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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 Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...
) and reject other signals nearby in
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, such as another broadcast on an
adjacent channel In broadcasting an adjacent channel is an AM, FM, or TV channel that is next to another channel. First-adjacent is immediately next to another channel, second-adjacent is two channels away, and so forth. Information on adjacent channels is ...
. Selectivity is usually measured as a
ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
in decibels (dB), comparing the signal strength received against that of a similar signal on another
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
. 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 Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples i ...
of external circuitry (see electronic oscillator). There are practical limits to the increase in selectivity with changing L/C ratio: *tuning capacitors of large values can be difficult to construct *stray capacitance, and capacitance within the transistors or valves of associated circuitry, may become significant (and vary with time) *the series resistance internal to the wire in the coil, may be significant (for parallel tuned circuits especially) *large inductances imply physically large (and expensive coils) and/or thinner wire (hence worse internal resistance). Therefore other methods may be used to increase selectivity, such as Q multiplier circuits and regenerative receivers. Superheterodyne receivers allow use one or more fixed intermediate frequency tuned circuits for selectivity. Fixed tuning eliminates the requirement that multiple tuning stages accurately match while being adjusted. The American Radio Relay League: "The Radio Amateur's Handbook, 1968", page 112


See also

* "Practical RF circuit design for modern wireless systems", volume I, by Les Besser and Rowan Gilmore; chapter 3.2.6, "Receiver selectivity" (p. 113), .


References

{{DEFAULTSORT:Selectivity (Electronic) Electrical parameters Broadcast engineering