Both AM and FM receivers incorporate automatic gain control (AGC), sometimes called
automatic volume control (AVC). If a strong station is tuned in, the volume of the sound
would tend to be overwhelming if the volume control had previously been set for a weak
station. This drawback is overcome by the use of negative feedback–a DC voltage is
developed at the detector and used to reduce automatically the gain, or amplification, of
the IF amplifiers.The prime advantage of FM, in addition to its fidelity, is its immunity to electrical
noise. Lightning storms superimpose noise on an AM signal by increasing the amplitude of
the signal. This effect shows up in a receiver as a crackling noise. An FM receiver,
because it decodes only the frequency variations, has a limiter circuit that restricts any
amplitude variations that may result from added noise.Single Sideband SystemsWhen an audio signal of 5 kHz is used to amplitude-modulate a carrier, the output of the
transmitter contains sideband frequencies in addition to the carrier frequency. The upper
sideband frequencies extend to 5 kHz higher than the carrier, and the lower sideband
frequencies extend to 5 kHz lower than the carrier. In normal AM broadcasts both sidebands
are transmitted, requiring a bandwidth in the frequency spectrum of 10 kHz, centered on
the carrier frequency. The audio signal, however, is contained in and may be retrieved
from either the upper or lower sideband. Furthermore, the carrier itself contains no
useful information. Therefore, the only part that needs to be transmitted is one of the
sidebands. A system designed to do this is called a single sideband suppressed carrier
(abbreviated SSBSC, or SSB for short). This is an important system because it requires
only half of the bandwidth needed for ordinary AM, thus allowing more channels to be
assigned in any given portion of the frequency spectrum. Also, because of the reduced
power requirements, a 110-watt SSB transmitter may have a range as great as that of a
1,000-watt conventional AM transmitter. Almost all ham radios, commercial radiotelephones,
and marine-band radios, as well as citizens band radios, use SSB systems. Receivers for
such systems are more complex, however, than those for other systems. The receiver must
reinsert the nontransmitted carrier before successful heterodyning can take place.Radio has become a sophisticated and complex area of electrical engineering, especially
when compared to its elementary origin. Every day new radio applications are being found,
ranging from digital radio-controlled garage-door openers to weather satellites and from
tracking systems for polar bear migrations to radio telescope investigations of the
universe. This multiplicity of uses demonstrates the important part radio plays in the
world today.