Amplitude Modulation Circuit Implementation For Use In A Communication .

Paper ID #18541Amplitude Modulation Circuit Implementation for use in a CommunicationCourse for Electrical Engineering StudentsDr. Robert J. Barsanti Jr., The CitadelRobert Barsanti is a Professor in the Department of Electrical and Computer Engineering at The Citadelwhere he teaches and does research in the area of target tracking and signal processing. Since 2015,Dr. Barsanti has served as the William States Lee Professor and Department Head. Before joining TheCitadel in 2002, he served on the faculty and as a member of the mission analysis design team at theNaval Postgraduate School in Monterey, CA. Dr. Barsanti is a retired United States Naval Officer. Hismemberships include the Eta Kappa Nu, and Tau Beta Pi honor societies.Dr. Jason S. Skinner, The CitadelJason S. Skinner was born in Marion, South Carolina on December 10, 1975. He received the B.S. degree(with departmental honors) in electrical engineering in 1998 from The Citadel, The Military College ofSouth Carolina, Charleston, South Carolina. He received the M.S. degree in 2002 and the Ph.D. degree in2005, both in electrical engineering, from Clemson University, Clemson, South Carolina. He joined theDepartment of Electrical and Computer Engineering at The Citadel in January 2006, where he is currentlyan associate professor. From May 2006 to July 2007, he also held the position of senior engineer withScientific Research Corporation, North Charleston, South Carolina. His current research interests includemobile wireless communication systems and networks, spread-spectrum communications, adaptive protocols for packet radio networks, and applications of error-control coding. Dr. Skinner is a member ofAFCEA, ASEE, Tau Beta Pi, and Phi Kappa Phi. He served as president of the South Carolina Gammachapter of Tau Beta Pi from 1997 to 1998. He was an M.I.T. Lincoln Laboratory Fellow from 2002 to2005 and a Multidisciplinary University Research Initiative Fellow from 2004 to 2005. In 1998, he received the George E. Reves award for outstanding achievements in mathematics and computer science atThe Citadel.c American Society for Engineering Education, 2017

Amplitude Modulation Circuit Implementation for use in an UndergraduateCommunication Course for Electrical Engineering StudentsAbstract – Modern descriptions of analog communication schemes are mathematics based usingtransform theory and block diagrams. This presentation style leaves undergraduate students withthe challenge of relating these theories to real world circuit implementations. This is particularlytrue if the lecture class does not have a complementary laboratory component. This paperattempts to bridge this gap by presenting a basic yet comprehensive project that can be used todemonstrate amplitude modulation and demodulation theory. It is specifically designed to stir theinterest of junior or senior level electronics minded electrical engineering students. In thisproject, a double sideband large carrier waveform is produced using a simple switchingmodulator circuit. The resulting amplitude modulation (AM) waveform is then demodulatedusing an envelope detector circuit. The proposed project requests that students perform a circuitsimulation as well as an actual circuit implementation. The circuit behavior is studied via bothanalysis using software tools and measurement using hardware components. The project furtherrequires that the electrical signals are visualized in both the time and frequency domain toenhance concept understanding. The paper outlines an introduction to the modulation theoryalong with an overview of the necessary circuits and concepts. Additionally, suggested studentactivities, project assignment alternatives, along with detailed mathematical solutions areprovided.Keywords: Engineering communications, Circuit Projects, PSpice software.BACKGROUNDCourse projects are one of the seven high impact practices discussed by Koh in [1]. Additionally,hands on activities are noted to improve learning motivation and retention. For example, it isnoted by Zhan in [2] that the use of real world examples in the classroom improves studentinvolvement and enhances the learning experience. In that regard, the electrical engineeringcurriculum has used simulations to assist student learning for more than two decades. A strongargument for the use of circuit simulators in the classroom can be found in [3], where the authorsargue the superiority of the ‘learn by doing” approach to teaching circuit analysis. A more recentexample of this teaching paradigm can be found in [4] where circuit simulation software iscombined with Mathcad to permit student interactive experimentation.Incorporation of projects into lecture classes provides an added mechanism to align thecurriculum with the Accreditation Board for Engineering and Technology (ABET) programoutcomes. Four of the relevant program outcomes are listed below. Outcome a: "an ability to apply knowledge of mathematics, science, and engineering"The proposed project requires the student to apply communications theory to a practicalcircuit implementation.

