FMsignaali

FMsignaali refers to the frequency-modulated electrical signal used in many radio communications, including FM broadcasting. In frequency modulation, the carrier frequency varies in proportion to the instantaneous amplitude of the input signal, so the instantaneous frequency is f_i(t) = f_c + k_f m(t), where f_c is the carrier and k_f is a sensitivity constant. The resulting waveform typically maintains constant amplitude while its frequency content expands into a wider bandwidth, determined by the modulation index.

The bandwidth of an FM signal is commonly estimated by Carson's rule: B ≈ 2(Δf + f_m), where Δf is the peak frequency deviation and f_m is the highest frequency of the modulating signal. In broadcasting, Δf is often specified as the maximum deviation (for example ±75 kHz in North America), and the audio bandwidth is typically about 15 kHz. Standards also define channel spacing and additional features such as pre-emphasis of high frequencies before modulation and de-emphasis in receivers.

FMsignaali is transmitted using high-frequency carriers in various bands, with receivers employing discriminators or ratio detectors to recover the audio. Modern receivers often integrate stereo and data features, such as the 19 kHz pilot tone for stereo decoding and 57 kHz RDS data for auxiliary information. The wideband nature of FM provides improved resistance to amplitude noise and multiplexing of stereo information, at the cost of requiring a larger occupied bandwidth compared to amplitude modulation.

Beyond broadcasting, FMsignaali is used in two-way radio, avionics, telemetry, and other communications systems where robust demodulation and fidelity are important. See also: frequency modulation, Carson's rule.