Selectivity (electronic)

Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to (such as a radio station) and reject other signals nearby in frequency, such as another broadcast on an adjacent channel.

Selectivity is usually measured as a ratio in decibels (dBs), comparing the signal strength received against that of a similar signal on another frequency. If the signal is at the adjacent channel of the selected signal, this measurement is also known as adjacent-channel rejection ratio (ACRR).

Selectivity also provides some immunity to blanketing interference.

LC circuits are often used as filters; the Q ("Quality" factor) determines the bandwidth of each LC tuned circuit in the radio. The L/C ratio, in turn, determines their Q and so their selectivity, because the rest of the circuit - the aerial or amplifier feeding the tuned circuit for example - will contain present resistance. For a series resonant circuit, the higher the inductance and the lower the capacitance, the narrower the filter bandwidth (meaning the reactance of the inductance, L, and the capacitance, C, at resonant frequency will be relatively high compared with the series source/load resistances). For a parallel resonant circuit the opposite applies; small inductances reduce the damping of external circuitry.

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