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AM Modulation

Amplitude Modulation - encoding information in signal amplitude

100 Hz
10 Hz
0.5 (50%)

Message Signal m(t)

Carrier Signal c(t) = cos(2πf_c t)

AM Signal s(t) = [1 + m·m(t)] · cos(2πf_c t)

Message Spectrum

AM Spectrum (Carrier + Sidebands)

s(t) = A_c [1 + m·cos(2πf_m t)] · cos(2πf_c t)

How AM Works

Modulation Index (m): The ratio of message amplitude to carrier amplitude. m > 1 causes overmodulation and distortion.

Sidebands: AM creates two sidebands at f_c ± f_m. Each sideband carries the same information.

Carrier Power: At least 50% of AM power is in the carrier, which carries no information - this is inefficient!

Applications: AM radio (535-1605 kHz), aviation radio, CB radio.

Variants: DSB-SC (suppress carrier), SSB (single sideband) are more efficient.