Electronics notes/signal reflection

From Helpful
Jump to navigation Jump to search

⚠ This is for beginners and very much by a beginner / hobbyist

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.

Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors and transformers · ground

Slightly less basic: amplifier notes · varistors · changing voltage · baluns · frequency generation · Transmission lines · skin effect

And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio and video notes: See avnotes

Platform specific

Arduino and AVR notes · (Ethernet)
Microcontroller and computer platforms ··· ESP series notes · STM32 series notes

Less sorted: Ground · device voltage and impedance (+ audio-specific) · electricity and humans · Common terms, useful basics, soldering · landline phones · pulse modulation · signal reflection · Project boxes · resource metering · SDR · PLL · vacuum tubes · Multimeter notes Unsorted 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 — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Shortish version

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

If you put an AC signal on a wire, it will be met with something that impedes it - something that is more complex than just the wire's resistance.

We call that impedance.

Yes, that plain (DC) resistance is part of that, and in many situations the largest part.

Another ingredient is the result of things like the skin effect - and that will vary with frequency.

Implication one is that one wire is more fit to transmit a (higher) frequency than others

More than that, if you pay no attention to what you're doing, there are reasons characteristic impedance might change somewhere in a path

which leads to even more specific issues


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.

When source and load devices have a designed purpose that is the same, then they are often also impedance-matched, because some engineer did this for you.

In which case impedance matching comes down to using the right cable for the job.

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

To minimize that effect, it helps a lot to have the end of the wire show a resistance that is the similar 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.


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.

Mixing impedances would be like combining thick/stiff and thin/loose rope, and giving that a whip. At the point where they connect, things happen that are no longer the wave you put in - a little energy goes back, the amplitude is different, etc.

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

Impedance matching would be not mixing rope.

Termination then 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 spring strength is critical, though - it's too stiff/loose for your particular hand-wave, it won't do much or even introduce trouble.

This is why terminators have a specific impedance.

Less analogous