Question

How do you calculate the speed of a wave from an oscilloscope?

Answer 1

You can't find spread velocity from oscilloscope but Maximum speed of wave is
Vman=AW =2piA/T
so it's easy to find it out. A=Amplitude and T=period Time of wave because both of them is clear to figure out in oscilloscope.

Answer 2

It would be tricky. The speed is given by the product of the frequency and the wavelength. You can obviously measure the frequency on the oscilloscope. To measure the wavelength, however, the only thing I can think of is to have the voltage at one point be the trigger for the trace, and then take the voltage actually displayed from some other point. By studying how the waveform changes as you vary the distance between these two points, you could determine the wavelength. For example, if the trace on the scope starts at the bottom of the sine wave when the measurment points are 10mm apart, and then 3 degrees up from the bottom of the sine wave when the points are 20mm apart, then the wavelength is (20 - 10 = 10mm)/(3/360) = 1.2m.

But I'm not sure this can be done with a standard scope, since I don't know if they generally allow for a separate trigger signal. But maybe.

Answer 3

Maybe I'm missing something here but why not just get the frequency from the o-scope (I assume you know how to do that) and calculate the speed using:

\[f=\frac{c}{\lambda}\]\[v=\lambda f\]
where c = speed of light = 3x10^8m/s.

Answer 4

Shane, I think you're missing the fact that you've assumed the speed of the wave is c in the process of calculating its speed. That is, you've assumed the answer.

It's actually generally the case that electromagnetic waves in circuits are not traveling at c.

Answer 5

Yea...I guess I did assume that. I'm thinking of an o-scope connected to an AC circuit (1ie 20VAC sine wave) in which case the velocity should be just about equal to c.

Answer 6

err 120VAC...typo there.

Answer 7

Not usually, no. The speed of signals in electronic circuits is typically about half of c.

Answer 8

Why would that be?

Answer 9

Er...how detailed do you want? The general answer is, because metal is not a vacuum, and EM waves always travel slower through a medium than through a vacuum. At a more detailed level, because what's actually passing through the metal is a wave of perturbation of the location of electrons, and electrons can only move so fast, in a way that depends on the exact properties of the material. Does that help?

Answer 10

I understand what you're saying, but it still doesn't make sense to me in this context.

The flow (propagation) of charge should be almost exactly the speed of light...it flows on the outside of the conductors. Maybe I slept that day in class but that's how I recall it.

Answer 11

The charge isn't generally going anywhere, it's the signal -- the change in the electric field -- that's moving. Furthermore, the usual prescription in electrostatics that there is no E field inside a conductor doesn't apply here, because that assumes equilibrium, and if there's a signal propagating down the wire, clearly we're not in equilibrium.

What happens is you shove some electrons in one end of the wire. That electric field propagates into the wire and exerts a force on nearby electrons, which then move appropriately. That, in turn, produces an addition to the electric field, which induces electrons further down the wire to start moving, and so forth. How fast the signal moves is a complicated question of how fast the field itself travels in the metal, and how fast the electrons respond to the field, both of which depends on the exact properties of the metal, and also on things like its shape and size, what its boundary looks like, e.g. whether it's insulated or not. It will always be less than c, but how much less depends. I see Wikipedia asserts about 66% of c for ordinary coaxial cable. I found an article in Scientific American that asserts about 50% of c for the traces inside a chip. But neither, unfortunately, points to any derivation.

Certainly for most practical purposes you might as well retriceume c, since the difference between c and 50% of c for any ordinary purpose is zip. In very precise electronics you would need to know, of course. And in this context, I'm assuming the OP wants to measure the actual velocity of the signal, and not just assume it's c.

Answer 12

That's a very good explanation...I guess I was looking at it the wrong way (for years).

It's not like this comes up in day-to-day electrical work very often...just theory stuff for most I assume. I've used an o-scope plenty but I've never really cared about the velocity of the wave...just the frequency, shape and to look for noise.

I will say as far as I can recall the o-scopes I've used do allow for a second trigger...not a function I ever used though or played with though.

Answer 13

Here's a macroscopic analysis that I found interesting:

http://alignment.hep.brandeis.edu/Lab/XLine/XLine.html

If you have access to a scope with a second trigger input, maybe you should try it!

Answer 14

Thanks, I'll check that out. I do have a few different types of o-scopes at work...maybe something to do when I get bored :)