The
earliest AM signal was broadcasted in the year 1901 by an engineer Reginald
Fessenden. He is a Canadian and he took a nonstop sparkle transmission as well
as located a carbon-based microphone within the lead of an antenna. The sound
waves affect the microphone by changing its resistance, and transmission
intensity. Even though very simple, signals were easy to hear over a few
hundred meters of distance, though there was a harsh sound will occur with the
sparkle. By the beginning of nonstop sine wave signals, broadcasting improved
extensively, and amplitude modulation will become common for voice
transmissions. Currently, the amplitude is used in broadcasting the audio on
the short-wave, long medium bands, as well as for bi-directional radio
communication on VHF used for aircraft.
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What is Amplitude Modulation?
The
amplitude modulation definition is, an amplitude of the carrier signal is
proportional to (in accordance with) the amplitude of the input modulating
signal. In AM, there is a modulating signal. This is also called an input
signal or baseband signal (Speech for example). This is a low-frequency signal
as we have seen earlier. There is another high-frequency signal called carrier.
The purpose of AM is to translate the low-frequency baseband signal to a higher
freq signal using the carrier. As discussed earlier, high-frequency signals can
be propagated over longer distances than lower frequency signals. The
derivatives of amplitude modulation include the following.
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The
modulating Signal (Input Signal) Vm = Vm sin ωmt
Where
Vm is the instantaneous value and Vm is the maximum value of the modulating
(input) signal.
fm
is the frequency of the modulating (input) signal and ωm = 2π fm
The
Carrier Signal Vc = Vc sin ωct
Where
Vc is the instantaneous value and Vc is the maximum value of the carrier
signal, fc is the frequency of the carrier signal and ωc = 2π fc.
The amplitude modulation equation is:
VAM
= Vc +Vm= Vc + Vm sin ωmt
vAM
= VAM sin θ = VAM sin ωct
=
(Vc + Vm sin ωmt) sin ωct
=
Vc (1+m sin ωmt) sin ωct where m is given by m = Vm/Vc
Modulation Index
Modulation
Index is defined as the ratio of the amplitude of the modulating signal and the
amplitude of the carrier signal. It is denoted by ‘m’
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Modulation
Index m = Vm/Vc
Modulation
Index is also known as Modulation factor, Modulation coefficient or degree of
modulation
“m” shall have a value between 0 and 1.
“m” when expressed as a percentage is called % modulation.
Vm
= Vmax-Vmin/2
Vc
= Vmax-Vm
Vc
= Vmax- (Vmax-Vmin/2) = Vmax + Vmin/2
Therefore,
Vm/Vc = (Vmax-Vmin/ Vmax + Vmin)
Critical Modulation
It
happens when modulation Index (m) =1. Note, during critical modulation Vmin =0
M
= Vm/Vc = (Vmax-Vmin/ Vmax + Vmin) = (Vmax/Vmax) = 1
Substitute
V m = 0 Therefore at critical modulation m = Vm/Vc
Substitute
m = 1. Therefore at critical modulation Vm = Vc
What is Over Modulation and Sidebands
of AM?
This
can occur when m>1
That
is (Vm / Vc) > 1. Therefore Vm > Vc. In other words, the modulating
signal is greater than the carrier signal.
The
AM signal will generate new signals called sidebands, at frequencies other than
fc or fm.
We
know that VAM = (Vc + m Vm sin ωmt) sin ωct
We
also know that m = Vm / Vc. Therefore Vm = m.Vc
Therefore,
Case1: Both input signal and carrier
signal are sine waves.
VAM = (Vc + m Vc sin ωmt) sin ωct
= Vc sin ωct + m Vc sin ωmt . Sin ωct
Recall
SinA SinB = 1/2 [ cos (A-B) – cos (A + B)]
Therefore
VAM = Vc sin ωct + [ mVc/2 cos (ωc – wm)t] ─ [mVc/2 cos (ωc + wm)t]
Where
Vc sin ωct is carrier
mVc/2
cos (ωc – wm)t is lower side band
mVc/2
cos (ωc + wm)t I supper sideband
Therefore
AM signal has three frequency components, Carrier, Upper Sideband and Lower
Side Band.
Case 2: Both input signal and carrier
signal are cos waves.
VAM
= (Vc + m Vc cos ωmt) cos ωct
=
Vc cos ωct + mVc cos ωmt. cos ωct
Recall
Cos A Cos B =1/2 [cos (A ─B) + cos (A + B)]
Therefore
VAM = Vc cos ωct + [mVc/2 cos (ωc – wm)t] + [mVc/2 cos (ωc + wm)t]
Where
Vc cos ωct
mVc/2
cos (ωc – wm)t is lower sideband
mVc/2
cos (ωc + wm)t supper sideband
Therefore
AM signal has three frequency components, Carrier, Upper Sideband and lower
side Band
Bandwidth of AM
The
bandwidth of a complex signal like AM is the difference between its highest and
lowest frequency components and is expressed in Hertz (Hz). Bandwidth deals
with only frequencies.
