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King Fahd University of Petroleum & Minerals Electrical Engineering Department EE370 Communications Engineering LAB Manual Dr. Maan A. Kousa & Dr. Ali H. Muqaibel January 2011 (version 1.1) EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Contents INTRODUCTION TO “COMMUNICATION ENGINEERING I” LABORATORY ............................................ 3 EXP 1: GETTING FAMILIAR WITH THE LABORATORY EQUIPMENT....................................................... 7 EXP 2: SIMULATION OF COMMUNICATION SYSTEMS USING MATLAB ............................................. 11 EXP 3: REPRESENTATION OF SIGNALS & SYSTEMS ............................................................................ 15 EXP 4: SPEECH SIGNALS .................................................................................................................... 19 EXP 5: DSBSC MODULATION & DEMODULATION ............................................................................. 23 EXP 6: AM AND QAM ....................................................................................................................... 27 EXP 7: FM MODULATION.................................................................................................................. 31 EXP 8: FM DEMODULATION ............................................................................................................. 35 EXP 9: PCM ENCODING ..................................................................................................................... 39 EXP 10: PCM DECODING ................................................................................................................... 43 EXP 11: LINE CODING ....................................................................................................................... 47 EXP 12: DIGITAL MODULATION: FSK................................................................................................. 51 APPENDIX A: LABORATORY REGULATIONS AND SAFETY RULES ....................................................... 55 APPENDIX B: SAMPLE REPORT ......................................................................................................... 56 Kousa & Muqaibel Contents 2 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Introduction to “Communication Engineering I” Laboratory Purpose of “Communication Engineering I” Laboratory The goals of the communication laboratory are: 1. to allow you to perform experiments that demonstrate the theory of signals and communication systems that are discussed in course, 2. to introduce you to some of the electronic blocks that make up communication systems (which may not be discussed in the lecture course because of time limitations) , and 3. to familiarize you with proper laboratory procedure, including precise record- keeping, logical troubleshooting, safety, and learning about the capabilities and limitations of your equipment. Introduction This document contains the laboratory experiments to accompany the course EE 370 “Communications Engineering I”, offered by Electrical Engineering Department, KFUPM. The document contains twelve experiments, four on basic and general background, four on analog modulation, and four on digital modulation. The four basic experiments cover introduction to the laboratory equipment, simulation of communication systems using MATLAB, time- and frequency-domain representation of signals, and processing of speech signals. The analog modulation part covers the generation and detection of Double-Side Band Suppressed Carrier (DSBSC) modulation, Double-Side Band With Carrier (also known as AM) modulation, Quadrature Amplitude Modulation (QAM), and Frequency Modulation. The digital modulation experiments include PCM encoding and decoding, line codes and digital carrier modulation (ASK and FSK). Each experiment, whenever applicable, contains the following sections: Objectives: where the expected achievements by the end of the experiment are stated. Introduction: where the theory of the subject is reviewed. The introduction is kept brief, assuming the student has covered the material in detail in class, or can refer to his textbook for further reading. System Modules: where the main new modules to be used in the experiment are described. Lab Work: leading the student on how to run the experiment. The lab work is organized in parts in order to have a clear and integrated structure of the work. Kousa & Muqaibel Introduction to “Communication Engineering I” Laboratory 