View source for Engineering:Selectivity (radio)

'''Selectivity''' is a measure of the [[Engineering:Performance|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 [[Physics:Frequency|frequency]], such as another [[Engineering:Broadcasting|broadcast]] on an adjacent channel.
Selectivity is usually measured as a [[Ratio|ratio]] in [[Physics:Decibel|decibel]]s (dB), comparing the signal strength received against that of a similar signal on another [[Physics:Frequency|frequency]]. If the signal is at the adjacent channel of the selected signal, this [[Measurement|measurement]] is also known as '''adjacent-channel rejection ratio''' ('''ACRR''').

Selectivity also provides some immunity to [[Engineering:Blanketing|blanketing]] interference.

[[Engineering:LC circuit|LC circuit]]s are often used as filters; the [[Physics:Q factor|Q]] ("Quality" factor) determines the [[Bandwidth (signal processing)|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 [[Physics:Electrical reactance|reactance]] of the inductance, L, and the capacitance, C, at [[Physics:Resonant frequency|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 ratio|damping]] of external circuitry (see [[Engineering:Electronic oscillator|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 [[Engineering:Q multiplier|Q multiplier]] circuits and [[Engineering:Regenerative circuit|regenerative receivers]]. [[Engineering:Superheterodyne receiver|Superheterodyne receiver]]s  allow use one or more fixed [[Engineering:Intermediate frequency|intermediate frequency]] tuned circuits for selectivity. Fixed tuning eliminates the requirement that multiple tuning stages accurately match while being adjusted. <ref>The American Radio Relay League: "The Radio Amateur's Handbook, 1968", page 112</ref>

== 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.&nbsp;113), {{ISBN|1-58053-521-6}}.

== References ==
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[[Category:Electrical parameters]]
[[Category:Broadcast engineering]]

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