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EET 2351
Lecture 2
Professor: Dr. Miguel Alonso Jr.
Outline
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Baseband signals
Carrier Modulation of Baseband Signals
Types of Modulation Methods
Frequency, Spectrum, and Bandwidth
Generation of Baseband Signals
Baseband Signals
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Baseband signals are defined as the band of
frequencies delivered by the source in a
communication system
Examples are:
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Voice
Composite Video Signal
Carrier Modulation of
Baseband Signals
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In order for transmission over any medium,
the baseband signal is used to modulate a
carrier signal
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Carrier is typically a sine wave of a higher
frequency than that of the largest frequency
Examples are:
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Frequency Modulation
Amplitude Modulation
Types of Modulation Methods
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Several Types of Modulation methods exist
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Analog Modulation
Digital Modulation
Digital Baseband Modulation
Pulse Modulation
We will be covering the last three types of
modulation schemes
Digital Modulation
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The aim is to transmit a digital bit stream over
an analog band-passed communication
channel
Examples include:
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Phase Shift Keying
Frequency Shift Keying
Amplitude Shift Keying
Digital Baseband Modulation
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The aim is to transmit digital bit streams over
an analog low-pass channel
Examples include:
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Unipolar Coding
Non-return-to-zero (NRZ) coding
Manchester Coding
Pulse Modulation
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Pulse modulation usually aims at transmitting
an analog signal over an analog low-pass
channel as a quantized signal by modulation
a pulse train
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Examples include:
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PAM
PCM
PWM
Frequency, Spectrum, and
Bandwidth, and misc topics
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Guided media – waves are guided along a
physical path
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Unguided media – provide a means for
tranmission, but no guide
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Exp: twiste pair, coaxial cable, optical fiber
Exp: air, vacuum, sea water
Direct link – no intermediate devices
Point – to point – direct link with only two
devices sharing the medium
Digital and Analog Signals
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Discrete, continuous
Periodic, A periodic
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
80
90
100
Characteristics
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Peak Amplitude
Frequency : ω = 2*π*f (Period T = 1/f)
Phase
Frequency Domain Concepts
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Fundamental Frequency
Spectrum: Range of frequencies contained in
a signal
Absolute bandwidth: width of the spectrum
Effective bandwidth: bandwidth where the
majority of the energy in a signal is contained
DC component
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s(t) = A*sin(ω*t + φ)
Power Calculations, SNR and
Dynamic Range
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P=IV, P=V2/R, P=I2/R
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SNRdB = 10*log10 (SNR)
PdB = 10*log10 (P / 1 Watt)
VdB = 20*log10 (V/V0) : Gain In= 1volt, Out=
10volts. What is the gain in dB’s ?
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Sampling Frequency
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The sampling rate, sample rate, or sampling
frequency is defined as the number of
samples per second (or per other unit) taken
from a continuous signal to make a discrete
signal.
Nyquist criteria: Perfect reconstruction of a
signal is possible when the sampling
frequency is greater than twice the maximum
frequency of the signal being sampled.
f s  2  f max
Pulse Width
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Pulse width is measured at about 50% of the
amplitude of the pulse. Exp: 0.25ms or
250us.
Rise Time and Fall Time
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Rise Time is the time it takes to go from 10% to
90 % of the pulse amplitude.
Fall Time is the time it takes to go from 90% to
10 % of the pulse amplitude.
The rise time below is about 0.01ms or 10us.
The fall time is similar.
Duty Cycle
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The period, T, of the pulse train above is 1ms.
It can be measured from rising edge to rising edge,
or from falling edge to falling edge.
The first pulse occurs from 23.0ms to 23.5ms, so the
pulse width, pw or tp, is 0.5ms. The Duty Cycle, D, is
defined as the pulse width divided by the period.
D = pw/T = 0.5ms/1ms = 0.5
The Duty Cycle is frequently expressed as a
percentage. In this case, D = 50%.
RMS Voltage of Square
Pulses
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Frequency 1kHz. )eriod, T, is 1ms
The pulse width, pw, is 0.25ms
The duty cycle, D, is therefore 25%. The
steps to find the r.m.s. value are given below.
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1. Square it.
2. Mean it.
3. Root it.
Square it
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1. Square it: Square the positive voltage. Call
this Vp2. Square the negative voltage. Call
this Vn2.
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Vp2 = 5.02 = 25
Vn2= (-2.0)2 = 4.0
Vp and Vm are the high and low voltages
respectively.
Mean it
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2. Mean it: Compute the mean, or average.
Multiply the square the positive voltage, Vp2,
by the duty cycle, D. Multiply the square the
negative voltage, Vn2, by one minus the duty
cycle, (1-D). Add these two quantities. This is
the mean or average of the squares.
(Vp2 *D+ Vn2*(1-D))
= (25*.25+4.0*.75)
= 9.25
Root it
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3. Root it: Take the square root of the above mean.
This is the r.m.s. voltage.
9.251/2 = 3.0v
Note: The mean dc level or average voltage is
determined by both the voltage levels, and the duration
of these levels.
Mean DC = D * Vp + (D-1) * Vm
In this case, the signal spends 25% (0.25) of the time at
5v, and 75% (0.75) at -2v. D is 0.25, and (1-D) is 0.75.
Mean DC = 0.25 * 5v + 0.75 * (-2v) = -0.25v = -250mv
Generation of Baseband
Signals
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Baseband signals can come from many
sources
They are essentially the information or
intelligence that is to be transmitted.
Examples:
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Voice
Video
Tire Air Pressure: Car telemetry system
Keystrokes
Lab Reports
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Title Page
Table of Contents
Abstract/Summary
Introduction
Experimental Procedure
Results: data, figures, graphs, tables, etc.
Discussion
Conclusions
Simulation Using Pspice or
Multisim