3 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Post-Lab Work: extra questions and tasks for the student to carry after the lab, and include in the lab report. General Laboratory Procedure While there is no specific document to be submitted at the beginning of the Lab –unless your instructor advises you otherwise-, you are expected to read the experiment fully before you come to the laboratory. Interestingly, you can even try parts of the experiment at home. Here is a list of programs that will equip you with a virtual lab at your home: Tool Function Link TutorTIMS® Virtual Lab (Modules,..etc) Picoscope® Oscilloscope & Spectrum Analyzer Matlab® Simulation Tool In addition to the experiment write up, a Lab Sheet has been prepared for every experiment. The Lab Sheet is a working document, designed to help students record all lab activities (measurements, observations, answers to questions in the lab manual, …). The student must have his instructor sign the sheet before he leaves. The material in the sheet shall be utilized in writing the report. Plots from the PC-based oscilloscope and spectrum analyzer may be saved on a storage media (or student file-box if network is available) to reproduce them later in the report. The lab sheets for the 12 experiments are collected in one booklet separate from this document. A set of Laboratory Regulations and Safety Rules are attached in Appendix A. All students have to observe them carefully. MATLAB will be frequently invoked as part of the post-lab work, mainly in the form of designing a simulation counterpart for the experimental work. Such exercise will improve the student programming skills, and acquaint him with the most frequently-encountered functions and techniques for simulating communication systems. It is the sole responsibility of the student to learn the basics of MATLAB. Every student should submit a report on each experiment. The report must be self- contained, and can be read independent from the lab manual. All axes in all graphs should be clearly labeled. If there is more than one trace in the plot, they should be clearly labeled. A sample report is attached in Appendix B. Troubleshooting Things will not always go as expected; this is the nature of the learning process. While testing a communication block, if the output signal is not what you expect, don't just try things at random, i.e replacing wires, rotating knobs, and toggling switches, hoping to get lucky. Rather, think before you do anything. If you do so you will avoid wasting time going down dead-end streets. Kousa & Muqaibel Introduction to “Communication Engineering I” Laboratory 4 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Be logical and systematic. First, look for obvious errors that are easy to fix. Is your measuring device correctly set and connected? Are you looking at the proper scale? Is the power supply set for the correct voltage? Is the signal generator correctly set and connected? And so on. Next, check for obvious misconnections or broken connections, at least in simple circuits. As you work through your circuit, use your lab sheet to record tests and changes that you make as you go along; don't rely on your memory for what you have tried. Identify some test points in the system at which you know what the signal should be, and work your way backwards from the output through the test points until you find a good signal. Now you have a section of the system to focus your efforts on. Here is where a little thought about laying out your board before connecting it up will pay off; if your system looks like a jungle, it is going to be very hard to troubleshoot, but if it is well organized and if the wires are short, it is going to make your job a lot easier. Final remark: if you do discover a bad module or wire, do not just throw it back in the box. Tell your instructor or the lab technician about it. Neatness When you have finished for the day, return all modules to their proper storage bins, return all test leads and probes to their storage racks, return all equipment to its correct location, and clean up the lab station. If appropriate switch off the unneeded equipments. We hope you an enjoyable learning experience Kousa & Muqaibel Introduction to “Communication Engineering I” Laboratory 5 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL This page is intentionally blank. All Experiments start with odd pages for double-sided printing Kousa & Muqaibel Introduction to “Communication Engineering I” Laboratory 6 