Scope Wars

[nctnico]

Not just that. A step (from an infinite time -1 to an infinite time +1) contains a very wide frequency spectrum. Somewhat repetitive signals OTOH consist of harmonics of their fundamental frequency. In order to have problems with aliasing and/or sharp roll-off from the anti-aliasing filter the harmonics have to be in the frequency band where the anti-aliasing filter has an effect on the signal which is large enough to make it visible.

A fast (e.g 100 ps) pulse is an almost flat spectrum over a large BW.  This is why the seismic industry records the impulse responses  of the recording system input amplifiers and filters.  It verifies that all the channels are working properly which is a *big* deal when you're being paid $10-20 million to acquire the survey.  The client oil company has a person on board, the observer, whose sole job is to make sure that all the many pages of specifications about positioning, sea state and other factors are adhered to.  There are contractual allowances for how many guns and receivers may be inoperable.

If you want an accurate waveform, there are certain constraints about filter roll off and stop band at Nyquist which *must* be met.  Of course, if you don't care, then it hardly matters, does it?

Again: It all depends on whether the waveform has frequency content in the area around the Nyquist frequency. If you look at a 400MHz signal using a 1GHz scope like the RTM3004 (using 2.5Gs/s) then the 2nd harmonic is well within the pass-band of the anti aliasing filter and the 3rd harmonic is well beyond. It is just as easy to argue that having a sharp roll-off in the anti-aliasing filter gives you the widest bandwidth for the lowest samplerate. Having a more gradual roll-off (with less bandwidth) also distorts a signal by cutting out the high frequencies. Sure, you won't get ringing but you'll also lose bandwidth. It is a typical case of picking your poison depending on the application. There is no free lunch here. On many oscilloscopes you have several bandwidth limiters; you can use these to create a more gradual roll-off.

40MHz signal with steep edges from an RF synthesizer with full bandwidth (and aliasing artefacts):


Bandwidth limited to 500MHz to get a more gradual roll-off:

[rhb]

Don't you think it is sort of cheating to say we only give you an accurate waveform under certain conditions?  And leave the user to figure out when their waveform reflects reality?

[rhb]

BTW "Infinite +/- 1" does not exist.

Sure it exists. It's infinite.

It violates the definition of "infinite".

[David Hess]

Got a link for the Soviet solution?

I managed to find it:

https://groups.io/g/TekScopes/message/79995

With this providing some additional context:

https://groups.io/g/TekScopes/message/89431

The US Government required instruments to fit inside a 19" rack while the Soviet solution was to make their scan converter CRTs 6 meters long to reduce deflection requirements.

[nctnico]

BTW "Infinite +/- 1" does not exist.

Sure it exists. It's infinite.

It violates the definition of "infinite".

No. You removed the words time which turns the statement into nonsense.

[tv84]

It violates the definition of "infinite".

Not true. If you go that route then even "infinite" doesn't exist. (good read)

[SilverSolder]

Don't you think it is sort of cheating to say we only give you an accurate waveform under certain conditions?  And leave the user to figure out when their waveform reflects reality?

It seems a 100% valid question to ask what a particular scope actually does when pushed to the limit -  no different from wanting to find the limits of a camera or lens by testing with a resolution chart. 

Where things become a judgement call, is whether something is "good enough" for what you are trying to do.  Even in the seismic world, there must come a point where enough is enough and you use the equipment you have, understanding its limitations?

[nctnico]

Don't you think it is sort of cheating to say we only give you an accurate waveform under certain conditions?  And leave the user to figure out when their waveform reflects reality?

Read my signature 

You have to define 'accurate waveform' first. If you are going after a step response or square wave then the image on the screen will never be accurate simply because both types of signals need infinite bandwidth to reproduce. Read my example very carefully again. On the RTM3004 (just an example) you can cater to both situations where you want a gradual roll-of OR the most bandwidth. Show a step response without ringing by enabling the bandwidth limit OR look at a 400MHz square wave in a way it still looks (somewhat) like a square wave instead of a sine wave using full bandwidth. Which case is cheating?

[rhb]

SilverSolder: Yes, but aliasing is *not* acceptable and at 24 bit acquisition challenging.  I presume that they are actually substantially oversampling and using digital filters as pure analog filters would be quite impossible.

Compromises are operational.  The air guns fail and receiver and recording channels fail.  Air guns being the biggest issue.  I've never been on a boat, but I presume that they can retrieve an individual air gun from the array, service it and replace it without stopping.  The financial incentive for being able to do that  is huge as you can't stop the boat.    So you have to reshoot the line.  At somewhere in the neighborhood of a half million a day for the boat and crew turning the boat with 6-12 25 km streamers towed behind it around twice to reshoot a segment is *very* expensive and it comes out of the acquisition contractor's pocket.  The boat is only making a few knots and has to make very wide turns to avoid tangling the streamers.

The location of the receivers towed behind the boat at the time of each shot is specified as a few meters.  If the contractor doesn't achieve that they are obliged to reshoot until they do or not get paid.  With billion dollar decisions hanging on the results, they are not playing.  The only DSP that gets more serious is war.