Outcome b: "an ability to design and conduct experiments, as well as to analyze andinterpret data"The proposed project provides the opportunity for the student to experiment with the circuitparameters and evaluate the circuit response. Outcome e: "an ability to identify, formulate, and solve engineering problems"The proposed project gives the student a chance to solve for a number of circuit componentsand signal parameters associated with the assignment. Outcome k: "an ability to use the techniques, skills, and modern engineering tools necessaryfor engineering practice"The proposed project uses modern simulation software and basic circuit measurementtechniques to produce the requested results.INTRODUCTIONSenior level undergraduate electrical engineering students at The Citadel may elect to take a onesemester course in Communications Engineering as part of their degree requirements. This threecredit hour course presents the basic principles of analog communications systems includingsignal flow and processing in amplitude, frequency and pulse modulation systems. This course istypically taught using one of the popular Communication Engineering textbooks such as ref [5].Unfortunately, these texts can be overly mathematical, leaving the student mystified by themodulation and demodulation process. The purpose of this paper is to describe a simple circuitsimulation project that demonstrates the relevant concepts in an intuitive manner.This project covers amplitude modulation and demodulation. A double sideband large carrierwaveform is produced using a simple switching modulator circuit. The resulting AM waveformis then demodulated using an envelope detector circuit. It requires the students to simulate thecircuit and then construct the circuit and monitor signal in both the time and frequency domain.Plots and discussion are required at each stage to show understanding of the relevant modulationconcepts.The learning objectives for the proposed project covering six levels of Bloom’s taxonomy are:1. The student should be able to list the necessary components of the AM switching modulatorand the associated demodulator circuit.2. The student should be able to explain the operation of the switching modulator anddemodulator.3. The student should be able to use simulation software to describe the signal flow thorough thecircuit.4. The student should be able to compute required values for various circuit components.5. The student should be able to anticipate how changes in the signal or circuit will affect theresults.6. The student should be able to be able to suggest improvements to the circuit.

BASIC AMPLITUDE MODULATION THEORYAmplitude modulation is the process of transferring information signals to the amplitude of ahigh-frequency continuous-wave carrier. The modulated AM waveform can be described by(1) cos 2 ,where Ac is the carrier amplitude, m(t) is the arbitrary message signal, and fc is the carrierfrequency. As a result of the modulation property of the Fourier transform, the signal spectrumis given by ,2where the carrier spectrum is composed of two Dirac delta functions at fc and the messagesignal spectrum is translated to fc. (2)Creation of the AM waveform of Equation (1) can be realized in a three-step process depicted infigure 1. )Filterc(t)Figure 1: Amplitude modulation block diagramThe Project AssignmentThe ModulatorAs discussed in ref [5], page 79, a switching modulator circuit can be constructed as shown infigure 2. The large signal carrier V1 and single tone message V2 are placed in series. The carriersignal causes the diode D1 to turn on and off periodically at the carrier frequency resulting in themodulation of the message signal m(t) onto the carrier c(t). The frequencies and amplitudes werechosen for illustration purposes, not to simulate any particular AM system.The project directions have the student use PSpice software (Orcad PSpiceTM) to generate thecircuit of figure 2 to implement the signal 2 1 0.8 2 10 2 10 .(3)The assignment directs them to reproduce and explain the time-domain and frequency-domainplots and to relate them to the circuit implementation. The explanation should include the reasonfor the spectral replication and why the replicas are reduced in amplitude. Extra credit isprovided to those who take the effort to compute the Fourier series coefficients as

)(*# "# %&(4). '("# VR1D1D1N40025002V2VOFF 1VAMPL 0.8FREQ 1kL12.5mHR21kC1100n1V1VOFF 0VAMPL 2FREQ 10k0Figure 3: Switching modulator with bandpass filterFrom Fourier theory, we know that periodic sampling of a continuous message signal willproduce a periodic repetition of the message signal spectrum. These replica spectra will occur atthe sampling frequency and will be scaled by the Fourier series coefficients of the samplingpulses. Therefore in order to capture the double-sideband large-carrier (DSB-LC) signal atfrequency fc, and reject all others, a bandpass filter is required to be centered at fc.The students are directed to compute a bandpass filter centered at the carrier frequency fc. Theyshould have the requisite knowledge to know that , 2 ' .(5)- 2 ., 2 ,(6)And, if they are given thatThe students should be able to compute one choice of solution to be R 1kΩ, L 2.5mH, C 100nF. Added credit could be given for computing the 3 dB down bandwidth using the filtertheory equation#2/ 01 234.After selecting the R, L, and C values, the plot of the DBS-LC waveform of figure 4 should beproduced.(7)

0msV(L1:2)TimeFigure 4: AM DSB-LC waveform in the time domainThe Envelope DetectorRecovery of the message signal m(t) from the modulated waveform s(t) is accomplished for largesignal AM via an envelope detector, or peak-following circuit. Since the information of themessage will reside in the amplitude variations of the AM wave, by tracing the amplitudevariations of the high-frequency carrier, the message signal is recovered. Not coincidentally, thesimplicity of the demodulation is the reason for the popularity of broadcast AM. Figure 5 showsthe addition of a diode and RC circuit to accomplish the demodulation and recovery of themessage signal.R1VD1D1N4002500D2D1N40022V2VOFF 1VAMPL 0.8FREQ 1kR2L1C11kR31.5kC21u1V1VOFF 0VAMPL 2FREQ 10k0Figure 5: Addition of the peak detector circuitProper selection of the RC time constant will permit fast charging and slow discharge of theoutput capacitor. This results in an output voltage that will follow the peak of the AM waveform,thereby recovering the message signal. This results in an output voltage that will follow the peakof the AM waveform, thereby recovering the message signal. Typically, the value of the RC time