Modulation is the process of varying a property of a carrier wave — its amplitude, frequency, or phase — in step with a message so that information can travel over radio.¹

Modulation encodes a message by varying the carrier — its amplitude (AM), frequency (FM), or phase.

How it works

Analog modulation varies a property continuously: AM, FM, and SSB. Digital modulation switches the carrier between discrete states (symbols): FSK, PSK, and QAM.

Relevance to SDR

Choosing the matching demodulator for a signal’s modulation is the heart of decoding. The same three carrier properties reappear as the axes of the IQ plane.

Sources

1. Modulation — Wikipedia, overview of analog and digital modulation methods. ↩

See also

• Carrier wave
• Amplitude modulation (AM)
• Frequency modulation (FM)
• Phase-shift keying (PSK)
• Frequency-shift keying (FSK)
• Quadrature amplitude modulation (QAM)

Learn it step by step

• Digital modulation & constellations
• Analog modulation — AM, FM, SSB