Bridging Communication Systems And Circuits With PSPICE

Session 4DBridging Communication Systems and Circuitswith PSPICEAndrew Rusek and Subramaniam Ganesan,Oakland University, rusek@oakland.edu, Ganesan@oakland.eduAbstract-- PSPICE has many interesting systemblocks and circuit components which can beconnected together or used separately. We havedeveloped a broad variety of student edition PSPICEmacromodules for use in class room and laboratoryteaching, including units that simulate pulse widthmodulators and demodulators, delta encoders anddecoders, generic class C amplifiers, frequencysynthesizers, noise generators, band pass filters, AMmodulator/ demodulators with noise and interference,analog correlators, oscillators, and phase lock loops.This paper presents details of the above modules, andalso describes the use of the PSPICE modules in thecommunication courses and circuit courses.IntroductionPSPICE has many interesting system blocks andcircuit components which can be connected togetheror used separately Blocks such as Analog Multipliers,Amplitude Limiters, Generic Amplifiers, LaplaceBlocks, Ideal Transmission Lines, CoupledTransmission Lines, Arbitrary Function Blocks withTwo or Three Inputs, Summing and DifferenceNodes, Generic Filters, Trigonometric and LogFunction Blocks, etc. are available in addition tomany passive and active circuit components create anexcellent base for system/circuit preparation. Thiscould be controlled by built-in FFT analysis,parametric sweep of component/system parametervalues, time-domain, frequency-domain analyses,possibility of reading external tables (eg. pseudorandom numbers for noise signal generation),optimization procedures could show max or minvalues while sweeping, etc.Obviously, direct circuit implementations are verynatural, since even active circuit components can bemodified to create new microwave circuits from LFbasic diodes and/or transistors. Typical example: old2N2222 transistor operating within MHz frequencyranges can be "reshaped" by changing its parametersto operate within GHz ranges. Some instruments e.g.Network Analyzers can be simulated to extract Sparameters for further analysis or design ofmicrowave amplifiers using other special softwarethat is based on S-parameters. The S parametersextracted by PSPICE Network Analyzer can be alsoused in many available MATLAB programs tocontinue design processes.We have developed a broad variety of PSPICEmacromodules for use in class room and laboratoryteaching, including units that simulate pulse widthmodulators and demodulators, delta encoders anddecoders, generic class C amplifiers, frequencysynthesizers, noise generators, band pass filters, AMmodulator/ demodulators with noise and interference,analog correlators, oscillators, and phase lock loops.Some selected circuits and the use of the PSPICEmodules in the communication courses and circuitcourses are given in the following section.The PSPICE circuit and simulation discussed in thepaper would make excellent additions to the classroom or laboratory of any undergraduatecommunications systems course. Frequency hoppingCDMA and other wireless techniques are difficultconcepts to grasp and difficult to obtain practicalexperimental experience. Demonstrating many of providesinvaluableeducationalopportunity.Why PSPICE was appliedPittsburgh, PAMarch 26-27, 2010ASEE North Central Sectional conference4D-1

Session 4DThe PSPICE educational version is free and prettyeffective; number of components is limited, butsufficient in all presented cases. Each student can usePSPICE anywhere, as long as he or she has acomputer, laptop, desktop, or net-book. Some of thefeatures of PSPICE are:1.2.3.4.5.Basic blocks are available, such as analogmultipliers, summing blocks, differenceblocks, integrators, differentiators, Laplaceblocks, arbitrary function blocks; circuits,both analog and digital. Easy parameterchanges, value sweeps, etc. Larger circuitscan be integrated into blocks – book likeunits.Time-domain, frequency-domain, Fourieranalysis, Math operations on signals arepossiblePost-processing is el scopes, multichannel spectrumanalyzers, y-x displays with easy scalechanges.Easy documentation collectionOther options: System View of Elanix, andVisSim/Comm of Visual Solutions, Inc. are not freeand they do not include circuit components that couldshow possibility of laboratory implementation.Selected Circuit ExamplesFigure 1: AM modulator and DemodulatorThe circuit in Figure 1 includes AM modulator(transmitter) and demodulator. On the left is thetransmitter, on the right – two types of demodulators.Lower right corner circuit shows the demodulatorthat includes the simple carrier reconstruction circuitand low pass filter (synchronous demodulator), upperright image is asynchronous demodulator with anideal rectifier and a low pass filter. Center lowercircuit is a pseudo random noise generator – the noisesource “reads” the numbers from the text file andproduces the signal whose amplitude and “spike”repetition could be modified, as well as the otherpseudorandom sequence could be easily introducedas the new text file.In this section we illustrate a few basic functions.The circuit potential and noise level can be easilymodified, interference added, more filtering effectsare easy to modify to observe possible signaldistortions. More noise effects could be alsodemonstrated. The material supports our DigitalCommunication Systems course, ECE 534 (graduatecourse). Initial part reviews classical material fromECE 437, Communication Systems (undergraduatecourse).Figure 2: Time domain signalThe time-domain signals, shown in Figure 2, includethe information signal, modulated carrier, and twodemodulated waves – from upper and lowerdemodulators.Pittsburgh, PAMarch 26-27, 2010ASEE North Central Sectional conference4D-2

