EEVblog #601 - Why Digital Oscilloscopes Appear Noisy

[EEVblog]

Why do digital oscilloscopes appear noisier than traditional analog oscilloscopes?
Dave busts the myth that digital scopes are noiser than analog scopes, and demonstrates what inherent advantages digital scopes can have over analog scopes in terms of true waveform capture. And also why your analog scope may be hiding important signal detail from you.
Demonstrations of how memory depth, analog bandwidth, averaging, and intensity graded displays can all effect the signal detail you see on your digital oscilloscope.
And how long exposure camera shots on analog oscilloscopes can reveal detail you can't see with your eyes.

Demonstrations are done on the new Tektronix MDO3000, the Rigol DS1052E, the Tektronix TDS220, and Tektronix 2225 analog oscilloscope.
Previous video on common mode noise measurement:

[BravoV]

Noob question, so how do you tell if the noise is came naturally like your example there vs noise generated by crap/inferior scope front end or it's ADC ?

[c4757p]

Noob question, so how do you tell if the noise is came naturally like your example there vs noise generated by crap/inferior scope front end or it's ADC ?

Coin toss?

[EEVblog]

Noob question, so how do you tell if the noise is came naturally like your example there vs noise generated by crap/inferior scope front end or it's ADC ?

No input = noise from scope front end/sampling system.
When viewing a signal, you'd need to know how much your scope contributes at the given settings.

[electronics man]

I Alwase knew digital scopes were better than Analog ones, but don't digital scopes produce a lot more crap than analog ones

[valentinc]

       Dave, you are right about all you said in the video, BUT there is an inherent problem with digital scopes, that is not on an analog and that is: almost always the vertical resolution of the LCD screen is higher than the ADC resolution (8 bits, 256 samples)... And those pixels going all over the place are exactly from that: the vertical resolution is larger than 256 pixels, and there is some "noise" shown on the screen that there isn't present there in reality...

       I've seen some USB scopes that have 10 bit ADCs, but that is rare...

[Tek_TDS220]

Informative and entertaining as always.  I agree with Valentinc that analog scopes have higher resolution on all axes: x, y, and z. 

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

[madires]

Actually you have to take the vertical screen resolution combined with the quantization noise also into account for DSOs. If the signal wiggles between two quantization steps of the ADC you could get up to 4 steps noise (in a simplified case). Now, multiply that with the ratio of the screen resolution to the ADC resolution. Take a small 800x600 screen for example. Substract some space for menus and stuff and you got about 500 pixels vertical resolution for the signal. The ADC is 8 bit. That makes 2 pixels per bit or 8 pixels for 4 bits. That way the noise looks worse than it is. On a CRO you'll see a smooth signal, because you won't notice such a small change in the signal.

[Rigby]

Dave, I absolutely love it when you tell everyone they're wrong, then you go and prove it.  I absolutely love it.

It's always the things everyone "knows" to be true that are false, because no one ever challenges them.  No one challenges them because they're "known."

Serial contrarians like yourself (and myself, though I'm not as skilled) don't get a lot of love when we prove the opposite of common knowledge, but you do it anyway.

I love it.

Thank you.  Bravo.

[GK]

If an analogue oscilloscope "appears" less noisy then, for all sakes and purposes, it is less noisy. I guess it all boils down to how anal you want to be over the use of the word noise in this context.

I often find myself firing up an analogue scope when there are details to a waveform that are just too fine to properly discern on a DSO. In such instances the "ability" of the analogue scope to hide what Dave refers to as "uncorrelated noise" in the video is actually a blessing, not a negative. The effective resolution of a good analogue CRT display is less than the actual width of the trace. Changing the DSO's memory depth setting only works to a degree and averaging is completely useless on any signal that isn't completely periodic and stable. Hitting the BW limit button is also no help if the waveforms you are viewing are higher in frequency than the BW limit!

It wouldn't be too hard to set up some waveforms for demonstration purposes.

[EEVblog]

       Dave, you are right about all you said in the video, BUT there is an inherent problem with digital scopes, that is not on an analog and that is: almost always the vertical resolution of the LCD screen is higher than the ADC resolution (8 bits, 256 samples)...

Not on many of the low end scopes. The Rigol for example is 320x240 for the entire window.

[EEVblog]

If an analogue oscilloscope "appears" less noisy then, for all sakes and purposes, it is less noisy.

So can be a digital scope if you lower the memory depth, turn on hi-res mode, and limit the bandwidth.

In such instances the "ability" of the analogue scope to hide what Dave refers to as "uncorrelated noise" in the video is actually a blessing, not a negative.

