Amplitude Modulation

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 (see 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 .

AM vs. FM

AM radio's main limitation is its susceptibility to atmospheric Interference , which is heard as Static from the receiver. The narrow Bandwidth traditionally used for AM broadcasts further limits the quality of sound that can be received. Since the 1970s, Wideband FM has been preferred for musical broadcasts, due to its higher audio fidelity and noise-suppression characteristics.

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.

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. Unlike other modulation techniques, 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 .

of a simple AM receiver. A diode functions as the Envelope Detector , with the recovered audio fed directly to an Earphone .]]

To make a good AM receiver an Automatic Gain Control loop is essential; this requires good design. To make a good FM receiver a large number of RF amps which are driven into limiting are required to create a receiver which can take advantage of the Capture Effect , one of the biggest advantages of FM. With valved (tube) systems it is more expensive to make active stages than it is to make the same number of stages with solid state parts, so for a valved Superhet it is simpler to make an AM receiver with the automatic gain control loop while for a solid state receiver it is simpler to make an FM unit. Hence even while the idea of FM was known before WWII its use was rare because of the cost of valves - in the UK the government had a Valve Holder Tax which encouraged radio receiver designers to use as few active stages as possible, - but when solid state parts became available FM started to gain favour.

FORMS OF AM

In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent Sideband s. 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'' (DSBSC) signal. If the carrier is only partially suppressed, a double-sideband reduced carrier (DSBRC) signal results. DSBSC and DSBRC 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.

EXAMPLE

Suppose we wish to modulate a simple sine wave on a carrier wave. The equation for the carrier wave of frequency

\omega_c

, taking its phase to be a reference phase of zero, is

:

c(t) = C \sin(\omega_c t)

.

The equation for the simple sine wave of frequency

\omega_m

(the signal we wish to broadcast) is

:

m(t) = M \sin(\omega_m t + \phi)

,
with

\phi

its phase offset relative to

c(t)

.

Amplitude modulation is performed simply by adding

m(t)

C

. The amplitude-modulated signal is then

:

y(t) = (C + M \sin(\omega_m t + \phi)) \sin(\omega_c t)

The formula for

y(t)

above may be written

:

y(t) = C \sin(\omega_c t) + M rac{\cos(\phi - (\omega_m - \omega_c) t)}{2} - M rac{\cos(\phi + (\omega_m + \omega_c) t)}{2}

The broadcast signal consists of the carrier wave plus two sinusoidal waves each with a frequency slightly different from

\omega_c

, known as sidebands. For the sinusoidal signals used here, these are at

\omega_c + \omega_m

and

\omega_c - \omega_m

. As long as the broadcast (carrier wave) frequencies are sufficiently spaced out so that these side bands do not overlap, stations will not interfere with one another.

A more general example

This relies on knowledge of the Fourier Transform . The discussion of the figure may prove more useful for a quicker understanding.

Consider a general modulating signal

m(t)

, which can now be anything at all. The same basic rules apply:
:

\cos(\omega_c t)

.
Or, in Complex form:
:

rac{e^{j\omega_c t} + e^{-j\omega_c t}}{2}

Taking Fourier Transforms, we get:

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Amplitude Modulation