Amplitudimodulaation

Amplitudimodulaation, or amplitude modulation, is a method of varying the strength of a carrier signal in proportion to the instantaneous amplitude of a modulating signal, while the carrier frequency remains constant. The transmitted signal can be written as s(t) = A_c [1 + μ m(t)] cos(ω_c t), where A_c is the carrier amplitude, m(t) is the normalized modulating signal with peak |m(t)| ≤ 1, and μ is the modulation index. Overmodulation occurs when μ > 1, which can distort the recovered signal.

In the frequency domain, amplitude modulation produces a carrier at f_c and two sidebands at f_c ± f_m, where f_m is the modulating frequency. For a single-tone modulator, each sideband carries power (μ^2/4) of the carrier power, so the total sideband power is (μ^2/2) P_c and the total transmitted power is P_t = P_c (1 + μ^2/2). The spectrum and power distribution depend on the modulation index and the modulating signal spectrum.

Demodulation can be accomplished with envelope detection, which requires a reasonably stable carrier and moderate modulation, or with synchronous detectors for suppressed-carrier forms. Envelope detection is simple and widely used in standard AM radio receivers.

Variants and bandwidth considerations: Double-sideband with carrier (DSB-FC) is the traditional form; double-sideband suppressed-carrier (DSB-SC) removes the carrier for power efficiency; single-sideband (SSB) and vestigial-sideband (VSB) reduce bandwidth further, at the cost of more complex transmission and reception.

Applications and history: Amplitude modulation has been widely used for AM radio broadcasting and some aviation and marine communications. It remains academically and technically important for illustrating modulation concepts and for certain legacy systems. Pros and cons include simplicity and robustness to noise versus power inefficiency and bandwidth use.