am lecture ruet
TRANSCRIPT
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Signal
Carrier
time
time
Modulating signal and carrier Wave
Vm Sin
t
Vc Sint
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time
Modulated wave
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If the equation of modulating signal is
And the carrier wave equation is
Then the modulated wave is the superposition of the above two waves.
Frequency spectrum of AM wave
The modulation index is defined as
c
m
V
Vm 0
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Expand the above equation using trigonometrically relation
The first term represent the un-modulated carrier.
Second and third terms represent the additional sidebands.
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6
Representation of AM wave
minmax
minmax
VV
VV
V
Vmc
m
2
2
minmax
minmaxmax
max
VV
VVV
VVV mc
2
minmaxVV
Vm
Modulation index
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Power relation in the AM wave
R
V
R
V
R
VP USBLSBcarrt
222
Total power in the modulated wave
R
V
R
V
R
VP cccarrc
2
2/ 22
2Carrier power
R
Vm
R
VmR
mV
R
VPP cccSBLSBUSB
2482
2/222222
Similarly
21
44
24242
2
22
22222
m
P
P
Pm
Pm
P
R
Vm
R
Vm
R
VP
c
t
ccc
ccc
t
We have from above relations
Maximum power in AM wave ct PP 5.1For m=1
means 100% modulation
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Current Calculation:It is the modulated current and Ic is the unmodulated current and R is the
radiation resistance of the antenna.
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Modulation by several sine waves
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643.04.0757.0222
12
2
2
2
2
1
2
mmm
mmm
t
t
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Generation of AM waves
Basic requirements
High Power
Maximum efficiency
Class C amplifier output is
current pulse, it can produce
complete pulse when passes
through the RL tune circuits.
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Generation of AM waves
Transmitter Block DiagramIf the output stage in a transmitter is
plate (collector) modulated in a lower
power transmitter , the system is called
High-level Modulation
If the modulation is applied at any other
point (base of transistor or grid of
Triode) of the output amplifier , the
system is called Low-level Modulation
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Transistor Collector Modulator
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SSB (single side band) techniques
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Why SSB?
Save power
Specially useful for
mobile systems
Disadvantages
Complicated
circuits needed in
both transmitter and
receiver.
For SSB, its
amplitude varies in
accordance with
modulating voltage,and its frequency
varies with frequency
of modulating signal.
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Suppression of carrier
bvi
bvai
Effect of nonlinear resistance on
added signals
In linear region
In diode or input of transistor
In the non linear portion
termshigherdvcvbvai 32
C
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If two signals are applied simultaneously then
)2()(
)()(
21222121
22121
vvvvcvvbai
vvcvvbai
If two signals are sinusoidal
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Conclusions
Inter-modulation occurs in audio and RF amplifier
In practical Modulator, out put is developed across a circuit tuned at carrier
frequency, and the bandwidth is just large enough to pass the two side bands.
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Output of LC tuned circuit
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The balanced Modulator uses the non linear
properties of diode or transistors
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Carrier frequency will be absent (heavily attenuated) from the output of theamplifier
Just two side bands appear at the out put
BUT How ? It needs mathematical conceptions
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How to suppress unwanted sidebands
The filter method
The phase shift method
The Third method
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Transmitted signal
bandwidth
Received signal
bandwidth
M th ti l t ti f FM
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Mathematical representation of FM
Instantaneous frequency of FM wave
)1( tCoskVff mmc
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Frequency Spectrum of FM
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Frequency Spectrum of FM
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