What are AM, FM, and signal modulation? | Explained Premium

Learn how AM and FM modulation work, and how they impact radio waves and modern communication technologies.

When you tune in to the radio, the announcer says, “Welcome to 100.1 FM Gold” or “You are listening to Akashvani at 783 AM”. The terms ‘AM’ and ‘FM’ refer to forms of signal modulation — a method at the heart of modern long-distance communications. Signal modulation simplifies the technologies required to send and receive good-quality signals carrying information, like the news and songs on the radio.

Imagine a vast, quiet lake. The water is as still as a statue. At the centre of the lake is a small island of pebbles. You pick up and drop a pebble into the water. A single wave ripples across the lake, forming a circle that expands in all directions. That is: a single, short disruption triggered the release of a single wave. After the wave passed, the lake became quiet again.

If you dropped a small gravel, the resulting wave would have had a short height. If the stone had been heavier, the impact would have been more energetic and the wave would have been taller. This height is called the wave’s amplitude. A high amplitude sound wave will be louder; a high amplitude light wave will be brighter.

Dropping a stone every second will create a series of waves, traversing the surface one after another in concentric circles. If I’m sitting in a boat at some distance from the tower, and the wave strikes the boat, it will heave up and down. By measuring how much the boat is lifted, I can calculate the amplitude and from that the weight of the rock you dropped. I can also determine how frequently you drop new stones by counting how often my boat seesaws

The number of successive crests arriving at a spot per second is the wave’s frequency, measured in hertz (Hz). One Hz equals one cycle per second. This means the Chennai Akashvani radio station produces radio waves with 783,000 crests in one second. The interval between two successive crests (or troughs) is called the wavelength, measured in metres.

Suppose we agree to use the Morse code: two stones dropped in quick succession equals a dot (•) and a longer gap between two stones is a dash (–).

When two stones are dropped one after the other, the waves are closely spaced, resulting in a higher frequency. If we drop the pebbles with a big time gap, the frequency drops.