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Exp 1: Getting Familiar with the Laboratory Equipment Objectives • Learn the various components and conventions of the lab equipment from TIMS. • Use the data sheets to learn about the operation, parameters and limitations of system modules. • Explore the features and capabilities of the PC-based oscilloscope and spectrum analyzer. • Perform basic modeling using TIMS. TIMS Overview Throughout the course, we will be using the laboratory equipment 301C PC-based from TIMS® to complement and demonstrate the theoretical part of the course. We will devote this experiment to introduce the equipment and get familiar with its usages. TIMS is a telecommunications modeling system that models block diagrams representing telecommunications systems. Physically, TIMS is a dual rack system; the upper rack accepts up to 12 plug-in cards, or modules; the lower rack houses a number of fixed modules, as well as the system power supply. Plug-in Modules Fixed Modules Figure 1: TIMS 301-C System Unit The modules are simple electronic circuits, which serve as basic communications building blocks. Each module, fixed or plug-in, has a specific function; functions fall into three categories: 1. Signal Generation - oscillators, variable DC, etc 2. Signal Processing - multipliers, filters, etc 3. Signal Measurement - frequency counter, PC-based instrument inputs. Kousa & Muqaibel Exp 1: Getting Familiar with the Laboratory Equipment 7 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Some of those modules are classified as basic modules while others are advanced modules. The fixed modules are all basic. They include: BUFFER AMPLIFIERS, FREQUENCY AND EVENT COUNTER, HEADPHONE AMPLIFIER, MASTER SIGNALS, TRUNK PANEL, VARIABLE DC and PC- BASED INSTRUMENT INPUT. The list of available plug-in modules is shown in the table below. Module Type Module Type 1 AUDIO OSCILLATOR Basic 12 VOLTAGE CONTROL OSCILLATOR 1 Basic 2 ADDER Basic 13 VOLTAGE CONTROL OSCILLATOR 2 Basic 3 DUAL ANALOG SWITCH Basic 14 60KHz LOW PASS FILTER Basic 4 MULTIPLIER Basic 15 QUADRATURE UTILITIES Advanced 5 PHASE SHIFTER Basic 16 LINE CODE ENCODER Advanced 6 QUADRATURE PHASE SHIFTER Basic 17 LINE CODE DECODER Advanced 7 SEQUENCE GENERATOR Basic 18 100KHz CHANNEL FILTER Advanced 8 UTILITIES Basic 19 PCM ENCODER Advanced 9 TUNEABLE LOW PASS FILTER 1 Basic 20 PCM DECODER Advanced 10 TUNEABLE LOW PASS FILTER 2 Basic 21 BIT CLOCK GENERATOR Advanced 11 TWIN PULSE GENERATOR Basic 22 SPEECH MODULE Advanced A data sheet for each module describing its input(s), output(s), configurable parameters and function can be found in the User Manuals (Basic and Advanced) available in the lab bench drawers. A soft copy is also available on all laboratory computers’ desktop. All TIMS modules conform to the following conventions: • Signal interconnections are made via front panel sockets • Sockets on the left hand side are for module inputs. • Sockets on the right hand side are for module outputs. • Yellow sockets are for analog signals. • Red sockets are for digital signals. • Analog signals are held near the level of 4V p-p. • Digital signals are TTL level, 0 to 5 V. • The green socket is the system Ground. • Any plug-in module may be placed in any of the 12 positions of the upper rack. • All modules use the back plane bus to obtain power supply. • The modules can be plugged-in or removed without turning off the power. It is important to note that: • The plug-in modules are not firmly locked in the rack, and need to be held in position while interconnecting leads are removed. • When removing the leads, hold them from their solid heads and DO NOT PULL them from the flexible segment, in order not to damage the wires. • There are 22 plug-in modules. Make sure you leave them in sequence in the storage shelves. Kousa & Muqaibel Exp 1: Getting Familiar with the Laboratory Equipment 8 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Oscilloscope and Spectrum Analyzer 1 TIMS is equipped with a fixed module, PC-BASED INSTRUMENT INPUTS , that provides interface with display devices, namely oscilloscope and spectrum analyzer. Either one can be physical stand-alone equipment or soft PC based. The connection to physical display devices is provided by coaxial cords, whereas the connection to the soft devices is provided through USB connection (already connected from the back panel). The application that runs the soft oscilloscope and spectrum analyzer in our lab is called picoscope®, and can be started from the shortcut on the