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