Radio is the name given to the system of transmission and reception of information by the propagation of electromagnetic radiation as radio waves through space. It is the most significant contemporary technique for the transmission of information over distances.
After amplification, a signal source is used to modulate a carrier wave, and after further amplification, it is fed to an antenna for transmission. At the receiver, the radio wave is selected (to the exclusion of all other radio waves), demodulated, amplified, and fed to the loudspeaker, which reproduces the original sound.
Modulation
If some means were not available for distinguishing between the desired radio signal and all other signals being broadcast, the result would be very poor reception. Furthermore, the situation would deteriorate with the number of transmitters in a given area, especially if the desired signal was from a less powerful or more distant transmitter.
To overcome this problem, modulation is used. First, each transmission is provided with a "signature tone" called the carrier frequency. Each transmitter has its own signature tone so that any transmission can be selected. Then the signal is in some way superimposed on this carrier wave before transmission—this process is modulation. The receiver is "tuned into" this frequency and, with suitable electronic circuitry, segregates the carrier wave from the signal (demodulation), amplifies the latter, and feeds it to the speaker.
In transmission, as with reception, a number of different techniques are possible. With one particular type of transmitter or receiver, the circuit design can have even greater variety. The description below refers only to the basic types, the major classifications being on the type of modulation employed.
AM radio transmitters
The first requirement of an amplitude-modulated (AM) transmitter is a stable carrier frequency. If it is not stable, the reception at the receiver will be inconsistent in quality and likely to fade and distort. Stability is provided by a crystal oscillator. Piezoelectric crystals are used, of which the quartz crystal has by far the best characteristics. The design of quartz oscillators for these applications is similar to those used in quartz clocks, where stability is of the utmost importance.
The voltage output from the crystal oscillator is a sine wave; this output is amplified to a high power level by several amplifiers in series. Such amplifiers require special design because of the high frequencies involved, from 30 kHz in the low frequency (LF) band to upwards of 30 MHz in the very high frequency (VHF) band. They are known as radio frequency (RF) amplifiers. The greatly amplified sine wave then passes to a modulated amplifier.
The signal to be transmitted is first amplified using an LF amplifier and then passes to the modulating amplifier. The output from this amplifier alters the amplitude of the high-power carrier sine wave in the modulated amplifier according to the instantaneous magnitude of the signal—this process is amplitude modulation. The AM signal then passes through a matching network and on to the antenna for transmission.
The type of circuit described above produces a double-sideband (DSB) AM transmission, so-called from the way in which the signal and carrier wave are combined—essentially by multiplication. In general, the multiplication of one sine wave (frequency f1) by another (frequency f2) produces a waveform containing two frequencies (f1 + f2) and (f1 – f2). Where the signal contains a range of frequencies, as in speech, the resulting AM signal contains the original carrier frequency, fc, with two sidebands about this frequency. If, for example, the signal has frequencies up to 4 kHz (typical speech) and the carrier frequency is 100 kHz, then the total frequency band width of the AM signal is 8 kHz about the 100 kHz mark—that is, from 96 kHz to 104 kHz. The frequencies on either side of the 100 kHz mark are known as the sidebands.
There is a certain redundancy in this situation, because the information about the original signal is contained separately in both sidebands. Also, the transmission bandwidth is twice what is really necessary. Where the radio spectrum is crowded, as it generally is between LF and VHF, there results an extravagant waste of band space.
Single-sideband (SSB) transmission is therefore used in some situations by filtering out one of the sidebands. Furthermore, using two such circuits with one common carrier frequency, two independent sidebands (representing two independent signals) can be superimposed about the carrier frequency for transmission. Such systems are used, for example, in remote-control models—providing two separate control signals in one transmission—and for stereo transmissions.