PC. The DISPLAY INTERFACE module can take up to 4 signals on channels A1, A2, B1 and B2, but allows 2 of them (one from A and one from B) to be viewed simultaneously. The channels can be selected by means of two mechanical switches on the front panel of the module. If the displayed signal seems to be sliding left and right or changing too fast, then the oscilloscope has to be triggered. Triggering is some form of synchronization that provides a reference point for a periodic waveform. Without triggering, each sweep starts from a different instant of the period, resulting in unstable display. It is important to consider which of the many signals present will be used to trigger the oscilloscope. Use a periodic signal with the longest period from among the displayed signals, or use an external signal if needed. External triggering is connected to Channel-E of the DISPLAY INTERAFCE module. You have been exposed to the oscilloscope before, but the spectrum analyzer may be new to you. The spectrum analyzer is a device that displays the frequency composition of the signal. The horizontal axis represents the frequency whereas the vertical axis represents the magnitude. Because of the large variation of the magnitude spectrum, the vertical axis is usually set to dB scale. Note that X = 20 log(X). For example, if A is 40 dB below B , then dB dB dB B/A = 100. The decibel symbol is often qualified with a suffix that indicates which reference quantity has been used. For example, dBm indicates that the reference quantity (0 dBm) is one milliwatt, while dBu indicates that the reference quantity (0 dBu) is one microwatt. When observing the signal spectrum on the spectrum analyzer, you will notice a lot of “noise” all over the frequency axis. This is due to the circuit components. However, the noise level is extremely low, in the range of -60 dB or even less, compared to the signal level (i.e. one thousands of the signal level); it can therefore be neglected. You have many options to plot the results you see on the picoscope. One option is to save the data in .mat or .csv. In this case you can import the data to MATLAB or MS Excel and reproduce the plot. You may, alternatively, save the plot directly as .gif. You can download a fully functioning demo version of PICOSCOPE (PICOSCOPE 3204) from the following site: http://www.picotech.html/software.html In this experiment, we will introduce the fixed modules in addition to the ADDER plug-in module. 1 The name of this module is not intuitive. We will instead refer to it as DISPLAY INTERFACE module. Kousa & Muqaibel Exp 1: Getting Familiar with the Laboratory Equipment 9 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Lab Work 1. Read the data sheet of the ADDER in the TIMS Manuals-Basic Modules. Which of the following equations can be implemented using the ADDER and which cannot? Write your answers in the Lab Sheet. -2 cos(2π 2x106t) - 1.5 cos(2π 2x105t); -1.3 cos(2π 2x104t)x(t) – 0.5 sin(2π 2x103t); -2.5 cos(2π 2x104t)x(t) – 10.5 sin(2π 2x103t); 1.3 cos(2π 2x104t)x(t) + 0.5 sin(2π 2x103t). 2. Use the FREQUENCY COUNTER module to verify the frequencies of the following four signals from the MASTER SIGNALS: 100 kHz sine, 8.3 kHz Clock, 2 kHz TTL and 2 kHz sinusoid. Note down the values. Warning: The FREQUENCY COUNETR module accepts TTL and analog inputs. ONLY ONE OF THEM SHOULD BE CONNECTED AT A TIME, otherwise you may get erroneous measurement. 3. Connect the previous four signals of the MASTER SIGNALS module to the four inputs of the DISPLAY INTERFACE. Use the switches to display them on the oscilloscope (picoscope). Measure the amplitude of each signal and note them down in the Lab Sheet. 4. Use the VARIABLE DC, BUFFER AMPLIFIERS and ADDER modules to generate the 3 signal 3cos(2πx2x10 t)+6 V. Draw the modules and show the connections. Let your instructor verify the waveform. 5. Observe and plot the spectra of each of the four signals of the MASTER SIGNALS module. a. Do the spectra plots coincide with your expectations? Explain. b. How far is the noise level below the signal level? 