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Chapter 5. Angle Modulation Chapter 5. Angle Modulation

Chapter 5. Angle Modulation - PowerPoint Presentation

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Chapter 5. Angle Modulation - PPT Presentation

Husheng Li The University of Tennessee Phase and Frequency Modulation Consider the standard CW signal We define the total instantaneous angle Phase and Frequency Modulation Phase modulation PM ID: 550111

modulation signal frequency phase signal modulation phase frequency modulated output spectrum angle modulator bandwidth wave tone carrier arbitrary circuit

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Slide1

Chapter 5. Angle Modulation

Husheng Li

The University of TennesseeSlide2

Phase and Frequency Modulation

Consider the standard CW signal

We define the total instantaneous angleSlide3

Phase and Frequency Modulation

Phase modulation (PM)

Frequency modulation (FM)Slide4

Characteristics of Angle Modulation

The amplitude of an angle modulated wave is constant.

The message resides in the zero crossings alone, providing the carrier frequency is large.

The modulated wave does not resemble the message waveform.Slide5

Narrowband PM and FM

We can expand the signal (using Taylor’s expansion)

The spectrum is given by

Hence, the signal has a bandwidth of 2W.Slide6

Example of Narrow Band Angle Modulation

Both PM and FM have carrier component.Slide7

Tone Modulation

We can allow a 90 degree difference in the modulating tones:

Β

is called the modulation index for PM or FM with tone modulation.Slide8

Spectrum of Narrowband Tone Modulation

When the modulation index is very small, we have

The spectrum is given bySlide9

Spectrum of Arbitrary Modulation Index

For a single tone signal with arbitrary modulation index, the modulated signal can be written as

w

here

j_n(β) is the Bessel function.Slide10

Bessel FunctionsSlide11

Characteristic of FM SpectrumSlide12

Homework 5

Deadline Oct. 14, 2013Slide13

Spectrum with Different Modulation Indices

We can either fix or fix Slide14

Multi-tone

Consider the case of multiple tones, e.g.,

The modulated signal can be written asSlide15

Periodic Modulation

When the signal is periodic, the Fourier series are given by

The modulated signal can be written asSlide16

Transmission Bandwidth

The generation and transmission of pure FM requires infinite bandwidth. Hence, our questions is: how much of the modulated signal spectrum is significant?

The Bessel function falls off rapidly for

There are M significant sideband pairs and 2M+1 significant lines all told. The bandwidth can be given by Slide17

IllustrationSlide18

Arbitrary Modulated Signal Bandwidth

For arbitrary modulating signal, the required bandwidth is given by

An approximation:

Carson’s rule

(deviation ratio)Slide19

Case of Phase Modulation

We can also define the phase deviation.

We haveSlide20

Linear Distortion

We consider an angle-modulated

bandpass

signal applied to a linear system:The

lowpass equivalent output spectrum is Slide21

Nonlinear Distortion

The output of signal through a nonlinear system is given bySlide22

Example: Clipper

A clipper has only two outputs

The output signal is given bySlide23

Procedure of ClipperSlide24

Direct FM

In direct FM, we use VCO to generate the frequency modulated by the signal.Slide25

Phase Modulator

Although we seldom transmit a PM wave, we are still interested in phase modulators because (1) the implementation is relatively easy; (2) the carrier can be supplied by a stable frequency source; (3) integrating the input signal to a phase modulator produces an FM output.Slide26

Switching-circuit Modulator

Larger phase shifts can be achieved by the switching-circuit modulator:Slide27

Indirect FM Transmitter

The integrator and phase modulator constitute a narrowband frequency modulator that generates an initial NBFM signal with instantaneous frequency:Slide28

Triangular-Wave FM

Triangular-wave FM is a modern and rather novel method for frequency modulation that overcomes the inherent problems of conventional CVOs and indirect FM systems.Slide29

Frequency Detection

A frequency detector, often called a discriminator, produces an output voltage that should vary linearly with the instantaneous frequency of the input.

Almost every circuit falls into one of the following four categories:

FM-to-AM conversion

Phase-shift discriminationZero-crossing detectionFrequency feedbackSlide30

FM-to-AM Conversion

Any device of circuit whose output equals the time derivative of the input produces FM-to-AM conversion:Slide31

PHASE-SHIFT Discriminators

Phase-shift discriminators involve circuits with linear phase response, in contrast to the linear amplitude response for slope detection:Slide32

Quadrature Detector

A phase-shift discriminator built with a network having group delay and carrier delay:Slide33

Zero Crossing DetectorSlide34

Interference

Interference refers to the contamination of an information-bearing signal by another similar signal, usually from a human source.

Interfering sinusoids: consider a receiver tuned to some carrier frequency. The total received signal isSlide35

Demodulated Output

Consider a weak interference. The demodulated output isSlide36

Deemphasis

The fact that detected FM interference is most severe at large values of |

f_i

| suggests a method for improving system performance with selective postdetection filtering, called

deemphasis filtering.