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What modulation is
Putting information onto a carrier wave.
Putting information onto a carrier wave.
Modulation is how information gets onto a radio wave. A transmitter makes a steady carrier at the operating frequency, then changes one of its properties in step with the message. A receiver measures that change and recovers the message, which is demodulation. Without it, a carrier says only "I am here".
A sine wave has only three properties, so there are only three things to vary. Every mode you will meet is one of them, or a mix:
Audio is far too low in frequency to radiate usefully, and every station would land on top of every other. A carrier moves the message up to an assigned frequency, so many stations can share the spectrum, each in its own slice, and so antennas can be a practical size.
A perfectly steady carrier occupies almost no spectrum, but it carries nothing. Any change you make creates new frequencies either side, the sidebands. A faster or bigger change means a wider signal. That is the central trade-off of every mode: information needs bandwidth. See Time domain, frequency domain and Fourier for why, and Bandwidth, symbol rate and data rate for the numbers.
| Mode | What varies | Typical width |
|---|---|---|
| CW (Morse) | amplitude, on and off | under about 200 Hz |
| SSB voice | amplitude and phase, one sideband | 2.4 to 3 kHz |
| AM voice | amplitude | about 6 kHz |
| FM voice | frequency | about 10 to 16 kHz |
| FT8 | frequency, 8 tones | 50 Hz |
Widths are typical, and depend on speed, filtering and settings.