As
shown in the following figure
Bandwidth
= (fc – fm) – (fc + fm) = 2 fm
The
power levels in carrier and sidebands
There
are three components in the AM wave. Unmodulated carrier, USB & LSB.
Total
Power of AM is = Power in the
Unmodulated
carrier + Power in USB +Power in LSB
If
R is the load, then Power in AM = V2c/R + VLSB2/R + VUSB2/2
Carrier Power
Peak
carrier Power = V2c/R
Peak
Voltage = Vc, therefore RMS voltage = Vc/√2
RMS
carrier power =1/R [Vc/√2]2= V2c/ 2R
RMS
Power in Side bands
PLSB
= PUSB = VSB2/R = 1/R [mVc/2/√2]2
= m2 (Vc)2 / 8R= m2/4 X V2c/2R
We
know that V2c/2R =Pc
Therefore
PLSB = m2/4 x Pc
Total
power = v2c/2R + m2Vc2/8R + m2Vc2/8R
v2c/2R
[ 1 + (m2/4) + (m2/4)] = Pc [ 1 + (m2/4)
+ (m2/4)]
PTotal
= Pc [ 1 + m2/2 ]
Modulation
Index in terms of Total Power (PTotal) and Carrier Power (Pc)
PTotal
= Pc [1+m2/2]
PTotal/
Pc = [1+m2/2]
m2/2
= PTotal/Pc – 1
m
= √2 (PTotal/Pc – 1)
Transmission Efficiency
In
AM there are three power components Pc, PLSB and PUSB
Out
of these Pc is an unmodulated carrier. It is wasteful as it carries no
information at all.
The
two sidebands carry, all the useful information and therefore useful power is spent
only in Sidebands
Efficiency (η)
A
ratio of transmitted power which contains the useful information (PLSB + PUSB)
to the total transmitted power.
Transmission
efficiency = (PLSB + PUSB) / (PTotal)
η = Pc [m2/4 + m2/4] / Pc [1 = m2/2] = m2/2+m2
η % = (m2/2+m2) X 100
Amplitude Demodulation
The
inverse of the modulator and it recovers (decodes) the original signal (what
was the modulating signal at the transmitter end) from the received AM signal.
Envelop Detector
AM
is a simple wave, and the detector is a demodulator. It recovers the original
signal (what was the modulating signal at the transmitter end) from the
received AM signal. The detector consists of a simple half-wave rectifier which
rectifies the received AM signal. This is followed by a low pass filter which
removes (bypasses) the high-frequency carrier waveform the received signal. The
resultant output of the low pass filter will be the original input (modulating)
signal.
The
incoming AM signal is transformer coupled HW rectifier conducts during positive
cycles of AM and cuts off negative cycles of AM. Filter capacitor C filters
(bypasses) the high-frequency carrier (fc) and allows only the lower frequency
(fm). Thus, the filter output is the original input (modulating) signal.
Types of Amplitude Modulation
The
different types of amplitude modulations include the following.
1)
Double sideband-suppressed carrier (DSB-SC) modulation
The
transmitted wave consists of only the upper and lower sidebands
But
the channel bandwidth requirement is the same as before.
2)
Single sideband (SSB) modulation
The
modulation wave consists only of the upper sideband or the lower sideband.
To
translate the spectrum of the modulating signal to a new location in the
frequency domain.
3)
Vestigial sideband (VSB) modulation
One
sideband is passed almost completely and just a trace of the other sideband is
retained.
The
required channel bandwidth is slightly in excess of the message bandwidth by an
amount equal to the width of the vestigial sideband.
Advantages & Disadvantages of
Amplitude Modulation
The
advantages of amplitude modulation include the following.
Amplitude
modulation is economical as well as easily obtainable
It
is so simple to implement, and by using a circuit with fewer components it can
be demodulated.
The
receivers of AM are inexpensive because it doesn’t require any specialized
components.
The
disadvantages of amplitude modulation include the following.
The
efficiency of this modulation is very low because it uses a lot of power
This
modulation uses amplitude frequency several times to modulate the signal by a
carrier signal.
This
declines the original signal quality on the receiving end & causes troubles
in the signal quality.
AM
systems are susceptible toward the generation of noise generation.
The
applications of amplitude modulation limits to VHF, radios, & applicable
one to one communication only
Thus,
this is all about an overview of amplitude modulation. The main advantage is
that since a coherent reference is not required for demodulation as long as 0
< u < 1, the demodulator becomes simple and inexpensive. The main
disadvantage of this modulation is the wastage of carrier power. It is used in
many commercial broadcast applications, it is sufficient to justify its use.
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CONTACT
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Company
Address: Buliding A,the first industry park of Guanlong,Xili
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Source:
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