Signals & Wireless (RF)
Amplitude modulation (AM) explained
Amplitude modulation (AM) sends a message by turning a steady carrier wave's height up and down in step with the signal, like a voice. Trace the tops of the modulated wave and you get the voice back: that outline is the envelope. How far the voice swings the height is the modulation depth, and it must stay at or below 100%.
A carrier says nothing
A carrier is a steady wave at one frequency, a million times a second for a 1 MHz station. Every cycle is the same, so on its own it carries no message. To send something you have to change it, and the oldest way is to change its height.
The voice changes far more slowly than the carrier, which is what lets a receiver separate the two.
Modulation depth
Max and min are the envelope's biggest and smallest heights. Bursts swinging between 1 V and 3 V give (3 − 1) ÷ (3 + 1) = 50%. More depth makes a louder voice at the receiver; at 100%, the troughs just touch zero.
Live experiment
Set the modulation depth
A voice and a steady bias meet at a MOSFET that a carrier switches 20,000 times a second, chopping the output into bursts whose heights trace the voice. Pick R1: 10 kΩ gives 50% depth, bursts from 1 to 3 V; 4.7 kΩ is just over 100%; 2.2 kΩ tears the envelope.
“Ride the carrier” is lesson 1 of 6 in Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 14 before it first.
Sidebands and bandwidth
A carrier at fc whose height swings at a note fm is exactly the same as three steady tones added together: the carrier, plus two sidebands at fc − fm and fc + fm. A 1 MHz station playing a 3 kHz note puts sidebands at 997 and 1003 kHz.
So a station isn't one frequency but a slice of the dial, twice its highest audio frequency wide. AM stations sit 9 kHz apart in Europe and 10 kHz apart in the Americas.
The voice lives entirely in the sidebands; the carrier never changes. Even at 100% depth, the sidebands hold only a third of the transmitted power, which is why AM is inefficient and why single-sideband radio drops the carrier and one sideband.
Getting the voice back
The simplest receiver is an envelope detector: a diode and a capacitor with a resistor across it. The diode lets through the carrier's peaks; the capacitor holds each one and the resistor lets it sag, so the output follows the envelope. Choose R × C long against a carrier cycle but short against the voice: for a 50 kHz carrier and a 1 kHz voice, about 100 µs.
AM broadcast radio runs from about 0.5 to 1.7 MHz, and aircraft voice radio, from 118 to 137 MHz, uses AM too.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Ride the carrier” is lesson 1 of 6 in Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 14 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: Zero: the carrier vanishes for an instant. 100 % means the height swings all the way down to nothing. Any further and it would have to go below zero.
- Why not “Twice the peak”? Twice the resting height is reached at the crest, not the trough. At 100% the trough falls all the way to zero.
- Why not “The same as at the peak”? Bursts stay the same height only with no modulation at all, 0% depth.
- Why not “Negative”? Negative bursts would be over 100% depth, overmodulation. Exactly 100% just reaches zero.
Show the answer
Answer: A torn, buzzing copy of the voice. Past 100 % the envelope no longer matches the voice: the troughs fold over, and the detector hears buzz. It also splashes energy onto neighbouring stations.
- Why not “Silence”? The receiver still hears something: a torn, distorted version of the voice.
- Why not “Only the carrier”? The voice is still on the carrier, just distorted, so it's heard as a buzz rather than only the carrier.
- Why not “A louder, cleaner voice”? Overmodulation distorts the envelope, so the voice comes out worse, not cleaner.
Show the answer
Answer: 997 kHz and 1003 kHz. Each note appears below and above the carrier, as far out as its pitch: 1000 − 3 and 1000 + 3 kHz.
- Why not “3 kHz and 1 MHz”? The 3 kHz note isn't sent at 3 kHz. It appears as two sidebands, 3 kHz either side of the carrier.
- Why not “1003 kHz only”? AM makes two sidebands, one on each side of the carrier: 997 kHz as well as 1003 kHz.
- Why not “1 MHz only”? A carrier alone carries no message. The 3 kHz note appears as sidebands either side of it.
“Ride the carrier” is lesson 1 of 6 in Airwave Atoll, Signals & Wireless (RF) World 3. Free with an account. Lessons unlock in order, so you’ll do the 14 before it first.
Questions people ask
What is amplitude modulation?
Sending a message by varying the height, the amplitude, of a steady carrier wave in step with the signal. Trace the tops of the modulated wave and you get the signal back: that outline is the envelope. AM broadcast radio and aircraft voice radio both use it.
How do you calculate modulation depth?
m = (max − min) ÷ (max + min), using the envelope's biggest and smallest heights. Bursts swinging between 1 V and 3 V give (3 − 1) ÷ (3 + 1) = 50%. At 100% the envelope just touches zero at each trough.
What is overmodulation?
Modulation deeper than 100%. The troughs would need a height below zero, which a carrier can't have, so the envelope tears. The receiver hears a harsh, buzzing copy of the voice, and the distortion splashes noise onto neighbouring stations.
How much bandwidth does an AM signal need?
Twice its highest audio frequency. A note of f_m on a carrier f_c makes sidebands at f_c − f_m and f_c + f_m. Speech up to about 4.5 kHz needs about 9 kHz, which is why AM stations sit 9 or 10 kHz apart.
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