Selectivity is usually measured as a ratio in decibels (dB), 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 (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. [1]
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), ISBN1-58053-521-6.
References
^The American Radio Relay League: "The Radio Amateur's Handbook, 1968", page 112
April 28, 2023
selectivity, radio, selectivity, measure, performance, radio, receiver, respond, only, radio, signal, tuned, such, radio, station, reject, other, signals, nearby, frequency, such, another, broadcast, adjacent, channel, selectivity, usually, measured, ratio, de. 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 dB 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 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 1 See also Edit Practical RF circuit design for modern wireless systems volume I by Les Besser and Rowan Gilmore chapter 3 2 6 Receiver selectivity p 113 ISBN 1 58053 521 6 References Edit The American Radio Relay League The Radio Amateur s Handbook 1968 page 112 Retrieved from https en wikipedia org w index php title Selectivity radio amp oldid 1081797356, wikipedia, wiki, book, books, library,