6. Using a 2 kHz sinusoidal signal on one channel and 8.33 kHz digital signal on the other channel, familiarize yourself with the picoscope by exploring the following features. Feature Switch between oscilloscope and frequency analyzer on the same view Display one or both channels on the same view (window) Separate the two channels on the same view so that they are non- overlapping (do it manually and auto) Change the setting of the axes. Take a snap shot or continuous scan Zoom in a specific segment of the graph Display measurements of DC value, frequency, period, … Use horizontal and vertical markers Set the oscilloscope on external triggering Create time view and spectrum view and save them Kousa & Muqaibel Exp 1: Getting Familiar with the Laboratory Equipment 10 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Exp 2: Simulation of Communication Systems Using MATLAB Objectives: The main objective of this session is to learn the basic tools and concepts for simulating communication systems using MATLAB. Introduction MATLAB is a user-friendly, widely used software for numerical computations. MATLAB is vector-oriented, that is, it mainly deals with vectors (or matrices). It is assumed that you have used MATLAB before, and you can do simple operations, as well as create and run .m files. Some useful tutorials can be found on the EE 370 course/lab website. If you need help on how to start working on MATLAB, we advise you to read Matlab Primer available in the internet. Our focus in this session will be on using MATLAB for simulating communication systems. Instead of going in the traditional approach of explaining items individually, we will work through one complete example, and introduce the application as we go. Case Study: Write a MATLAB program to simulate the following system z(t) Low Pass Full-Wave y(t) g(t) m(t) Filter B = 1 kHz Rectifier c(t) 3 where m(t) = exp(-100t) ; c(t) = cos(2π 10 t) m-File: % Define the time interval ts=0.00001; t= -0.1:ts:0.1; % Define the functions m(t) and c(t) m=exp(-100abs(t)); c=cos(2pi1000t); Kousa & Muqaibel Exp 2: Simulation of Communication Systems Using MATLAB 11 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL % Performe the multiplication g=m.c; % Perform full-wave rectification y=abs(g); % Create the filter cutoff=1000; a b=butter(5,2cutoffts); % Get the output after the filter; z=filter(a,b,y); % Plot the input and output on the same graph figure (1) plot(t,m,t,z); legend('Input Signal','Output Signal') xlabel ('time') ylabel('amplitude') title ('Case Study') % Finding the FT of the signals M=abs(fftshift(fft(m))); G=abs(fftshift(fft(g))); Y=abs(fftshift(fft(y))); Z=abs(fftshift(fft(z))); % Creating the vector for the frequency axis f=-length(t)/2:length(t)/2-1/(length(t)ts); % Plotting all FT on one sheet, in a 2x2 matrix format figure (2) subplot (221) plot(f,M) subplot(222) plot(f,G) subplot (223) plot(f,Y) subplot(224) plot(f,Z) Discussion % Define the time interval This is usually the first step in any simulation. There are three parameters to define: the beginning of the interval, the step size, the end of the interval. The beginning and end of the Kousa & Muqaibel Exp 2: Simulation of Communication Systems Using MATLAB 12 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL interval are intuitive; for periodic signals you want to cover 3-5 periods; for non-periodic signals, you usually want to cover the non-zero part of the signal. The selection of the step size is crucial for the accuracy of the simulation. You need enough sample points to represent the signal. Usually, the step size is taken to be of the order of one hundredth of the smallest period in the program (Or, the sampling frequency f = 1/ts should s be 100 times the frequency of the signal). In our example, since we are having c(t) of frequency 1000 Hz, we selected f = 100000, or ts = 0.00001. s % Define the functions m(t) and c(t) This is a straightforward step. The function abs stands for , while pi=π. Note that the signals m and c are now vectors of the same size as t. % Perform the multiplication This is also a straightforward step. However note the dot after m. Why this is necessary here? What would happen if you remove the dot? % Perform full-wave rectification This is again a straightforward step, provided you recognize that full-wave rectification is mathematically equivalent to taking the absolute value. % Create the LPF This operation is frequently encountered in simulating communication systems. A LPF is defined by one parameter, the cutoff frequency. A filter in MATLAB is represented by its transfer function. The transfer function is in general in the form of the division of two polynomials. The filter is completely defined by the coefficients of the polynomial at the numerator and the polynomial at the denominator. These are the vectors a and b respectively in the program. There are many realizations for designing filters. One common realization is Butterworth, which is the one used