- Absence of an Autoset button 
- (Edit:) You can do all sorts of 'magic' if you're long experienced with that type of scope (learned out of necessity). DSOs these days are easier for the normal user.

[tggzzz]

So other than a cathode ray display an analog scope has no actual benefit over a digital scope?

All tools have characteristics. Whether a particular characteristic is an advantage or disadvantage depends on the circumstances. One recent thread about scopes, for example, was https://www.eevblog.com/forum/projects/5mhz-ttl-clock-oscillator-giving-20hz-what%27s-wrong/

Ignore people that make absolute claims that X is better than Y. Take notice of people that give a balanced view of a tool's characteristics in different circumstances.

If you want to stretch your imagination and capabilities, then imagine how you would develop test and measure a new tool such as the world's fastest scope. In doing that you will begin to appreciate that, with imagination and understanding, you can often use a relatively simple tools to do remarkable things; you don't always need the latest and greatest.

[Keysight DanielBogdanoff]

I personally prefer the digital screen over the CRT because you have a lot more flexibility with persistence and more capabilities like cursors, automatic measurements, etc. 

Working with an analog scope has some unidentifiable draw though. But I'll invariably pull out a digital scope before an analog scope if I have to get stuff done.

[System Error Message]

thanks for the info, im trying to compare how an analog scope performs compared to a digital scope in measuring an input signal when both scopes have equal frequency and digital has high sampling rate.

So an analog scope would be better when measuring very tiny changes and over a large timebase because of no aliasing and theoratically infinite y resolution? It seems you can take a signal generator and use an oscilloscope to look like an arcade video game.

[nctnico]

So an analog scope would be better when measuring very tiny changes and over a large timebase because of no aliasing and theoratically infinite y resolution?

When using a DSO make sure to enable peak-detect and you won't have aliasing problems.

[Gyro]

So an analog scope would be better when measuring very tiny changes and over a large timebase because of no aliasing and theoratically infinite y resolution? It seems you can take a signal generator and use an oscilloscope to look like an arcade video game.

Oh yes, you can make a lot of very pretty pictures (and hopefully still understand what's going on)!

I should probably qualify my points with the statement that a DSO is better for pretty much everything else these days, but for these few items I still think and analogue scope excels.

[guenthert]

I've seen many videos about analog scopes but the one main thing that bugs me is about the sampling rate and how blind an oscilloscope is.

As nctnico said, every scope has blind time, there's no way around that. Analog scopes were pretty fast (up to 600k wfms/s) but that didn't compensate for the long list of shortcomings compared to a decent digital scope.

What good will do me such a high update rate, unless I attach a (very) high speed camera to the screen?  How unlikely a glitch will I be able to identify without burning away the phosphor with the periodic signal? 

I wonder whether there existed a device where an e-beam is projected on a narrow (quasi single dimensional) screen, deflected in one axis and a (long) film was moved continuously along the other?  I'm sure it did, but how was it called (if it didn't exist, let me travel back in time, invent it and call it 'permascope' ) ?

[nctnico]

- Potentially lower noise floor - it has no bits so 'theoretically' infinite Y resolution.

If you forget about the fosfor being too slow to catch a lot of the spikes and non-linearities and distortion in the Y amplifiers. Don't forget the signal fed into an analog scope has to be amplified a lot before it has enough amplitude to drive the deflection plates.

[Wuerstchenhund]

So other than a cathode ray display an analog scope has no actual benefit over a digital scope?

Not sure I would call a CRT a "benefit". I certainly won't miss the blooming, the flickering, the burn-in, the fragility and (on color CRTs) the often dreadful convergence problems of a fish bowl.

As to the benefits of an analog scope, the only advantage I can think of is X/Y/Z mode. Almost all modern DSOs can do X/Y mode (and many do it nicely) but hardly any DSO does Z mode (brightness modulation) as well. In fact, the only digital scopes I'm aware of that can do that are the Rohde & Schwarz RTM/RTE/RTO scopes. Which is very likely because there is little demand for X/Y/Z mode these days.

- Potentially lower noise floor - it has no bits so 'theoretically' infinite Y resolution.

But it requires a lot of amplification of the original signal to get it to display on the screen, and that comes with added noise (which isn't necessarily visible on the CRT!), non-linearities and other issues.

As to the resolution, it is only 'infinite' in theory because the CRT alone limits what can be displayed - and there are many things that an analog scope will hide from the user.

- No aliasing when the timebase is set too slow (potential trap for the unwary).

True in general, but that comes down to knowing your kit, its limitations and what you're doing. And without that there are lots of traps that an analog scope offers which can easily mislead you.

- Easier to see mixed low and hight frequency signals eg. when you have mains hum superimposed on a faster signal.

Can't say that this is true. A somewhat modern, decent DSO shouldn't have much problems resolving mains hum on a fast signal.

- Potentially faster to interpret IF you're experienced with them ie. Easier to twiddle a few knobs by 'feel' to work out what's going on in a strange situation.

Not sure how a graticule showing the waveform only on a CRT (analog scope) should be easier/quicker to interpret than a graticule with the waveform plus all relevant signal parameters shown on a color display.

Also, interpretation by the user is pretty much the only way to get sensible information out of an analog scope, as there isn't a lot in terms of 'measuring (and even if there was, the poor specs of most analog scopes would make them of little use).

Analog scope vs. DSO is like measuring voltage with a light bulb instead of a DVM.

- Absence of an Autoset button 

I guess you also frown upon computers with graphical user interfaces then? 

