EEVblog #442 - Analog Vs Digital Oscilloscope Noise

[EEVblog]

Is a modern digital oscilloscope noisier than an old analog CRT oscilloscope?
Are analog scopes better at displaying signals?
You might be surprised.when Dave demonstrates the display of some common mode noise on both types of oscilloscope.
An aside video from the

[jancumps]

Why would the more expensive DSOs show less switching noise? Is it the sample rate?

[Bored@Work]

Why would the more expensive DSOs show less switching noise? Is it the sample rate?

It is the attempt to mimic an analog scope which backfires in this particular case.

[EEVblog]

Why would the more expensive DSOs show less switching noise? Is it the sample rate?

I thought I explained that, but probably not well enough.
The more expensive digital scopes have an intensity graded display, so they try to work as much as possible like analog scopes.
i.e. showing less frequent noise pulses as a dimmer waveform.
So with high end digital scope with such a display, you can miss the noise (or it doesn't show up as readily) if you don't have the waveform "brightness" turned up enough, just like on an analog scope.
And then waveform update rate plays into this as well too of course.
And as mentioned, in this instance it can be disadvantage over a cheaper digital scope that does not have variable persistence. Depends on your needs and what you are looking for.

[cyr]

In other words, a higher-end scope shouldn't be a disadvantage as long as you know how to use it.

[Ketturi]

Sometimes that analog persistence, or its equivalence in DSOs is useful and that is probably why more expensive modern scopes try to emulate that. Not showing switching noise may be wanted effect, if it is known to be external, non-removable and unimportant factor. And there is also difference in analog scopes, my scope shows often strongly that noise(cheap UPS is huge noise generator), and sometimes I have mistaken that noise to be coming from actual device under test, which gives gray hairs.

Thanks Dave for making these vblogs, now I know to start looking elsewhere when I see that next time!

[EEVblog]

Sometimes that analog persistence, or its equivalence in DSOs is useful and that is probably why more expensive modern scopes try to emulate that.

Yes, basically there are more advantages to having it than not. I'm showing probably the only downside to having it.
But it's not actually a downside when (almost all) digital scopes also have an infinite persistence mode you can switch on. I forgot to mention this at the time of this little aside. In this case if you were serious about deliberately capturing the noise envelope you would most certainly turn on infinite persistence mode.

[jancumps]

Ah, I misunderstood what persistance means in this context. I didn't make the link with intensity of the displayed waveform.

[jahonen]

A good way to identify common-mode noise presence is to connect both signal and ground of the scope connection (either ordinary probe or a coax) to same point on DUT ground. If there then is noise, it is common-mode noise. Note that the DUT itself may generate common-mode noise so this test should be done with DUT on and off.

And like I said in the other thread, a ferrite clamp on scope probe/coax cable may help to attenuate this noise.

Regards,
Janne

[mikeselectricstuff]

So with high end digital scope with such a display, you can miss the noise (or it doesn't show up as readily) if you don't have the waveform "brightness" turned up enough, just like on an analog scope.

Yes, although the high-end digital has the advantage that you can turn the brightness fully up to see the infrequently-occurring noise/events without the brighter part of the trace swamping the smaller details, which is what typically happens on an analogue scope.

In other words, a higher-end scope shouldn't be a disadvantage as long as you know how to use it.

Exactly.
I have no doubt that there are some people out there who regularly use good digital scopes with  the intensity always on maxiumum & don't even know about the intensity graded display!

[BravoV]

Noob question, does it matter when comparing the analog Tek 2225 which is only a 50 MHz scope vs the Rigol DS2202 that capable of 200 MHz at those really high frequency noise ? 

[SeanB]

No, the bandwidth limiter was switched on, to make it effectively a 20MHz unit. In any case even my 2MHz scope would have displayed a 66kHz noise signal just fine.

[Salas]

X10 setting worsens the SNR to the scope's residual by 20dB. But necessary if we must scan over some tens of MHZ of quoted RMS noise spec.

[grenert]

What about looking at the front end noise with a 50-ohm termination on the input?
Turn off the high-res mode on the DSO, of course 

[Gridstop]

No, the bandwidth limiter was switched on, to make it effectively a 20MHz unit. In any case even my 2MHz scope would have displayed a 66kHz noise signal just fine.

It had a repetition rate of 66khz, but the actual rise time of the signal would be order(s) of magnitude faster than that?

[AndyC_772]

Correct, the repetition rate doesn't matter. What you're interested in is the rise time and harmonic content of those spikes, and the video shows quite clearly the difference between what you see with 20MHz bandwidth vs the more accurate picture you see when the scope is set to 500 MHz.

[Gridstop]

And that's one thing I like about analog scopes, that the intensity of the vertical line tells you about the rise time relative to the rest of the signal. But, that's kinda negated these days by digital scopes actually having reasonable amounts of storage memory so you can single-shot an event then zoom in and inspect the rise time directly.

[vk6zgo]

What about looking at the front end noise with a 50-ohm termination on the input?
Turn off the high-res mode on the DSO, of course 

You'd have to make sure that you did this with the PSU off,or you'd let the magic smoke out of the termination!

[vk6zgo]

Dave,I kept waiting for you to go to delayed timebase & have a look at one of those noise bursts-----then I remembered that model Tektronix doesn't have it!

You could probably produce a fairly similar display to that of the DSO using,for instance,my Tek 7613 with the 7B53A plugin.

The best test for frequency content of the noise,however, would be the FFT function of the DSO.