Session 4DFigure 6: Time-domain waves from circuits shownin Figure 5Figure 3: Spectra of the waves: information signal,modulated wave, demodulated signals after filtering.Figure 7: Spectral representationsFigure 4: Modulated signal spectrum with noise afterfilteringSpectral representations as shown in Figure 7 are :top- rectangular (rich spectrum as far as the numberof harmonics, broad main lobe (1/pulse durationwidth), second for square wave shows eliminatedeven harmonics, trapezoidal – “slower” wavetransitions show smaller peaks in comparison withrectangles, triangle wave spectrum decays quickly forHF due to slowest transitions.Next simulations introduce the students to basebandmodulators and demodulators such as delta and pulsewith modulation (PWM) units.Figure 5: Multiple circuits to generate different waveforms.Four circuits as shown in Figure 5 are usedsimultaneously to demonstrate Fourier analysis:square, rectangular, trapezoidal, and triangularperiodic waves are generated and spectra aredisplayed in Figure 6.Pittsburgh, PAMarch 26-27, 2010ASEE North Central Sectional conference4D-3

Session 4DFigure 8: Modulator/demodulatorFigure 10: PWM circuit with modulator anddemodulatorDelta modulator (upper part) and demodulator (lowerpart), both “unpacked to show details. The wavesshown in Figure 9 demonstrates analog into “zeroone” signal conversion, and “zero-one” conversion,back into analog signal to reconstruct originalinformation. The output signal can be more filtered tomake it smoother, but there could be a problem withencoding signal of a higher frequency.Figure 10 shows PWM Pulse Width Modulator –larger signal levels correspond to broader pulses,smaller values – narrower pulses, Lower Circuit –demodulator.Figure 11: Output waveform for Figure 10.Figure 9: Output waveform for Figure 8Figure 11 shows: Top- information signal, secondPWM wave, third – reconstructed original signal,bottom – difference between original information andreconstructed signal.Pittsburgh, PAMarch 26-27, 2010ASEE North Central Sectional conference4D-4

Session 4DSeveral examples, shown below demonstrate bandpass modulation and demodulation processes.Figure 12: QPSK circuitQPSK – quadrature phase shift keying system shown in Figure 12, is fully integrated. Transmitter(two level system with two sub-blocks shown inFigures 13 and 14), on the left splits the digitalarbitrary signal from the source (intell) to save thebandwidth, sine and cosine carriers “carry” even andodd digital signal components, both modulatedsignals are added and transmitted, noise is added(from the bottom circuit). QPSK receiver reconstructsthe intelligence signal.Figure 14. Receiver circuitIn Figure 14, Receiver splits the channels, decodesodd and even parts of the intelligence signal in twocoherent demodulators, and combines these parts toreconstruct original information.Figure 15. Output waveforms for Figure 14.Figure 13: Transmitter with two channels and sine,cosine carriers.Figure 13 shows the way the digital intelligencesignal coming from the source V1 (Fig. 12) is dividedbetween two separate channels (D-flip-flops) and theupper part of intelligence signal (even) modulates thesine carrier, lower part (odd) modulates the cosinecarrier. Both modulated signals are combinedtogether (added) for final transmission.In Figure 15, the top signal is the original digitalsignal (intelligence), second and third from the topare even and odd parts of the intelligence, split in thetransmitter to slow down modulation process andsave spectrum, fourth signal is the modulated carrier,and the last signalis reconstructed originalintelligence.Pittsburgh, PAMarch 26-27, 2010ASEE North Central Sectional conference4D-5

Session 4DFigure 18: FSK transmitter sub blockFigure 16: Output waveformsIn Figure 16, the top signal is the even part ofintelligence, second signal is the modulated carrier(sine) by the even part, third signal is the odd part ofintelligence, fourth signal is the cosine carriermodulated by odd part, the last signal is the –reconstructed intelligence.The FSK system – frequency shift keying –Integrated System, and two sub-blocks are shown inFigures 17 to 19. The Voltage Controlled Oscillator(VCO) is modulated by the info source. Large levelof noise is added within the transmission channel.Phase Locked Loop (PLL) system receives andreconstructs the info(rmation) signal.Figure 19: FSK receiver sub block with PLL systemFigure 20: Output waveformsFigure 17: FSK circuit with transmitter and receiversub blocksDigital information signal is encoded in frequencychanges, and this digital signal is decoded in thereceiver. Noise signal is added to the modulatedsignal.The following Figures 21 to 23 demonstrate theoperation of CDMA (Code Division MultipleAccess) systems.Pittsburgh, PAMarch 26-27, 2010ASEE North Central Sectional conference4D-6