- (Edit:) You can do all sorts of 'magic' if you're long experienced with that type of scope (learned out of necessity). DSOs these days are easier for the normal user.

Not just easier, a decent DSO can offer a lot of information that is extremely difficult (best case) or completely impossible to gain from an analog scope.

Analog scopes have been the mainstay of electronics for several decades simply because back then they were the best tools available, but technology has made a lot of progress in the last few decades which is why world & dog has moved on to DSOs. Most people that still use them either want them for their nostalgic feeling, because their knowledge about DSOs is still stuck in the late 80's or because they can't afford something better.

[Wuerstchenhund]

I've seen many videos about analog scopes but the one main thing that bugs me is about the sampling rate and how blind an oscilloscope is.

As nctnico said, every scope has blind time, there's no way around that. Analog scopes were pretty fast (up to 600k wfms/s) but that didn't compensate for the long list of shortcomings compared to a decent digital scope.

What good will do me such a high update rate, unless I attach a (very) high speed camera to the screen?  How unlikely a glitch will I be able to identify without burning away the phosphor with the periodic signal?

Very little (and frankly, these days we have better methods to find glitches than staring at a fishbowl).

But then, the waveform update rate is generally overvalued anyways, and is so because a certain manufacturer that shall not be named has made update rates its main marketing argument. After all, high waveform rates are no longer the domain of analog scopes, in fact some DSOs vastly exceed the max waveform rate an analog scope can achieve.

[Gyro]

Analog scope vs. DSO is like measuring voltage with a light bulb instead of a DVM.

I knew I would regret replying to this thread!

I probably used the wrong words when I said " 'theoretically' infinite Y resolution" what I should have said was absence of visible steps (against an 8 bit scope anyway).

[DimitriP]

I knew I would regret replying to this thread!

[guido]

Turned into an analog vs digital discussion again    Obviously a digital scope has way more options, but an analog one could also get the job done, depending on what the job is. And i could not determine the job at hand.

So if i have a variable wave coming in and i want to measure in real time to see changes (i dont want to store data) and put it through an oscilloscope.

What is variable in this case? Frequency? Amplitude? Both?  I'm assuming a wave is a sinewave (?). Are you sure you need a scope in the first place?

E.g. if it is only amplitude you might be better off with a modern dmm with logging capabilities. Zero blind-time and a nice rolling graph (ok, you are blind inbetween samples but it is nothing like the blind time of a scope).

As to the light bulb statement , i think the better comparisson is between an analog volt meter and a modern dmm. Where the dmm has peak hold, freeze, min-max, logging etc etc. But both can measure voltage. And if you want to track slow changes, the analog one might be better for the job. Or a digital one with a graph-bar next to digits 

PS some analog scopes even have an auto setup button 

[FlyingHacker]

Some scopes, like the Tek 2232, even have both a standard analog sweep AND a digital storage function (displayed on the CRT) with at least basic measurements like voltage, period, frequency, etc. It has averaging and peak detection as well.

[tautech]

PS some analog scopes even have an auto setup button 

Yes they are DSO's with a CRT display. 

[CustomEngineerer]

Turned into an analog vs digital discussion again

Pretty sure that was the whole point of the thread? "Questions about analog oscilloscopes vs digital oscilloscopes"

[nctnico]

PS some analog scopes even have an auto setup button 

Yes they are DSO's with a CRT display. 

Guido wasn't joking: the Tektronix 2445A comes to mind.

[joeqsmith]

Rather than permascope how about calling it a strip chart recorder.   

I am not sure what defines an oscilloscope.   A box with some sort of programmable input, DAS and display?   I would guess the dead time is not always going to be the case and more dependent on your requirements.  For example, if you only needed a 1 hour sample rate, I am pretty sure I could get you something with reasonable sample jitter and store for a very long time with no dead time.   

What good will do me such a high update rate, unless I attach a (very) high speed camera to the screen?  How unlikely a glitch will I be able to identify without burning away the phosphor with the periodic signal? 

I wonder whether there existed a device where an e-beam is projected on a narrow (quasi single dimensional) screen, deflected in one axis and a (long) film was moved continuously along the other?  I'm sure it did, but how was it called (if it didn't exist, let me travel back in time, invent it and call it 'permascope' ) ?

[guido]

Yes, tek 24x5A or B. Or the last ranges of Iwatsu analog scopes.

I know the question is about scopes, i'm wondering if that is the right question in the first place. For sure it was not the generic beginners question we see over and over. Were the answer is obvious.

[System Error Message]

Turned into an analog vs digital discussion again    Obviously a digital scope has way more options, but an analog one could also get the job done, depending on what the job is. And i could not determine the job at hand.

So if i have a variable wave coming in and i want to measure in real time to see changes (i dont want to store data) and put it through an oscilloscope.

What is variable in this case? Frequency? Amplitude? Both?  I'm assuming a wave is a sinewave (?). Are you sure you need a scope in the first place?

E.g. if it is only amplitude you might be better off with a modern dmm with logging capabilities. Zero blind-time and a nice rolling graph (ok, you are blind inbetween samples but it is nothing like the blind time of a scope).

As to the light bulb statement , i think the better comparisson is between an analog volt meter and a modern dmm. Where the dmm has peak hold, freeze, min-max, logging etc etc. But both can measure voltage. And if you want to track slow changes, the analog one might be better for the job. Or a digital one with a graph-bar next to digits 

PS some analog scopes even have an auto setup button 

Are you saying that a bench DMM with graphing capability has almost no blind time?

By variable wave as in both frequency and amplitude.