Amplitudemodulated

Amplitude-modulated, or amplitude-modulated (AM), refers to a method of conveying information by varying the amplitude of a carrier wave in proportion to a modulating signal. In the common case of a sinusoidal carrier, the transmitted waveform can be written as s(t) = A_c [1 + m cos(ω_m t)] cos(ω_c t), where A_c is the carrier amplitude, ω_c is the carrier angular frequency, ω_m is the modulating angular frequency, and m is the modulation index (0 ≤ m ≤ 1 for undistorted modulation).

Principles: The modulating signal controls the envelope of the carrier while the carrier’s frequency and phase remain essentially constant. The instantaneous amplitude of s(t) follows the modulating signal, producing an audible or data-carrying envelope.

Bandwidth and sidebands: For a single-tone modulating signal, AM creates two sidebands around the carrier with total transmitted bandwidth B = 2 f_m; for a general modulating spectrum, the AM bandwidth equals twice the highest modulating frequency. The spectrum consists of a carrier at f_c and two symmetric sidebands.

Types and efficiency: DSB-FC (standard AM with full carrier) retains the carrier, which simplifies envelope detection but wastes carrier power. DSB-SC (suppressed-carrier) removes the carrier for higher efficiency but requires coherent demodulation. SSB (single-sideband) further reduces bandwidth by suppressing one sideband. Conventional AM is power-inefficient; overmodulation (m > 1) causes envelope distortion.

Demodulation: Envelope detection uses a diode and RC network to recover the modulating signal; DSB-SC and SSB require coherent demodulation with a synchronized carrier.

Applications and history: AM has a long history in radio broadcasting, aviation communications, and audio transmission. Although modern systems favor more bandwidth-efficient schemes, AM remains a foundational concept in analog communications.