EEVblog #601 - Why Digital Oscilloscopes Appear Noisy

[leppie]

It hurts to hear you describe the TDS220 as 'ancient' - I still use one every day...

I have TDS340 with a TDS360 mainboard. Dave will probably call this prehistoric!

But I am a notch under 40, so I am probably "qualified" to use one 

[waspinator]

the closeup of the $10K Tektronix 3000 screen is really disappointing. Looks just as bad as the cheap Rigol. An HDPI+ screen is probably less than $100, why haven't they bothered upgrading that?

[Rigby]

the closeup of the $10K Tektronix 3000 screen is really disappointing. Looks just as bad as the cheap Rigol. An HDPI+ screen is probably less than $100, why haven't they bothered upgrading that?

they optimize for display speed over display quality, and rightly so.  Oscilloscopes are not meant for precise measurement, they're meant to be used to observe the shape of a waveform.

The display included lets one easily see the shape of the waveform.

[senso]

It will always look "bad", its only a 8 bits ADC.

[Legion]

For the Rigol 2000 series is the intensity grading option controlled by the "Waveform Intensity" or the "Persistence Time" setting? I'd check it myself but mine is in for warranty repair.

[KedasProbe]

Like shown in the video it's just a matter of representing the (noisy/extra) data.
Like this image below: (the gray or colour levels on the hameg are limited though.)


It can also help if you select to show dots only so these isn't a long line drawn.

edit: Notice that hameg decided to draw the grid on top of the noisy part. (blue part)

[M0BSW]

 I will no doubt buy a digital Scope, however, nothing smells as good as my  Telequipment 61A or my New scope a Tektronix 465B,beautiful smell when there warmed up, no reason given accept I just love the old birds.

[Hydrawerk]

Have you seen this old thread? http://www.home.agilent.com/owc_discussions/thread.jspa?threadID=17549&start=90&tstart=0

[David Hess]

I thought of this discussion thread where I linked a video and included a pair of Tektronix articles which discuss using analog oscilloscopes for noise measurement:

https://www.eevblog.com/forum/chat/will-your-scope-lie/msg420557/#msg420557

A good DSO can make this sort of noise measurement directly but not all of them can do it accurately.

[R_G_B_]

Granny bashing the TDS210 lol still a handy scope better than no scope I guess.

[miguelvp]

It will always look "bad", its only a 8 bits ADC.

Vertical resolution 8 bits (11 bits with Hi Res)

I know 11 is not that much but in reality it's 3 orders of magnitude bigger.

And don't forget the 2.5 Gsps, show me a DAC that's 0.4 ns per sample, say you get the 500 MHz scope that's 2ns per point with 5 samples to determine that point.

As others mention, the purpose is not precision but to be able to see the waves so say you get this:
DAC5670IGDJ (14bit, 2.4 GSPS ) That's 0.417 ns per sample so you will have a 480 MHz scope with more vertical resolution, but it will cost way more because you have to add support logic for the extra 6bits or extra 3 bits for the high resolution.

But what does the vertical resolution give you? more precise measurements? sure but it comes at an exponential cost. Each bit doubles the resolution and math functions etc, so 2^6 =64 times the detail that you have to support, that's going to cost quite a bit of money, power and resources, and for what?

Then consider that because of the extra resolution your noise gets bigger so you have to deal with that as well.

Edit: by bigger I mean, it renders the lower end bits useless.

Edit: think about this, how come we don't have 10 based digital chips, heck we can measure 0.1 volt increments right? so 0-1V at 0.1V and we can have decimal computers. But it's really not that simple.

Edit: also consider the setting time of that fast 14 bit chip at 3.5ns, so you couldn't do better than 285 MHz.

Argh, edit again: The thing is that oscilloscopes are tools for the time domain. You want to use them to see what is happening in the smallest time possible for high speed communications or what have you.

I mean what is more important anyways, being able to see your signals changing states or to measure the voltages missing the signals?

[VK3DRB]

(In the following, "CRO" is Australian slang for oscilloscope. It means "Cathode Ray Oscilloscope" but these days it means any oscilloscope - cathode ray tube or LCD.)

I came across a bloke working in the Australian army at an Agilent test equipment show. He said to me "Digital CRO's are for pooftas. Anyone who knows what they are doing will only use an analogue CRO. I only use analogue because I know a lot more than any engineer." He was a bad advertisement for the military. (I have come across others just as stupid and arrogant.)

Sure, analogue CRO's can have advantages. But these days, the digital CRO is a FAR more a useful instrument to me. I have one that is a combined digital CRO and logic analyser. Try to get that in an analogue CRO.

 

[rigrunner]

Very informative video 

Bearing in mind that i'm a hobbyist and not a professional. I have basic scopes - Hantek hacked to 200Mhz and an old Iwatsu 5710.
Where i find the digital scope lets me down is lag.

If i'm looking at an RF waveform and tuning an RF stage the lag on the DSO gets annoying and i switch to the analogue every time,

I'd be interested to see how the lag has changed through the different generations of DSO.