The entire issue is under what conditions is discretely sampled data *accurately* representative of the analog reality?  Seismic was recorded on 2" analog tape with 21 tracks before digital became possible.  And *no one* was recording digital data for actual exploration work until it was as good as the analog data.

The more I learn about the  DSP skills of the typical EE the more agog I am.  And *not* in a good way.  There are certainly EEs who can match the seismic community, but I've come to conclude they all work in the defense industry on highly classified projects.  And most of the rest wouldn't recognize a clue if they fell over it.

nctnico:  I've read your signature, many times.  I now understand it much better. It's very simple. What would an analog scope of adequate BW for the task display? .  That's what a proper DSO should show.  I bought my 485 and 7104 precisely because I came to realize how bad DSOs were.   That made me want an arbitrar I could trust. As there is *no* physical system that can *produce* an infinite BW step you are citing a red herring.

Have you done a full cal on an analog scope in the 465, 475, 485 class?  A key requirement is a fast step generator.  I tried to build one which it turns out was pretty good, but not good enough.  Neither I nor the EE who helped me test it at work recognized the impedance matching issue so it seemed to ring excessively and I concluded I had failed.  It actually does about 1.5 ns rise time on a 5 V step quite nicely.  Ultimately I bought a Tek 106 <1 ns step generator to calibrate my 465.

This is intended to be about evaluating DSOs, not "DSP 101".

Have Fun!
Reg

[StillTrying]

40MHz signal with steep edges from an RF synthesizer with full bandwidth (and aliasing artefacts):

It looks like just sinx to me, improving with 2 samples on the fast the edge instead of 1 or none.

[Elasia]

Not too far off the mark... But I would exchange spying for war and the kind that looks down

[Fungus]

Don't you think it is sort of cheating to say we only give you an accurate waveform under certain conditions?  And leave the user to figure out when their waveform reflects reality?

Yes, but what's the alternative?

[Fungus]

40MHz signal with steep edges from an RF synthesizer with full bandwidth (and aliasing artefacts):

This image works better for me. I can look at that and immediately see a bandwidth limited square wave.

I have no idea what the hell's going on in this one (and your description makes no sense):

Bandwidth limited to 500MHz to get a more gradual roll-off:

[nctnico]

@Fungus: The second screen shot is taken with the 500MHz bandwidth limit on. This mimics the behaviour of an oscilloscope with a more gradual roll-off (like analog oscilloscopes and the older equivalent time sampling oscilloscopes have).

[nctnico]

nctnico:  I've read your signature, many times.  I now understand it much better. It's very simple. What would an analog scope of adequate BW for the task display? .  That's what a proper DSO should show. 

No because this would mean that you'll need a way higher samplerate to achieve the same bandwidth. 5 to 10 times more if you want to allow for a Gaussian roll-off. However for a lot of measurements this doesn't make sense so with a factor of 2.5 and a steeper anti-aliasing filter you can get way more bandwidth from the same sampling system. And if you want a more gradual roll-off you enable the bandwidth limiter (at the expense of bandwidth) which will make a DSO behave more like an analog oscilloscope where it comes to the frequency response. IMHO the real problem is that you expect the frequency response from a DSO to be the same compared to an analog scope for a given bandwidth. That just isn't the case. BTW several of the higher end oscilloscopes do some signal processing to improve the frequency response. There is a video from HP/Agilent/Keysight floating around somewhere on how this is done.

This is intended to be about evaluating DSOs, not "DSP 101".

Unfortunately DSO and understanding of DSP (or better put: how signals are sampled) can not be seperated. To really understand the limits of a DSO you need some understanding of signal sampling theory.

[RoGeorge]

This image works better for me. I can look at that and immediately see a bandwidth limited square wave.

But then, the question is:  was that signal generated like that by the generator, with small wiggles before each edge, or that wiggle before each edge is a DSP artifact introduced by the oscilloscope?

On an analog oscilloscope, there will be no doubt that the signal wiggles right before each edge.  On a digital oscilloscope, we don't know.  Might be a perfectly square wave with digital artifacts (because of the oscilloscope's DSP), or it might be because the signal really wiggles before each edge.

On the second screen capture, the same problem:  before each edge, there is some small wiggle.
- Is that a DSP artifact?
- Is that how the signal really is from the generator?
- Is that because of some reflections?

Most probably it's the first, oscilloscope's DSP artifacts, but we can not say for sure from the screen capture alone, unless we already know the signal.  But if we already know for sure the signal's shape, then why would we even bother looking at it.   

[Fungus]

@Fungus: The second screen shot is taken with the 500MHz bandwidth limit on.

This is what confuses me. The first one looks more like I'd expect a "bandwidth limited" signal to look like and the second one is more like a gradual rolloff.

[nctnico]

@Fungus: The second screen shot is taken with the 500MHz bandwidth limit on.

This is what confuses me. The first one looks more like I'd expect a "bandwidth limited" signal to look like and the second one is more like a gradual rolloff.

The first picture shows ringing which is not in the original signal. The bottom picture looks much more like the original signal. As far as I understand rhb, this is exactly the issue he is addressing.

