‫دانشکده مهندس ی کامپیوتر‬
‫ارتباطات داده (‪)40-883‬‬
‫مدوالسیونهای موجپیوسته‬
‫نیمسال ّ‬
‫دوم ‪92-93‬‬
‫افشین ّ‬
‫همتیار‬
‫‪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
,
Wideband 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: N0/2
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 modulationdemodulation 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: 10-11 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