here, hence the function name butter. The butter function has two arguments. The first argument is the order of the filter. The larger the order the sharper the filter (closer to ideal), but more processing is required. For most of our applications an order of 3-5 should be sufficient. The second argument is a coefficient related to the cutoff frequency. Without going into the details of the derivation, to design a LPF filter of cutoff frequency W, the argument should be set to 2Wts, where ts is the time step size of the program. For more details about the command butter , type: help butter ; in the MATLAB prompt How many arguments would a BPF require? What are they? Kousa & Muqaibel Exp 2: Simulation of Communication Systems Using MATLAB 13 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL % Get the output after the filter; In the previous step we have only created the filter. To apply the filter to a given signal, we use the function filter. This function has three parameters: the coefficients of the filter a and b, and the vector to be filtered. Note that although we think of the filter operation in frequency domain, the filter function operates on a time-domain vector. The output should as well be taken as a time-domain vector. % Finding the FT of the signals The Fourier Transform of signals can be found in MATLAB using the function fft. It can be used with a single argument, which is the time-domain vector. The fft function yields only the positive side of the spectrum. To get the double-sided spectrum, augment fft by fftshift. Finally, if you are only interested in the amplitude spectrum, augment all by the function abs. The resulting frequency-domain vector will have the same size as the size of the input time-domain vector. % Creating the vector for the frequency axis To plot the frequency spectrum as a function of frequency, you need to create the frequency axis. The available range of frequencies depends on ts, and is given by the relation: f=-length(t)/2:length(t)/2-1/(length(t)ts); % Plotting We leave this step to the student to explore. Use the help command to read about plot subplot, figure, legend, xlabel, ylabel, title and axis commands Lab Work 1. Create and run the m-file above, and produce Figure (1) and (2). 4 2. Change m(t) to 2+ sin(2π 1000t) and c(t) to cos(2π 10 t) and the cutoff frequency of the filter to 2 kHz. Redo part 1. Post-Lab Work 1. Include the m-file and the figures for the work you did in the lab in your report. 3 2. Using MATLAB, add the signals m(t) = exp(-100t) and c(t) = cos(2π 10 t) then separate them by means of filtering only (LPF and BPF). Provide the m-file and a plot of the sum in time and frequency, and of each of the recovered signals in time and frequency. Kousa & Muqaibel Exp 2: Simulation of Communication Systems Using MATLAB 14 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Exp 3: Representation of Signals & Systems Objective: By the end of this experiment, the student should be able to: • verify experimentally the relation between frequency and time domain representation of signals. • observe some of none idealities related to noise floor and harmonics. • measure the transfer function of a given system (filter) using narrow pulses. Introduction A signal is a function that symbolizes a physical variable of interest. Signals can be represented in time or frequency domains (Remember this is only representation). The two representations are related by Fourier Transformation. In this experiment we are going to examine some Fourier Transform properties, namely: Property Time Frequency Fourier transform of sinusoids cos𝜔𝑡 𝜋 𝛿 (𝜔−𝜔 )+𝛿 (𝜔 +𝜔 ) 000 Linearity ( ) ( ) 𝑎𝑔𝑡 +𝑎𝑔 (𝑡 ) 𝑎𝐺𝜔 +𝑎𝐺 (𝜔 ) 11221122 Modulation 𝑔 (𝑡 )𝑠𝑜𝑐 (𝜔𝑡 ) 0 Time Scaling g(at) Fill in the missing blocks in the table (See Lab Sheet). A system, on the other hand, is a combination and interconnection of several components to perform a desired task. Systems can be characterized by their impulse responses in time domain, or transfer functions in frequency domain. For a subclass of systems, the linear systems, the impulse response (or transfer function) provides a very convenient and straightforward relation between the input and out of the system. One type of system that is frequently-encountered in communications is the filter. A filter is a frequency-selective device that allows a certain frequency band to pass (with high gain), and blocks other bands. Depending on which band it passes, a filter can be classified as low pass (LPF), band pass (BPF) or