Short Notes: Amplitude Modulation-Analog Communication | Communication System - Electronics and Communication Engineering (ECE) PDF Download

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Amplitude Mo dulation: Anal og Comm unication
Amplitude Mo dulation (AM) is a fundamen tal tec hnique in analog comm unication where the amplitude
of a high-frequency carrier signal is v aried in accordance with a lo w-frequency message signal. This
section co v ers the principles, mathematical represen tation, and k ey c haracteristics of AM.
Key Concepts
• Carrier Signal : A high-frequency sin usoidal w a v e, t ypically represen ted as c(t) = A
c
cos(2pf
c
t) ,
where A
c
is the carrier amplitude and f
c
is the carrier frequency .
• Message Signal : The informa tion signal, denoted as m(t) , usually a lo w-frequency signal.
• Mo dulation : The pro cess of em b edding the message signal on to the carrier signal b y v arying its
amplitude.
Mathematical Represen tation
The AM signal is expressed as:
s(t) = [A
c
+m(t)]cos(2pf
c
t)
where:
• A
c
: Carrier amplitude (unmo dulated).
• m(t) : Message signal.
Assuming m(t) =A
m
cos(2pf
m
t) , where A
m
is the message amplitude and f
m
is the message frequency ,
the AM signal b ecomes:
s(t) =A
c
[1+µcos(2pf
m
t)]cos(2pf
c
t)
where µ =
Am
Ac
is the mo dulation index (0=µ= 1 for distortion -free AM).
Mo d ulation Index
The mo dulation index µ indicates the degree of mo dulation:
µ =
A
m
A
c
=
V
max
-V
min
V
max
+V
min
where V
max
and V
min
are the maxim um and minim um amplitudes of the mo dulated signal.
• µ< 1 : Under-mo dulation (desired).
• µ = 1 : F ull mo dulation.
• µ> 1 : Ov er-mo dulation (causes distortion).
F requency Sp ectrum
The AM signal consists of:
• Carrier F requency : f
c
.
• Sidebands :
– Upp er Sideband (USB): f
c
+f
m
.
– Lo w er Sideband (LSB): f
c
-f
m
.
The frequency domain represen tation is:
S(f) =
A
c
2
d(f-f
c
)+
A
c
2
d(f+f
c
)+
µA
c
4
[d(f-(f
c
+f
m
))+d(f-(f
c
-f
m
))+d(f +(f
c
+f
m
))+d(f +(f
c
-f
m
))]
Bandwidth
The bandwidth of an AM signal is:
BW = 2f
m
where f
m
is the maxim um freque ncy comp onen t of the message signal.
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Amplitude Mo dulation: Anal og Comm unication
Amplitude Mo dulation (AM) is a fundamen tal tec hnique in analog comm unication where the amplitude
of a high-frequency carrier signal is v aried in accordance with a lo w-frequency message signal. This
section co v ers the principles, mathematical represen tation, and k ey c haracteristics of AM.
Key Concepts
• Carrier Signal : A high-frequency sin usoidal w a v e, t ypically represen ted as c(t) = A
c
cos(2pf
c
t) ,
where A
c
is the carrier amplitude and f
c
is the carrier frequency .
• Message Signal : The informa tion signal, denoted as m(t) , usually a lo w-frequency signal.
• Mo dulation : The pro cess of em b edding the message signal on to the carrier signal b y v arying its
amplitude.
Mathematical Represen tation
The AM signal is expressed as:
s(t) = [A
c
+m(t)]cos(2pf
c
t)
where:
• A
c
: Carrier amplitude (unmo dulated).
• m(t) : Message signal.
Assuming m(t) =A
m
cos(2pf
m
t) , where A
m
is the message amplitude and f
m
is the message frequency ,
the AM signal b ecomes:
s(t) =A
c
[1+µcos(2pf
m
t)]cos(2pf
c
t)
where µ =
Am
Ac
is the mo dulation index (0=µ= 1 for distortion -free AM).
Mo d ulation Index
The mo dulation index µ indicates the degree of mo dulation:
µ =
A
m
A
c
=
V
max
-V
min
V
max
+V
min
where V
max
and V
min
are the maxim um and minim um amplitudes of the mo dulated signal.
• µ< 1 : Under-mo dulation (desired).
• µ = 1 : F ull mo dulation.
• µ> 1 : Ov er-mo dulation (causes distortion).
F requency Sp ectrum
The AM signal consists of:
• Carrier F requency : f
c
.
• Sidebands :
– Upp er Sideband (USB): f
c
+f
m
.
– Lo w er Sideband (LSB): f
c
-f
m
.
The frequency domain represen tation is:
S(f) =
A
c
2
d(f-f
c
)+
A
c
2
d(f+f
c
)+
µA
c
4
[d(f-(f
c
+f
m
))+d(f-(f
c
-f
m
))+d(f +(f
c
+f
m
))+d(f +(f
c
-f
m
))]
Bandwidth
The bandwidth of an AM signal is:
BW = 2f
m
where f
m
is the maxim um freque ncy comp onen t of the message signal.
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P o w er in AM Signal
T otal p o w er in the AM signal:
P
total
=P
c
(
1+
µ
2
2
)
where:
• P
c
=
A
2
c
2
: Carrier p o w er.
• Sideband p o w er:
µ
2
Pc
4
p er sideband.
T yp es of AM
• Standard AM : Includes carrier and b oth sidebands.
• Double Sideband Suppressed Carrier (DSB-SC) : Carrier suppressed, only sidebands trans-
mitted.
• Single Sideband (SSB) : Only one sideband transmitted, carrier suppressed.
• V estigial Sideband (VSB) : One sideband and a p ortion of the other transmitted.
A dv an tages and Disadv an tages
• A dv an tages :
– Simple to implemen t.
– Compatible with simple receiv ers (en v elop e detectors).
• Disadv an tages :
– Ine?icien t p o w er usage (carrier carries no information).
– Susceptible to noise.
– Requires larger bandwidth than SSB or DSB-SC.
Applications
• AM radio broadcasting.
• Aircraft comm unication (VHF AM).
• Analog television (VSB mo dulation).
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