Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to
that of a modulating signal. (Contrast this with frequency modulation, in which the frequency of the carrier is varied; and
phase modulation, in which the phase is varied.)
AM is commonly used at radio frequencies and was the first method used to broadcast commercial radio. The term "AM" is sometimes
used generically to refer to the AM broadcast (mediumwave) band, "AM radio".
Applications in Radio
A basic AM radio transmitter works by first DC-shifting the modulating signal, then multiplying it with the carrier wave using a
frequency mixer. The output of this process is a signal with the same frequency as the carrier but with peaks and troughs that vary
in proportion to the strength of the modulating signal. This is amplified and fed to an antenna.
An AM receiver consists primarily of a tunable filter and an envelope detector, which in simpler sets is a single diode. Its output
is a signal at the carrier frequency, with peaks that trace the amplitude of the unmodulated signal. Amazingly, this is all that is
needed to recover the original audio! In practice, a capacitor is used to undo the DC shift introduced by the transmitter and to
eliminate the carrier frequency by connecting the signal peaks. The output is then fed to an audio amplifier.
The fact that signals can be decoded using very simple equipment is one of the primary advantages of amplitude modulation. This was
especially important in the early days of commercial radio, when electronic components were still quite expensive. This simplicity
and affordability helped make AM one of the most popular methods for sending voice and music over radio during the 20th century.
Forms of AM
In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands.
Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Thus, most of the power output
by an AM transmitter is effectively wasted: half the power is concentrated at the carrier frequency, which carries no useful information
(beyond the fact that a signal is present); the remaining power is split between two identical sidebands, only one of which is needed.
To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal. This produces a reduced-carrier transmission
or double-sideband suppressed carrier (DSSC) signal. If the carrier is only partially suppressed, a double-sideband reduced carrier (DSRC)
signal results. DSSC and DSRC signals need their carrier to be regenerated (by a beat frequency oscillator, for instance) to be demodulated
using conventional techniques.
Even greater efficiency is achieved—at the expense of increased transmitter and receiver complexity—by completely suppressing both the
carrier and one of the sidebands. This is single-sideband modulation, widely used in amateur radio due to its efficient use of both power
and bandwidth.
A simple form of AM often used for digital communications is on-off keying, a type of amplitude-shift keying by which binary data is
represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse code, referred to
as continuous wave (CW) operation.
Source: wikipedia.org
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