Electronic selectivity
Encyclopedia
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 decibel
s (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 circuit
s 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.
There are practical limits to the increase in selectivity with changing L/C ratio:
Therefore other methods may be used to increase selectivity, such as Q multiplier
circuits and regenerative receivers
, superhetrodyne receivers (to allow several stages of lower – or at least fixed – intermediate frequency
tuned circuits where smaller bandwidths are easier to obtain.
Performance
A performance, in performing arts, generally comprises an event in which a performer or group of performers behave in a particular way for another group of people, the audience. Choral music and ballet are examples. Usually the performers participate in rehearsals beforehand. Afterwards audience...
of a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
to respond only to the radio signal it is tuned to (such as a radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
) and reject other signals nearby in frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, such as another broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
on an adjacent channel
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 used in keeping stations from interfering with one...
.
Selectivity is usually measured as a ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
in decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s (dBs), comparing the signal strength
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
received against that of a similar signal on another frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. If the signal is at the adjacent channel of the selected signal, this measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
is also known as adjacent-channel rejection ratio (ACRR).
Selectivity also provides some immunity to blanketing
Blanketing
Blanketing is interference caused by strong radio signals. Although the spectral mask of a radio station's transmitter suppresses spurious emissions on other frequencies in the band, being extremely close to a station may allow them to still be strong enough to cause significant interference...
interference.
LC circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
s are often used as filters; the Q
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
("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
Reactance
Reactance is the opposition of a circuit element to a change of electric current or voltage, due to that element's capacitance or inductance. A built-up electric field resists the change of voltage on the element, while a magnetic field resists the change of current...
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
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
of external circuitry.
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
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
circuits and regenerative receivers
Regenerative circuit
The regenerative circuit or "autodyne" allows an electronic signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback...
, superhetrodyne receivers (to allow several stages of lower – or at least fixed – intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
tuned circuits where smaller bandwidths are easier to obtain.
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), ISBN 158053521-6.