EET 2351 Lecture 2

1. EET 2351Lecture 2 Professor: Dr. Miguel Alonso Jr.

2. Outline • Baseband signals • Carrier Modulation of Baseband Signals • Types of Modulation Methods • Frequency, Spectrum, and Bandwidth • Generation of Baseband Signals

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Digital and Analog Signals • Discrete, continuous • Periodic, A periodic

11. Characteristics • Peak Amplitude • Frequency : ω = 2*π*f (Period T = 1/f) • Phase

12. 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

13. s(t) = A*sin(ω*t + φ)

14. 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 ?

15. 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.

16. Pulse Width • Pulse width is measured at about 50% of the amplitude of the pulse. Exp: 0.25ms or 250us.

17. 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.

18. 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%.

19. 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.

20. 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.

21. 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

22. 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

23. 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

24. Lab Reports • Title Page • Table of Contents • Abstract/Summary • Introduction • Experimental Procedure • Results: data, figures, graphs, tables, etc. • Discussion • Conclusions

25. Simulation Using Pspice or Multisim