high pass (HPF). Lab Work There are three parts in this experiment. In part I, we verify some of Fourier transform properties. In the second part, we study the effect of filtering on periodic signals. The last part is devoted to identify unknown systems by measuring their impulse response and transfer function. Kousa & Muqaibel Exp 3: Representation of Signals & Systems 15 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL To conduct the experiment, the following modules are needed: TUNABLE LPF, AUDIO OSCILLATOR, TWIN PULSE GENERATOR, 100 kHz CHANNEL FILTER. Part I: Verification of Fourier Transform Properties 1. Select and connect the proper modules to implement the following block diagram: x(t) G m(t) X z(t) + y(t) g cos (2π f t) Draw the equivalent Modules and show their interconnection. 2. Using the frequency counter, set the AUDIO OSCILLATOR module to produce a 5-kHz sinusoidal signal and connect it to the system as x(t). 3. Set y(t) as a sinusoidal signal of frequency 2 kHz from the MASTER SIGNALS module. 4. Set f in the above block to 100 kHz. 5. Set g to zero (full counter clockwise) and G to maximum. 6. Obtain the plot of m(t) from both the spectrum analyzer and the scope and compare with your theoretical expectations. Comment on the noise level and harmonics. 7. Vary the frequency of x(t) and observe the impact on both frequency and time domain. Describe what you observe in light of the time scaling property. 8. Re-adjust the frequency of x(t) to 5 kHz and increase g gradually. Observe the change in m(t) on the spectrum analyzer and the oscilloscope. When g is maximum, obtain plots of m(t) waveform and spectrum. What is the property we are trying to prove? 9. Plot the waveform and the spectrum of z(t). 10. Zoom the spectrum of z(t) around 100 kHz and observe its contents. 11. Compare the spectrum of m(t) and z(t) and comment on the modulation property. Part II: Filtering of Periodic Signals In this part we verify the Fourier Series representation of periodic signals, and examine the effect of filtering on the signal’s shape and spectrum. Tunable LPF ? 1. Apply a square wave signal with frequency of 2 kHz to the TUNABLE LPF module. Set the TUNE and GAIN knobs on the module to maximum (full clockwise), and set the toggle switch to WIDE. Observe the input and the out of the filter in both time and frequency domain. Are they similar? Why? Kousa & Muqaibel Exp 3: Representation of Signals & Systems 16 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL 2. Turn the TUNE knob to minimum (full counter clockwise). Observing the output, gradually increase the cutoff frequency to allow one harmonic, then two harmonics, then three, and so on. 3. Obtain time and frequency plots for three cases (one harmonic, two harmonics, max filter bandwidth). Adjust the axes to zoom-in the important data and get clear plots. 4. Explain the effect of the filter cutoff frequency on the output waveform. Part III: System Identification Systems, in general, are characterized by their impulse responses or transfer functions. An impulse is a non-realizable function. However, it can be approximated from a train of square pulses by making the pulse width as narrow as possible and the period as large as possible. This technique will be used to characterize different filters. 1. Use a TWIN PULSE GENERATOR module and clock it at 2 kHz using the AUDIO OSCILLATOR module (TTL output). Observe the output from Q1 on the scope and adjust the pulse width to minimum. 2. Connect the pulse train, Q1, to the input of the TUNABLE LPF. 3. Adjust the cutoff frequency of the TUNABLE LPF to a mid value. Plot the impulse response and the spectrum of the signal at the output of TUNABLE LPF. 4. Vary the cutoff frequency of the TUNABLE FILTER and verify that (the envelope of) the output spectrum approximates the transfer function of the filter. Get your instructor approval. Why the spectrum consists of spectral lines and not a continuous curve? What controls the spacing between the spectral lines? Test your hypothesis. 