am lecture ruet

7/28/2019 Am Lecture Ruet

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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

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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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