# Difference between revisions of "Electronics notes/signal reflection"

 This is for beginners and very much by a beginner. It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book. And some more applied stuff: Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer See also Category:Electronics.
 This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

## Shortish version

 This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

If you put an AC signal on a wire, it will be met with an effective impedance that is more complex than just a resistance.

It's plain resistance, plus things like the skin effect.

And these other effects vary with frequency, so the higher-frequency the signal you want to carry, the more it matters to get wires for the specific use.

More than that, there are a few reasons that such a characteristic impedance vary along a wire, which leads to specific problems.

Consider

the end of a wire, when there is nothing there, is effectively higher effective impedance than the rest (see below)
physically different kinds of cable will have different impedance for the same-frequency signal
(also, a short will be zero impedance)

When the impedance changes along a signal path changes, you see problems like

signal reflection
delayed versions of the signal (echo)
attenuation (e.g. simple loss)
attenuation distortion (different frequencies affected differently)
signal ringing
standing waves

Impedance matching means trying to make the whole signal path (all wiring) behave the same for the frequency you want, which avoids most such trouble.

Source and load devices that you want to connect are usually already impedance-matched, because they are have the same purpose (and are probably decently-designed products).

So impedance matching usually means using the right cable everywhere.

Termination solves problems where the end of the wire's higher effective impedance causes of problem.

To minimize that effect, you want the end of the wire to show a resistance that is the same as the characteristic impedance of your wire with your given use. If your frequency use is well defined (it usually is) you can do this with a simple resistor.

Impedance mismatch is less of a noticeable problem on very short lines - up to a few meters you rarely have to care much at all.

Reflection only start mattering when the wavelength starts resembling real-world cable lengths. For sine waves this is a one or two MHz. For block waves (including most serial communication) it starts mattering a few factors earlier (e.g. in the form of ringing).

For example

DMX's 250kHz rate, and the fact that applications easily use dozens of meter of wire, means it wants termination.
It's not relevant for audio frequencies - even 20kHz (which is higher than most people can hear) is ~15km long.

## Attempt at explanation

Most explanations just say that reflection happens, or give you a lot of words that associate it with various details, but which still come down to 'it happens'.

And yeah, you may want to choose to just trust that they do, because it is hard to get a thorough understanding of why things like reflections happen.

### Analogy

The best analogy I found so far is rope. Not because it's accurate, but because it pokes your intuition.

You may remember a physics lesson (about standing waves and such) where you tied a rope to the wall, and give it a single whip. The point was that the wave comes back. Conservation of energy stuff, really.

Termination would be something like a string damper on the end, which tries to dissipate the energy it gets so that it won't have to be hammered back. That damper's strength is critical, though - it's too stiff/loose for your particular hand-wave, it won't do much or even introduce trouble.

Mixing impedances would be like combining thick/stiff and thin/loose rope, and giving that a whip. At the point where they connect, bad things(tm) happen - a little energy goes back, the amplitude is different, etc.

If you're still swinging when the result reflects back from the wall, things get even messier.

Impedance matching would be not mixing rope.