5. Use the above approximate transfer function measurement method to discover the filter characteristics of the 100 kHz CHANNEL FILTER module. Obtain the transfer function for the three settings (1, 2, 3). What is the type of the filter in each case? Estimate the 3dB bandwidth of the filters. Use the horizontal markers of the picoscope to trap the 3 dB drop, then the vertical markers to measure the bandwidth. Post-Lab Work Use MATLAB to plot and compare the transfer function of (1) Butterworth LPF of cutoff frequency 1 kHz, and order 1, 3, 5. (2) Butterworth BPF of cutoff frequencies 5 kHz and 8 kHz, order 1, 3, 5. You can generate an impulse using the function rectpuls(t,W). You can make W as narrow as time step size ts in order to get an excellent approximation of an impulse. Kousa & Muqaibel Exp 3: Representation of Signals & Systems 17 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL This page is intentionally blank. All Experiments start with odd pages for double-sided printing Kousa & Muqaibel Exp 3: Representation of Signals & Systems 18 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Exp 4: Speech Signals Objectives • Understand the features and characteristics of speech signals. • Get acquainted with the SPEECH module from TIMS. • Perform simple processes on speech signals (filtering, frequency translation), and examine their effect on the sound. Introduction Speech is the most frequently encountered message in communication systems. Throughout the lab work we will use a real speech message, whenever appropriate. In order to be prepared, we devote this experiment to study the basic characteristics of speech signals and get acquainted with the SPEECH module of TIMS. We generate speech, or voice in general, by virtue of the vibration of our vocal cords. The sounds we produce are composed of many harmonics, or pitches. Typically, the significant part of human voice occupies the range from 300 Hz – 3 kHz. This can be seen from the spectrum of the voice signal. The low-end of the spectrum represents the low-pitch sounds, while the high-end of the spectrum represents the high or sharp pitches. We can hear sounds over a much wider frequency range than the ones we produce. These sounds are called audible signals. A healthy human being can hear frequencies up to 15-20 kHz. This is another proof that we are created to hear more than we talk The following plug-in modules will be needed for this experiment: SPEECH, TUNABLE LPF, MULTIPLIER, in addition to external signal generator. The SPEECH MODULE The SPEECH module allows speech and audio signals to be recorded and replayed. Three independent channels are provided: CHANNEL 1, CHANNEL 2 and LIVE. The module includes a built-in microphone. An EXTernal input is also provided for recording externally generated signals. The module front panel looks like that in Figure 1. Kousa & Muqaibel Exp 4: Speech Signals 19 EE370 COMMUNICATIONS ENGINEERING LAB MANUAL Figure 1: Speech Module Channels 1 and 2 can each record up to 32 seconds of speech from the common MICrophone input. To record speech or other sounds on either channel, set the front panel switch to RECORD and speak clearly into the microphone. The length of your message may vary from a few seconds up to 32 seconds. As soon as you have finished your message, set the switch to the PLAY position. The recorded content will automatically repeat upon switching to PLAY. Note that the length of the recorded message will only be the length of time the switch was in the RECORD position. A third non-recordable channel, LIVE, is also provided where the sound at the MICrophone is continuously output as an electrical signal. A pair of headphones is provided to allow the user to listen to the recorded messages by patching any one of the SPEECH module’s outputs to the HEADPHONE AMPLIFIER in the TIMS System Unit. WARNING: DO not put the headphone on if you are not having sound yet. A sudden high-volume sound may harm your inner ear. Lab Work This experiment consists of four parts. We start by measuring the audible range of our hearing system Then, in part II, we record few different voice signals and observe the variations in their spectra. In part III, we examine the effect of filtering on the sound quality. Finally we listen to the effect of slight frequency translation, or modulation. Part I: Audible Range of our Hearing System 1. Set an external power supply to sinusoidal signal of frequency 10 Hz, and peak value 2 V. 2. Connect the signal to the input of the HEADPHONE AMPLIFIER module. Make sure you connect the ground of the external signal generator to the ground of TIMS. 3. On the HEADPHONE AMPLIFER, keep the gain knob to a low setting, and set the LPF SELECT to OUT (i.e. no filtering). As we go on with the experiment the sound will become sharp and loud, and you may not feel comfortable to hear it. Therefore keep the headphone aside, but near enough to hear the sounds. Kousa & Muqaibel Exp 4: Speech Signals 20

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