Post on 23-Feb-2016
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EET 2351Lecture 2
Professor: Dr. Miguel Alonso Jr.
Outline
Baseband signals Carrier Modulation of Baseband Signals Types of Modulation Methods Frequency, Spectrum, and Bandwidth Generation of Baseband Signals
Baseband Signals
Baseband signals are defined as the band of frequencies delivered by the source in a communication system
Examples are: Voice Composite Video Signal
Carrier Modulation of Baseband Signals
In order for transmission over any medium, the baseband signal is used to modulate a carrier signal Carrier is typically a sine wave of a higher
frequency than that of the largest frequency Examples are:
Frequency Modulation Amplitude Modulation
Types of Modulation Methods
Several Types of Modulation methods exist Analog Modulation Digital Modulation Digital Baseband Modulation Pulse Modulation
We will be covering the last three types of modulation schemes
Digital Modulation
The aim is to transmit a digital bit stream over an analog band-passed communication channel
Examples include: Phase Shift Keying Frequency Shift Keying Amplitude Shift Keying
Digital Baseband Modulation
The aim is to transmit digital bit streams over an analog low-pass channel
Examples include: Unipolar Coding Non-return-to-zero (NRZ) coding Manchester Coding
Pulse Modulation
Pulse modulation usually aims at transmitting an analog signal over an analog low-pass channel as a quantized signal by modulation a pulse train Examples include:
PAM PCM PWM
Frequency, Spectrum, and Bandwidth, and misc topics
Guided media – waves are guided along a physical path Exp: twiste pair, coaxial cable, optical fiber
Unguided media – provide a means for tranmission, but no guide 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
Discrete, continuous Periodic, A periodic
0 10 20 30 40 50 60 70 80 90 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Characteristics
Peak Amplitude Frequency : ω = 2*π*f (Period T = 1/f) Phase
Frequency Domain Concepts
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
s(t) = A*sin(ω*t + φ)
Power Calculations, SNR and Dynamic Range
P=IV, P=V2/R, P=I2/R
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 ?
Sampling Frequency
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.
max2 ff s
Pulse Width
Pulse width is measured at about 50% of the amplitude of the pulse. Exp: 0.25ms or 250us.
Rise Time and Fall Time
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 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
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. 1. Square it. 2. Mean it. 3. Root it.
Square it
1. Square it: Square the positive voltage. Call this Vp2. Square the negative voltage. Call this Vn2. Vp2 = 5.02 = 25 Vn2= (-2.0)2 = 4.0
Vp and Vm are the high and low voltages respectively.
Mean it
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 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
Baseband signals can come from many sources
They are essentially the information or intelligence that is to be transmitted.
Examples: Voice Video Tire Air Pressure: Car telemetry system Keystrokes
Lab Reports
Title Page Table of Contents Abstract/Summary Introduction Experimental Procedure Results: data, figures, graphs, tables, etc. Discussion Conclusions
Simulation Using Pspice or Multisim