EEVblog #601 - Why Digital Oscilloscopes Appear Noisy

[pickle9000]

My Instek has other weird idiosyncrasies where if you zoom in really close on the noise (when paused) it looks like sin(x)/x pattern. Pretty sure it's a software glitch because it disappears when you change the timebase. Yikes!

Turn on the dot mode, to look at the unprocessed points.

[ornea]

Since we are getting nitty gritty ... the top and bottom of the waveforms/line appear to be slightly different. At 7:15 the top seems has 1-2 pixel height  notches missing while the bottom appears to have 1-3 pixel spikes.  I would have expected that the bottom and top of the line to look similar. It is very subtle but curious if it is explainable.

Justin

[David Hess]

I now wonder if the problem katzohki describes and excessive apparent noise on DSOs may be related to aliasing produced by nonlinearity and jitter in the ADC which will be worse if interleaving is used.  I brought this up recently in another discussion thread which includes a link to an Agilent application note mentioning the issue:

https://www.eevblog.com/forum/testgear/rigol-ds1074z-oscillosope/msg465619/#msg465619

My hypothesis is that the newer DSOs which accumulate multiple waveform acquisition records for the display record to yield a high waveform update rate look noisier than they should because in effect the envelope of the signal is expanded by the interaction of aliasing with the sin(x)/x reconstruction.

This effect is very apparent on my old 500 MSample/second DSO in which the digitizers use interleaving.  Fast edges or clean sine waves which are significantly below the Nyquist frequency show the effect but I never thought much of it because I normally use equivalent time sampling mode raising the sample rate into the 10+ GSamples/second range where aliasing is insignificant.

It might be possible to check this on a DSO which interleaves when only one channel is used by enabling the other channels and watching for a change in the display of the original waveform.  Alternatively make the DSO display just one acquisition record at a time and watch for the "wobbulation".

I do not think the noise tests that Dave did would have revealed this if it was happening.

[vokars]

I honestly regret, being so late with my comment here  .

I have an analog oscilloscope that is really nothing special: A rebranded GW Instek GOS-630 (Voltcraft 630-2). And nevertheless I love it. Compared to the 8 Bit DSO's I have worked with (yes I intend to buy the RTB2000 10Bit scope  ) it not only seems to show less noise. It has "no" noise. Why?

The basic message of David's video is at 5:29: "The bottom line is: Any noise on the analog amplifier down here or on the signal you are feeding in, any noise which is uncorrelated to your sweep speed or your trigger it just randomly appears, it's not synchronized with the sweep then it's goining to appear quite dim or non existent, because it is not gonna show up all the time."


a) Of course any noise (which is per definition not correlated with anything especially the trigger, sweep, ..) on an analog scope is visualized as beam width due to the screen luminescence /phosphorescence of the screen. The luminescence/phoshorescence layer of the screen is basically a memory storing excitation energy resulting in the typical persistance of scope- or (former) TV-screens. Depending on the material of the luminescence layer it typically stores the excitation energy for several milliseconds.

Because also the beam intensity has an impact on the beam width I prefer to adust the intensity as low as possible. Every change in beam width with fixed intensity /beam focus is: noise. And vice versa: If there is noise, the beam width increases. This can also be verified on a digital scope by switching on the persistance mode.

b) To really compare the noise produced by the measurement device: Using 1mV/div would make more sense instead of using 1V/div for a comparison as in the video. And with 1mV/div the noise shows up, if it would be hidden in the beam width of the 1V/div setting. With my analog scope it does not (older analog scope models I have worked with and which have worse amplifiers show a broad noise floor on the beam without hiding it). No increase in beam width. No "hidden data". That is my definition of "no" noise (of course an approximation that makes sense for practical usage). Crosscheck: Apply a noisy signal and watch the increase in beam width. Yes: Dave does that in the video and I am really astonished that he cannot see the increase in beam width as I do  .

c) Theoretical viewpoint: 8 bit scopes produce additional noise by their quantization process. An 8 bit scope can never be better than 48dB dynamic range (without Hi Res). Averaging can be applied but reduces the noise also from the signal, which is what you want to see. Means: With equal amplifier noise a digital scope has always more noise per design than an analog scope. Fully in line with experience.