[Rigby]

"lag" is a very fuzzy, often misused term.

Are you talking about latency between signal acquisition & signal display?  Are you talking about the screen update rate?  Are you talking about control responsiveness?  Can you be more specific?

[miguelvp]

Update lag is really not that important either, I mean if it was between two channels then yeah, you don't want that. But your eyes can't sync with an external device and the output of the scope so that doesn't matter.

The important thing is that when you trigger a signal you can inspect all those 10 million points in 1/50th of a second knowing that it didn't skip a beat. And I don't know if you get all 10 M at the higher BW, but just to illustrate.

[rigrunner]

> Can you be more specific?

I don't know exactly where the perceived lag lies.  I suspect it is latency between signal acquisition & signal display due to my DSO being low end, but i can't be certain.

>Update lag is really not that important either,

It is to me when i'm tuning an RF filter stage for max peak.

[miguelvp]

Average human reaction time (and trained at that) is about 200ms at best. Say that from your optics to your brain it's only 10ms and the 190ms is from the brain to the muscle.

10ms, that means that you can see at 100fps and notice if you skip a frame and that will be a very highly trained video game player, like in a fighting game that they can tell if a game glitches at 60fps. (16.666 ms per frame)

So the perceive lag is like the audiophoolery thread for sound. The thing is, that yeah, it's important that the scope is capable to display the smallest runt possible and your persistence of vision will detect it. Not because you can see that fast but because the display doesn't update so fast that you will miss it.

Edit: like subliminal messages, you are not aware of them even if your eye/brain detects them.

[rigrunner]

The perceived lag on the DSO causes me to overshoot when tuning for peaks. The delay i see is way more than 10ms, closer to 250ms i would think. I have to wait for the DSO to catch up and then retune. I switch to the analogue scope from that point onwards in the tuning process.
It's the only thing that keeps me holding on to my analogue scope. The DSO works fine for everything else.
 

[miguelvp]

Anything under 30 to 50ms as a runt might not be perceived, the 16.66ms (60Hz) is that you can notice a frame being gone from a sequential play, but showing a single frame that doesn't have anything to do with the normal sequence (therefore I use the runt as a sample) must be displayed for longer than 50ms for you to be aware of it.

Not sure what scope manufacturers use for displaying runts, but if the scope frame rate is higher than 30Hz you might not see the runt. Your eye/brain might detect it but your conscious mind wont.

Edit: Of course on CROs, the phosphor persistence will make sure you see it. On digital displays the faster they can really switch off the worse it would be for you to actually see the runt. But as a manufacturer, they can decide to display it longer so you can see it.

[rigrunner]

Runts and display persistence don't enter into what i'm observing.  It's a distinct delay between my hand turning a tuning core and what is displayed on the DSO screen.  The analogue scope appears to be much more real time than the DSO.

As i mentioned earlier - i'm happy to attribute this to my DSO being peasant specification but i would be interested in seeing how it has evolved though the generations of DSO.

[miguelvp]

So because of what I said before, maybe digital scope manufacturers choose their display so that a pixel will ghost the value for at least 50 or more milliseconds so that you can see it. I can see (no pun intended) the advantage of having displays that by nature lag their output for this purpose.

[miguelvp]

Runts and display persistence don't enter into what i'm observing.  It's a distinct delay between my hand turning a tuning core and what is displayed on the DSO screen.  The analogue scope appears to be much more real time than the DSO.

As i mentioned earlier - i'm happy to attribute this to my DSO being peasant specification but i would be interested in seeing how it has evolved though the generations of DSO.

There is no storage on analogue scopes, that's why they are more responsive to inputs. Analogs have to do computations before showing the results. But some high end ones can update that pretty quick, but since it's not analogue they have an inherit delay from input to output.

And by storage, even if they do have storage, they display the output instantly, the storage happens later.

[BravoV]

Miguelvp, you sound very experienced in this scope thingy, just curious, what scopes you've used In the past or own now ?

[miguelvp]

Miguelvp, you sound very experienced in this scope thingy, just curious, what scopes you've used In the past or own now ?

Not experienced on scopes at all, just common sense of what I would do to make sure a scope will work as intended.

I've been a game developer for many years so I know about perception and human interface. Did some racing and drag racing games (well simulations) so I know about the human aspect of reaction time. Just many years of experience in user interaction with computers. Before that I was on medical imaging and had to deal with a lot of custom hardware and research, meaning programming OS9 based 68000 custom hardware to deal with a lot of data back in the 90's. Again dealing with perception and how radiologists interact with systems.

Now I work for a AAA game studio and I'm pretty sure most of you have played games that I've worked on, but my engineering side always makes me dig deeper into how things work so I've always been keeping up with technology and what is possible.

As far as electronics I'm a veteran noob because I don't deal with electronics directly but I do know what they are capable of doing. In my work you either keep up or well work, making flash games

[kizzap]

This is possibly a stupid question, but wouldn't a lot of that "noise" on the DSOs come from the INL and DNL of the ADCs?