Yes, of course, it can be useful. But it can also be a hindrance like I showed in a previous video, and that is the point I made last time and tried to make again here.

[EEVblog]

Dave, I absolutely love it when you tell everyone they're wrong, then you go and prove it.  I absolutely love it.

I should clarify though that I'm not saying analog scopes are crap and useless, and that all digital scopes are as good as an analog, and hence everyone is plain wrong. I just wanted to point out that there is much more to the story than most people realise.

[EEVblog]

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

It's coming up to 20 years old. That's older than a lot of people on this forum

[GK]

If an analogue oscilloscope "appears" less noisy then, for all sakes and purposes, it is less noisy.

So can be a digital scope if you lower the memory depth, turn on hi-res mode, and limit the bandwidth.

To a degree. Such settings still don't make the DSO's that I either own or use at work effectively less "noisy" in the display or higher in perceived resolution under all circumstances than my best analogue scopes. That's just my practical experience. DSO's are a Lecroy LT364L Waverunner, Lecroy WaveJet portable and a Rigol 1202CA.
 

[Ketturi]

I still think that old high-end analog scopes have much lower noise floor compared to cheapest DSOs. Scope front ends are still as critical as before and you can't just make them dirt cheap. I can't see how that rigol could be better than equipment sold at many times higher price.

[Tek_TDS220]

Ketturi, I guess your point is that for some applications, it might be better to buy a used analog scope vs a new digital scope due to a lower 'noise floor'? 

On the surface, this makes some sense.  When you need to digitize a signal at 1 GS/sec and display it, you are going to make a lot of RF noise in the same box.  The more expensive digital scopes have better isolation between the digital circuitry and the analog front end.  However, consider the cheapest Rigol DS2000 (~$1300 US) with a 500 uV/div range.  They must be doing a good job of isolation.  Also, digital scopes have a lot of other advantages.

[pickle9000]

Knowing your tools is very important. Every brand type will have it's high points and issues. What's critical to to know they exist.

Certain tools just will not have the functionality required. Again you need to know your tools and the limitations. 

[max_torque]

What might have been interesting, and shown off the ability of the expensive digital scope, would have been to generate a 1khz (say) 500Mvpkp sine wave signal, and then "swamp it" under say 1Vpkp white noise, and use the display graduation adjustment to show the base signal!

[mike1305]

Food for thought. A video that explains how update rate can also effect the visual appearance of noise. Basic statistics really (since noise is Gaussian), but nice to see live.

[Tek_TDS220]

Nice video mike1305, but I was worried the entire time that he was going to knock over the expensive Agilent scope onto the floor. 

How do you know the noise is Gaussian?  I guess it doesn't matter as long as it is not correlated with the sweep rate (as Dave pointed out).

[David Hess]

Even with equivalent input referred noise, the quantization noise of a DSO limits performance.

I ran across this a couple years ago when I modified one of my function generators to produce a fast transition sync pulse output.  One thing I noticed during testing was a tiny little "blip" getting into the main function generator output which coincided with the sync output and probably had to do with the original design not using a single point ground.

The blip was plainly visible on all of my analog oscilloscopes and none of my DSOs even with averaging with one exception;  it was plainly visible on my ancient 7854 which has a 10 bit digitizer and display to match.  It was also plainly visible on my high bandwidth analog sampling oscilloscope.  High resolution mode on a modern DSO should have revealed it though assuming the display resolution was high enough.

[BravoV]

Noob question, so how do you tell if the noise is came naturally like your example there vs noise generated by crap/inferior scope front end or it's ADC ?

No input = noise from scope front end/sampling system.
When viewing a signal, you'd need to know how much your scope contributes at the given settings.

Great ! Just want to hear this cause worry after watching this video, many "young players" will just blindly shout out loud & proudly claim that their One Hung Low digital scope is "DEFINITELY NOT" noisier than a high end analog scope that probably has a cherry picked low noise components at it's front end, just because Dave said so in this video without reading this kind of _{small foot print note}. 

[Noise Floor]

I've had a conversation on this topic at least once a year for the past 5 years.  Thank you for making a video I can point people to.       

[99tito99]

Hi Dave:  As expected (and so you should never disappoint) a great primer on oscilloscope noise (or should I say data resolution).  I think you do a good job by providing a wide swath (but not noisy) when describing these type of principles and I always find them applicability.  Although I don’t own a digital, I do have an analog (TEK 2213A), and so, I now see what I could not see.  Cheers, Mark