Selectivity is a measure of the
performance
A performance is an act or process of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function.
Performance has evolved glo ...
of a
radio receiver
In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. ...
to respond only to the
radio signal it is tuned to (such as a
radio station
Radio broadcasting is the broadcasting 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 rad ...
) and reject other signals nearby in
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
, such as another
broadcast
Broadcasting is the data distribution, distribution of sound, audio audiovisual content to dispersed audiences via a electronic medium (communication), mass communications medium, typically one using the electromagnetic spectrum (radio waves), ...
on an
adjacent channel.
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
decibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whos ...
s (dB), comparing the
signal strength
In telecommunications, particularly in radio frequency engineering, signal strength is the transmitter power output as received by a reference antenna at a distance from the transmitting antenna. High-powered transmissions, such as those used i ...
received against that of a similar signal on another
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
. If the signal is at the adjacent channel of the selected signal, this
measurement
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determining how large or small a physical quantity is as compared to ...
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
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act ...
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
Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
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
An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found ...
).
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
In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in Transmission (telecommunications), transmission or reception. The intermediate frequency is ...
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