Radio receiver
The information produced by the receiver may be in the form of sound, video (television), or digital data.
The television channel received by a TV occupies a wider bandwidth than an audio signal, from 600 kHz to 6 MHz.
A two-way radio is an audio transceiver, a receiver and transmitter in the same device, used for bidirectional person-to-person voice communication.
Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; limiting reception distance to about 40 miles (64 km), and can be blocked by hills between the transmitter and receiver.
Like FM, DAB signals travel by line of sight so reception distances are limited by the visual horizon to about 30–40 miles (48–64 km).
Radio waves from many transmitters pass through the air simultaneously without interfering with each other and are received by the antenna.
The modulation signal output by the demodulator is usually amplified to increase its strength, then the information is converted back to a human-usable form by some type of transducer.
A video signal, representing moving images, as in a television receiver, is converted to light by a display.
Digital data, as in a wireless modem, is applied as input to a computer or microprocessor, which interacts with human users.
[3] The strength of the signal received from a given transmitter varies with time due to changing propagation conditions of the path through which the radio wave passes, such as multipath interference; this is called fading.
In addition as the receiver is tuned between strong and weak stations, the volume of the sound from the speaker would vary drastically.
Without an automatic system to handle it, in an AM receiver, constant adjustment of the volume control would be required.
In the simplest type of radio receiver, called a tuned radio frequency (TRF) receiver, the three functions above are performed consecutively:[4] (1) the mix of radio signals from the antenna is filtered to extract the signal of the desired transmitter; (2) this oscillating voltage is sent through a radio frequency (RF) amplifier to increase its strength to a level sufficient to drive the demodulator; (3) the demodulator recovers the modulation signal (which in broadcast receivers is an audio signal, a voltage oscillating at an audio frequency rate representing the sound waves) from the modulated radio carrier wave; (4) the modulation signal is amplified further in an audio amplifier, then is applied to a loudspeaker or earphone to convert it to sound waves.
[4] The drawbacks stem from the fact that in the TRF the filtering, amplification, and demodulation are done at the high frequency of the incoming radio signal.
The IF signal passes through filter and amplifier stages,[8] then is demodulated in a detector, recovering the original modulation.
The fixed frequency allows modern receivers to use sophisticated quartz crystal, ceramic resonator, or surface acoustic wave (SAW) IF filters that have very high Q factors, to improve selectivity.
The RF filter on the front end of the receiver is needed to prevent interference from any radio signals at the image frequency.
[9] This filter does not need great selectivity, but as the receiver is tuned to different frequencies it must "track" in tandem with the local oscillator.
To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a dual-conversion or double-conversion superheterodyne.
At the cost of the extra stages, the superheterodyne receiver provides the advantage of greater selectivity than can be achieved with a TRF design.
In order to reject nearby interfering stations or noise, a narrow bandwidth is required.
Modern FM and television broadcasting, cellphones and other communications services, with their narrow channel widths, would be impossible without the superheterodyne.
