Amplitude Modulation
or AM as it is often called is an electronic communication systems technique
wherein the baseband signal is superimposed with the amplitude of the carrier
wave i.e. the amplitude of the carrier wave varies with proportion to the base waveform
that is being transmitted. Amplitude Modulation has been in use since the
earliest days of radio technology. One of the main reasons for the use of
amplitude modulation was its ease of use. The system mainly required the
carrier amplitude to be modulated, additionally; the detector required in the
receiver could be a simple diode-based circuit. This meant that complicated
demodulators weren’t required and as a result, the costs were reduced – a key
requirement for the use of radio technology in the early days when ICs weren’t
available.
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What is Amplitude
Modulation (AM)?
When an amplitude
modulated signal is created, the amplitude of the created signal represents the
original baseband signal to be transmitted. This amplitude forms an envelope
over the underlying high-frequency carrier wave. Here, the overall envelope of
the carrier is modulated to carry the audio signal. AM is the simplest way of
modulating a signal. In short, amplitude modulation is defined as the
modulation in which the amplitude of the carrier wave is varied in accordance
with some characteristic of the modulating signal.
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Amplitude Modulation
History
The first recorded
instance of amplitude modulation of the baseband wave harks back to 1901 when a
Canadian man Reginald Fessenden used a continuous spark transmission to create
the first amplitude modulation ever. Into this continuous spark transmission,
he puts a carbon microphone in the antenna lead. The sound waves impacting on
the microphone varied its resistances and this, in turn, varied the intensity
of the transmission.
Though the accuracy
and the signal-to-noise ratio in this earlier method of transmission were very
low, with the advent of a continuous sine wave generator, the audio quality was
greatly improved. This led to Amplitude modulated waves becoming the standard
for voice transmission.
Amplitude Modulation Formula
Amplitude Modulation
expression is given by:
s(t)=[Ac+Amcos(2πfmt)]cos(2πfct)
Where,
- Am is the amplitude of
the modulating signal
- Ac is the amplitude of
the carrier signal
- fm is the frequency of
the modulating signal
- fc is the frequency of
the carrier signal
Amplitude
Demodulation
Demodulation or
detection is a process where the signal that is a mixture of the amplitude of
the baseband signal and the frequency of the carrier signal, is deconstructed
to yield the original signal that is to be transmitted. Simply, it is the
recovery of the modulating signal from the modulated wave.
Detection of
Amplitude Modulated Wave (Demodulation)
The amplitude modulation and demodulation are equally simple to
perform. The amplitude modulated signal needs just a simple diode detector
circuit to demodulate. The diode rectifies the incoming signal, allowing only
one-half of the signal waveform to pass through. The capacitor then is used to
remove the radio frequency parts of the signal, leaving just the original
waveform. As you see, the equipment for demodulation is very cheap, and this
enables the cost of the receivers to be kept low.
Thus, amplitude
modulated wave can be demodulated in two steps:
- Rectification of
modulated wave
- Elimination of the RF
component of the modulated wave
Advantages and Disadvantages of
Amplitude Modulation
Given below in a
tabular column are the various advantages and disadvantages of amplitude
modulation.
Advantages
It can be demodulated
using a circuit with fewer components.
It is easy to
implement.
AM receivers are
cheap and there is no requirement of specialized components.
Disadvantages
It is not efficient
in terms of its use of bandwidth, requiring a bandwidth equal to twice of the
highest audio frequency.
Not efficient in
terms of power usage.
Prone to high levels
of noise as most noise is amplitude based and AM detectors are sensitive to it.
Applications of
Amplitude Modulation
With the improvement
of the technology, the uses of amplitude modulation waves has become somewhat
less prevalent, nevertheless it can still be found playing an important role
in;
- Broadcast Transmission: AM is still widely
used for broadcasting either long or medium or short wave bands. The
received signal is simple to break down into the baseband signal and hence
the equipment cost to the user is very little and it is easy to
manufacture
- Air band Radio: The use of AM in
the aerospace industry is widespread. The VHF (Very High Frequency)
transmissions made by the airborne equipment still use AM. The radio
contact between ground to air and also ground to ground use AM signals.
- Quadrature Amplitude Modulation: Believe it or not, AM is used in the transmission of data of pretty much everything, from short range transmissions such as Wi-Fi to cellular communications and etc. Quadrature amplitude modulation is formed by mixing two carriers that are out of phase by 90o.
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Source: byjus
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