1

92-93نیمسال دّو�م یار افشین هم�ت

دانشکده مهندسی کامپیوتر

(40-883ارتباطات داده )

پیوسته های موج مدّوالسیون

2

Continuous-Wave Modulations• Introduction• Amplitude Modulation• Linear Modulation Schemes • Frequency Translation• Frequency -Division Multiplexing• Angle Modulation• Frequency Modulation• Nonlinear Effects in FM Systems• Superheterodyne Receiver • Noise in CW Modulation Systems• Noise in Linear Receivers Using Coherent Detection • Noise in AM Receivers Using Envelope Detection• Noise in FM Receivers

3

Introduction

Continuous-Wave Modulation System

Transmitter

Receiver

4

Amplitude Modulation (1)Sinusoid Carrier Wave:

Amplitude Modulation:

5

Amplitude Modulation (2)

• Oldest method of modulation• Simple in implementation• Wasteful of power• Wasteful of bandwidth

6

Linear Modulation Schemes (1)

– Double Side-Band Suppressed Carrier (DSB-SC) Modulation– Single Side-Band (SSB) Modulation– Vestigial Side-Band (VSB) Modulation

7

Linear Modulation Schemes (2)DSB-SC Modulation

8

Linear Modulation Schemes (3)DSB-SC Demodulation

9

Linear Modulation Schemes (4)SSB

A low-power pilot carrier is transmitted in addition to the selected sideband.

A highly stable oscillator, tuned to the same frequency as the carrier frequency, is used in the receiver.

10

Linear Modulation Schemes (5)VSB

m(t) >> >> m’(t)

11

Frequency Translation

12

Frequency-Division Multiplexing

13

Angle Modulation

Phase Modulation

Frequency Modulation

14

Frequency Modulation (1)

Narrowband FM: β << 1 , Wideband FM: β >> 1

15

Frequency Modulation (2)

1 percent bandwidth

16

Nonlinearity Effects in FM Systems

No sensitive to amplitude nonlinearities but sensitive to phase nonlinearities such as AM-to-PM Conversion

17

Superheterodyne Receiver

• Carrier–frequency tuning• Filtering• Amplification

18

Noise in CW Modulation Systems (1)Channel Model: Additive White Gaussian Noise (AWGN) Receiver Model: Ideal Band-pass filter + Ideal demodulator

Power Spectral Density (PSD) of the Noise: N0/2

19

Noise in CW Modulation Systems (2)(SNR)C : Ratio of the average power of the modulated signal to the average power of channel noise in the message bandwidth, both measured at the receiver input.(SNR)O: Ratio of the average power of the demodulated message signal to the average power of the filtered noise, both measured at the receiver output.(Depends on the type of modulation used in the transmitter and the type of the demodulation used in the receiver.)

• The figure of merit may equal one, be less than one, or be greater than one, depending on the type of modulation used.• The higher the figure of merit, the better will the noise performance of the receiver be.

20

Noise in CW Modulation Systems (3)

Requirements for comparison between different modulation-demodulation systems are:

• The modulated signal transmitted by each system has the same average power.

• The channel noise has the same average power measured in the message band-width.

21

Noise in Linear Receivers using Coherent Detection

• Coherent SSB Receiver same as a coherent DSB-SC receiver• Both cases same as transmission without modulation• No trade-off between noise performance and bandwidth

22

Noise in AM Receivers using Envelope Detector (1)

• The figure of merit of an AM receiver using envelope detection is always less than unity.• Due to wastage of transmitter power, the noise performance of a full AM receiver is always inferior to that of a DSB-SC receiver.

23

Noise in AM Receivers using Envelope Detector (2)

• The envelope detector favor strong signals and penalizes weak signals (weak signal suppression).

24

Noise in FM Receivers (1)

• Capture effect: The FM receiver locks to the stronger signal.• When the signals are nearly equal strength, the receiver fluctuates back and forth between them.

25

Noise in FM Receivers (2)

I : Unmodulated carrier

II: Sinusoidally modulated carrier

Threshold: 10-11 dB

26

Noise in FM Receivers (3)

Threshold Reduction

27

Noise in FM Receivers (4)Pre-emphasis and De-emphasis

Improvement Factor:

28

Noise in FM Receivers (5)Pre-emphasis and De-emphasis