[edigi]

But then, the question is:  was that signal generated like that by the generator, with small wiggles before each edge, or that wiggle before each edge is a DSP artifact introduced by the oscilloscope?

Search for Gibbs phenomenon.
Even the top of the affordable (?) scopes like MSO8204 cannot show a correct square (provided that the signal has the spectral content for this) because it's theoretically not possible (due to the BW required).

[2N3055]

Scopes with Gaussian response have gradual rolloff from very low frequencies, so amplitude accuracy will be impacted. -3dB is 30% percent amplitude error. So for a Gaussian response scope, 1GHz scope will have good amplitude accuracy up to cca 300MHz or about.
Scopes with brickwall response will have very good amplitude accuracy, up to cca 90% of it's stated bandwidth.

Also, scopes with Gaussian response will (by virtue of it's response) slow down any edges that are faster than it can handle (by filtering out all frequency components higher than some frequency).

It will behave same as if you took 1 GHz brickwall response scope and put in 500-600 MHz low pass filter in front of it. Simply as that. It won't show any artefacts (like ringing) because you filtered out parts of input signal that would make problems. Basically, it shows nice pulse from Leo's 30 ps pulser, because it took 30 ps rise time pulse, and filtered it (slowed it down, converted it ) to a pulse with 500ns edge, that is not faster that what is scope's rise time.
And that is exactly what Nico demonstrated (thanks for that, you saved me time of making filter, I had same idea, but not  filter at hand).

Basically, whenever you look at the pulse that is seriously faster than than rise time of your scope, you get overshoot. You can fight it by deliberately making analog part of scope slower, oversampling by much larger factor (not very practical or cheap at 1GHz and up), or just accepting the fact that it will overshoot if you try to drive it faster than spec.

Actually, I can argue that after learning of how it  all works, I like brick wall response better.
Here is why:
1. Good frequency amplitude accuracy up to 90 % of full bandwidth. I can accurately assess 868 MHz amplitude with a 1 GHz scope.
2. Good rise time measurement accuracy. It will overshoot if you drive it fast, close to the edge of spec , but it will give more accurate rise time measurements right there up to the limits of specification.
3. Overshoot is there only if edge is faster than scope's rise time. Which is actually useful information. It tells me edge is faster than what it can handle, by overshooting..

So overshoot is actually useful. It shows you you are trying to look at the signal that is too fast for your scope, instead of happily hiding anything it doesn't like... You fight it by getting faster scope, not one that hides it better...

[nfmax]

@Fungus: The second screen shot is taken with the 500MHz bandwidth limit on.

This is what confuses me. The first one looks more like I'd expect a "bandwidth limited" signal to look like and the second one is more like a gradual rolloff.

The first picture shows ringing which is not in the original signal. The bottom picture looks much more like the original signal. As far as I understand rhb, this is exactly the issue he is addressing.

But how do you know what the original signal looks like? You only know what some other oscilloscope showed you!

[tv84]

A newbie question:

So, we can definitely make a good comparison between 2 scopes just by looking at a (for example, Bodnar's) pulse response, right?  (in terms of signal fidelity)

Or, with only that, we'll be missing important factors?

[Gandalf_Sr]

A newbie question:

So, we can definitely make a good comparison between 2 scopes just by looking at a (for example, Bodnar's) pulse response, right?  (in terms of signal fidelity)

Or, with only that, we'll be missing important factors?

Yes, it's a definite maybe 

[2N3055]

A newbie question:

So, we can definitely make a good comparison between 2 scopes just by looking at a (for example, Bodnar's) pulse response, right?  (in terms of signal fidelity)

Or, with only that, we'll be missing important factors?

Yes, it's a definite maybe 

Step response characterization will only show step response. You should test 100 MHz scope with a pulse that has 1-3 ns risetime and you will get nice results,  for a 1 GHz 400ps scope 150-300 ps would show you nice response...
As I said Gaussian response scopes do exactly that, they convert very fast speed signal to slow enough it can handle it...

To compare brickwall response scopes, you need to do proper frequency sweep to get frequency flatness. Or use calibrated noise source..

While fun and quick and dirty way to check if you "upgraded" your scope from base 100 MHz to something else, pulse response is not very relatable to usual scope work we all do. Unless you're making pulsers and you need to check them.. In which case you get scope with adequate bandwidth and calibrated and well defined pulse response for that particular purpose.

Most of the time, most complicated part is how to get signal to scope input in a form that even cursory resembles what the signal looks like before you connect some probing crap to it...

On clocks and such, you need to verify rise time and how clean is the edge, somewhere around receiver sampling points. If there is some over/undershoot is mostly of no concern to how well system works.... You look at the inside of the eye, not outside...

[SilverSolder]

[...]
So overshoot is actually useful. It shows you you are trying to look at the signal that is too fast for your scope, instead of happily hiding anything it doesn't like... You fight it by getting faster scope, not one that hides it better...

Is that really a safe assumption?  - i.e. can the overshoot actually be present in the signal you are looking at...  so you actually don't really know what it causing it on your display?