Slide Notes Frequency Modulation

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Slide 1

Frequency ModulationBasic Theory

Najeeb Haider Zaidi

Slide Notes Frequency Modulation

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Slide 2

Frequency Modulators

Direct, Indirect FM Modulators

Slide Notes Frequency Modulation

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Slide 16

Basics

How would you define it?

Generation of Carrier

LC Oscillator

Crystal Oscillator

How to change the Frequency?

Varactor Diode

Slide Notes Frequency Modulation

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Slide 17

FM Modulators

FM Modulators

Direct FM Modulators

Varactor Diode Modulators

Reactance Modulator

Indirect FM Modulators

PM Modulators

Slide Notes Frequency Modulation

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Slide 18

Varactor Operation Variable Capacitance

Acts like a small Capacitor

Dielectric

Conducting Plates

Do all diodes posse characteristics?

Varactors are made with a wide range

of capacitance values, most units having

a nominal capacitance in the 1- to 200-

pF range.

Slide Notes Frequency Modulation

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Slide 19

Varactor Diode Modulator Circuit

Amplifier

DC Coupling Capacitors

BasebandProcessing

The capacitance of varactor diode D1 and L1 form the parallel tuned circuit of the oscillator.

The value of C1 is made very large so its reactance is very low.

C1 connects the tuned circuit to the oscillator and blocks the DC bias on the base of Q1.

The values of L1 and D1 fix the center carrier frequency.

The modulating signal varies the effective voltage applied to D1 and its capacitance varies.

Slide Notes Frequency Modulation

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Slide 20

Crystal Modulators

Crystal Oscillator

Crystal oscillators provide highly accurate frequencies and their stability is superior to LC

oscillators.

The frequency of a crystal oscillator can be varied by changing the value of capacitance in

series or parallel with the crystal.

By making the series capacitance a varactor diode, frequency modulation can be achieved.

The modulating signal is applied to the varactor diode which changes the oscillator frequency.

Slide Notes Frequency Modulation

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Slide 21

Reactance Modulator

A reactance modulator is a circuit that uses a transistor

amplifier that acts like either a variable capacitor or an

inductor.

Wide Range Frequency deviation

Highly Linear

Slide Notes Frequency Modulation

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Slide 22

Reactance Modulator

Class A Amplifier

Carrier Oscillator

Slide Notes Frequency Modulation

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Slide 23

Phase Modulators

Most modern FM transmitters use some form of phase modulation (PM) to produce indirect FM.

In PM the carrier oscillator can be optimized for frequency accuracy and stability.

Crystal oscillators or crystal-controlled frequency synthesizers can be used to set the carrier frequency accurately and maintain stability.

The output of the carrier oscillator is fed to a phase modulator where the phase shift is made to vary in accordance with the modulating signal.

Slide Notes Frequency Modulation

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Slide 24

Varactor Phase Modulator

A simple phase-shift circuit can be used as a phase modulator if the resistance or capacitance

can be made to vary with the modulating signal.

A varactor can be used to vary capacitance and achieve phase shift modulation.

Slide Notes Frequency Modulation

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Slide 25

Transistor Phase Modulator

A transistor can be used as a variable resistor to create a phase modulator.

A standard common emitter class A amplifier biased into the linear region is used in PM.

The transistor from collector to ground acts like a resistor.

The transistor’s resistance forms part of the phase shifting circuit.

Slide Notes Frequency Modulation

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Slide 26

Frequency Demodulators

Direct Demodulators, Indirect Demodulators

Slide Notes Frequency Modulation

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Slide 27

Slide Notes Frequency Modulation

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Slide 28

Slope Detector

The slope detector makes use of a tuned circuit and a diode detector to convert frequency

variations into voltage variations.

Slide Notes Frequency Modulation

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Slide 29

Pulse Averaging Discriminator

A pulse-averaging discriminator uses a zero crossing detector, a one shot multivibrator and a

low-pass filter in order to recover the original modulating signal.

The pulse-averaging discriminator is a very high-quality frequency demodulator.

Originally this discriminator was limited to expensive telemetry and industrial control

applications.

With availability of low-cost ICs, this discriminator is used in many electronic products.

Slide Notes Frequency Modulation

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Slide 30

Quadrature FM Detector

The quadrature detector is probably the single most widely used FM demodulator.

The quadrature detector is primarily used in TV demodulation.

This detector is used in some FM radio stations.

The quadrature detector uses a phase-shift circuit to produce a phase shift of 90 degrees at the

unmodulated carrier frequency.

Slide Notes Frequency Modulation

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Slide 31

Phase Locked Loop

Slide Notes Frequency Modulation

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Slide 32

Power of Angle Modulated Signal

1

2n

SBnRT thPPP

Power of the Rest Frequency Component

Power of nth Significant Sideband

Slide Notes Frequency Modulation

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Slide 33

References

Made By Najeeb Haider Zaidi

Wikipedia: FM Modulation, Modulation Index

Delmar Learning System: MU-04.pdf

FM Mathematics: http://cnx.org/content/m15482/latest/

Electronic Communication Systems by Wayne Tomasi