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Oscilloscope data

Roughness

This is probably a very basic question but I could not find the answer to it online. I just want to make sure - is the roughness caused by electromagnetic interference?

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    \$\begingroup\$ Yes (12 more to go) well.. that's a general term, any noise, power supply noise is usually the worst culprit. However, it can also be high frequency ringing on the signal. You need to zoom in and see if it is consistant with the edge, but it looks like noise to me. \$\endgroup\$ – Trevor_G Nov 15 '17 at 18:41
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    \$\begingroup\$ Either that's the noise laid on top of earlier periods (overlaid/overlayed?). Or that's just how every period actually looks. My guess is that it's overlayed noise. But you're the one who got the zoom knob and can actually verify this. \$\endgroup\$ – Harry Svensson Nov 15 '17 at 18:50
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    \$\begingroup\$ Two possibilities here its either noise on the signal or that is just the height of the line width. If it goes away when you set the coupling to GND/0V then its noise. You don't see it during the vertcal edges because this is up and down, not side to side so not visible. \$\endgroup\$ – Warren Hill Nov 15 '17 at 18:52
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    \$\begingroup\$ *Forehead Slap* Everything, everything is noisy \$\endgroup\$ – Voltage Spike Nov 15 '17 at 19:11
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    \$\begingroup\$ to make sure that it is not interference, do a "single-shot" sweep \$\endgroup\$ – jsotola Nov 16 '17 at 1:58
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Roughness means some variations of signal:
1. High frequency additive oscillations (or noise). Try using much lower time/div to see them. Try to limit input bandwidth to suppress them.
2. Low frequency addidive oscillations or amplitude modulation. Try using much higher time/div. Also, try single-shot trigger mode to suppress multiple waveform accumulation on the screen.

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Some of what you're seeing is undoubtedly due to simple ringing.

The simple fact is that a perfect square wave requires infinite frequency, which (obviously) isn't possible.

With a limited bandwidth, the signal will show some ringing, so (even at best) it looks something like this:

enter image description here

The red line shows the theoretical wave. The green shows up to the fifth harmonic of the fundamental, and the blue line up to the fifteenth harmonic.

Note how even in a simulated picture like this with no noise added at all, we still get what looks like a much thicker line at the peaks.

I must admit that I'm somewhat (extremely?) puzzled by some of the comments. Let's consider this:

enter image description here

Now, I'm the first to admit that this scope is fairly old (probably older than some of the people who wrote comments). I don't think it's being restricted to a mere 600 MHz bandwidth is quite enough to qualify as a "horrible oscilloscope" (you'd certainly think 600 MHz would be enough to show a 1 KHz signal fairly accurately). It's showing the 1 KHz square wave generated by the scope itself, so I don't think that's somehow started to produce output at 100 MHz or something a whole lot faster than 1 KHz. I'm using a direct coax connection from the generator to the input.

Does somebody here honestly believe that a 1 meter coax cable is going to let huge amounts of noise in to cause the peaks to be bright, even though the rise and fall are so dim they're nearly invisible? If so, how do they believe that anybody would ever stand any chance at all of measuring anything when using a normal scope probe, which inevitably has at least a little unshielded wiring involved?

For those who are convinced that you just can't see the overshoot and ringing without infinite bandwidth, let's take a look at a somewhat zoomed in view:

enter image description here

Now remember: this is a direct coax connection, so what we're seeing is only what's left when the signal has been terminated quite carefully (50 Ohm cable going to a 50 Ohm input). Even so, there's a little overshoot and ringing--under almost any normal circumstances, we'd expect to see (quite a bit) more.

Now, it is almost certainly true that with a phosphor display like this oscilloscope uses, part of the difference in brightness is due to the speed at which the electron beam is moving at a given time. We set a specific speed at which the beam traverses the display (e.g., 1 millisecond). When it's traveling (approximately) horizontally, it spends a relatively large amount of time lighting up the phosphor in a fairly small area. When it gets to the rising or falling part of the wave, it's still moving at the same speed horizontally, but it's also moving at relatively high speed vertically--thus, it's only spending a very small amount of time exciting the phosphor along those sections.

With a raster display, it's pretty much up to the people writing the software to decide whether to simulate this effect or not. I haven't looked at the software of the particular oscilloscope used by the OP, but it wouldn't strike me as terribly surprising if they decided to imitate what a conventional trace of the same signal would look like.

That does not, however, render what I said previously incorrect--that some of what you're seeing is from ringing in the circuit. No matter how careful you are to eliminate it, there's undoubtedly going to be at least some noise as well--but in the setup I've shown above, we see roughly the same effect in a signal that should be extremely low in noise--far below anything that could possibly account for the effect shown.

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    \$\begingroup\$ It's a 1 kHz square wave, for those artifacts to appear, it has to be a horrible oscilloscope, or a high (more than 1 kHz) frequency square wave. \$\endgroup\$ – Harry Svensson Nov 15 '17 at 20:21
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    \$\begingroup\$ That Gibbs phenomenon you are talking about is non-causal, i.e. needs a perfectly squared bandwidth and is hence theoretical only. Such a circuit cannot be realized and so Gibbs ringing cannot ever seen in real life. \$\endgroup\$ – carloc Nov 15 '17 at 20:34
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    \$\begingroup\$ @Jerry Coffin: limited bandwidth (or limited number of harmonics) does not at all explain why the graph is thick. The reason is simply noise (HF) or a low frequency added signal (e.g. added line voltage interference). See Eugene K's answer. \$\endgroup\$ – Curd Nov 15 '17 at 23:05
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    \$\begingroup\$ As you have demonstrated, overshoot and ringing decays. That's why it cannot explain the phenomena seen in OP's pictures. \$\endgroup\$ – pipe Nov 16 '17 at 8:06
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    \$\begingroup\$ @Jerry Coffin: the harmonics (or overshoot) not only decay very quickly (as your colored graph with the harmonics also shows) but harmonics are also in phase with the fundamental wave and therefore don't form a smeadred thick line like uncorrelated noise or interference does \$\endgroup\$ – Curd Nov 16 '17 at 9:22

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