نیمسال دوّم افشین همّت یار دانشکده مهندسی کامپیوتر ارتباطات داده (883-40) مدولاسیون های موج پیوسته 1

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 2

Introduction Continuous-Wave Modulation System Transmitter Receiver 3

Amplitude Modulation (1) Sinusoid Carrier Wave: Amplitude Modulation: 4

Amplitude Modulation (2) Oldest method of modulation Simple in implementation Wasteful of power Wasteful of bandwidth 5

Linear Modulation Schemes (1) – Double Side-Band Suppressed Carrier (DSB-SC) Modulation – Single Side-Band (SSB) Modulation – Vestigial Side-Band (VSB) Modulation 6

Linear Modulation Schemes (2) DSB-SC Modulation 7

Linear Modulation Schemes (3) DSB-SC Demodulation 8

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. 9

Linear Modulation Schemes (5) VSB m(t) >> >> m’(t) 10

Frequency Translation 11

Frequency-Division Multiplexing 12

Angle Modulation Phase Modulation Frequency Modulation 13

Frequency Modulation (1) Narrowband FM: β > 1 14

Frequency Modulation (2) 1 percent bandwidth 15

Nonlinearity Effects in FM Systems No sensitive to amplitude nonlinearities but sensitive to phase nonlinearities such as AM-to-PM Conversion 16

Superheterodyne Receiver Carrier–frequency tuning Filtering Amplification 17

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: N 0 /2 18

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. 19

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. 20

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  21

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.  22

Noise in AM Receivers using Envelope Detector (2) The envelope detector favor strong signals and penalizes weak signals (weak signal suppression). 23

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.  24

Noise in FM Receivers (2) I : Unmodulated carrier II : Sinusoidally modulated carrier Threshold: dB 25

Noise in FM Receivers (3) Threshold Reduction 26

Noise in FM Receivers (4) Pre-emphasis and De-emphasis Improvement Factor: 27

Noise in FM Receivers (5) Pre-emphasis and De-emphasis 28