Past Exam Paper & Memo N4

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T380(E)(J24)TNATIONAL CERTIFICATECOMMUNICATION-ELECTRONICS N4(8080224)24 July 2018 (X-Paper)09:00–12:00This question paper consists of 5 pages and 1 formula sheet.Copyright reservedPlease turn over

(8080224)-2-T380(E)(J24)TDEPARTMENT OF HIGHER EDUCATION AND TRAININGREPUBLIC OF SOUTH AFRICANATIONAL CERTIFICATECOMMUNICATION-ELECTRONICS N4TIME: 3 HOURSMARKS: 100INSTRUCTIONS AND INFORMATION1.Answer ALL the questions.2.Read ALL the questions carefully.3.Number the answers correctly according to the numbering system used in thisquestion paper.4.Write neatly and legibly.Copyright reservedPlease turn over

(8080224)-3-T380(E)(J24)TQUESTION 1: DIRECT CURRENT THEORY (NETWORK THEOREMS)1.11.21.3Differentiate between the TWO Kirchoff's laws:1.1.1Current law1.1.2Voltage law(2 3)(6)Explain the inter relation between the networks: nortons and thevenin, interms of impedance.(2)Define a phasor.(4)[12]QUESTION 2: ALTERNATING CURRENT THEORY (VECTORS)2.1An AC supply of 50 V is applied across a coil having an inductive reactance of12 Ω and a resistance of 5 Ω.Calculate the following:2.22.1.1Total current(5)2.1.2Phase angle(6)2.1.3Power factor(2)2.1.4Draw the phasor diagram(2)With the aid of THREE labelled diagrams explain the operation of a pureinductive circuit when an alternating voltage is applied across an inductor.(5)[20]QUESTION 3: FREQUENCY RESPONSE (Filters, Attenuators and RC Time Constants)3.1Explain in detail the construction of a composite filter.(5)3.2Explain the principle of piezoelectric effect on a crystal filter.(5)3.3Explain FOUR requirements an attenuator must provide as a resistancenetwork to reduce voltage, current or power in controllable and knownamounts that are independent of frequency.(4)Copyright reservedPlease turn over

(8080224)3.4-4-T380(E)(J24)TDifferentiate between the TWO RC circuits below in terms of the outputvoltage and the waveform of the output.3.4.1RC integrator circuit3.4.2RC differentiator circuit(2 4)(8)[22]QUESTION 4: MODULATION4.1Define phase modulation.(3)4.2Explain the process that takes place at the receiving end of the circuit duringthe transmission of the double side band transmission.(4)Explain THREE main reasons for using single-sideband (reduced carrier)working for point-to-point communication.(6)4.34.4Explain the purpose of a carrier wave in telecommunication.(3)[16]QUESTION 5: DEMODULATION5.1Explain the process of demodulation.(3)5.2With a neatly labelled circuit diagram, explain the operation of a ratio detector.(7)[10]QUESTION 6: COMMUNICATION SYSTEMSIdentify the block diagram below and explain the function of each stage.[10]Copyright reservedPlease turn over

(8080224)-5-T380(E)(J24)TQUESTION 7: RADIO WAVE PROPAGATIONDefine the concepts as it relate to antennas:7.1Power gain7.2Antenna losses7.3Beam width7.4Radiation resistance7.5PolarisationCopyright reserved(5 2)[10]TOTAL:100

(8080224)T380(E)(J24)TCOMMUNICATION-ELECTRONICS N4FORMULA SHEET1.I VR18.Z 2.X L 2 fL19.Ic I Q3.XC 12 fc20.IL I Q4.Z R 2 ( X L X C )221.f 5.I VZ22.B W 6.VR I . R23.VL I . X LVC I . X CRZ7.Cos 8.I I R IC29. Tan 110.I I R2 I L211.I I R2 ( I L LC ) 212. Tan 113.fr 14.C 15.L 16,Q 17.Vc V Q and VL V QICIR( I L IC )IR12 LC14 . . f r2 . L214 . 2 . f r2 . CXL 1 LfR RR C bandwidthCopyright reservedLCR R2 LVT VR 2 (VC VL )2

MARKING GUIDELINENATIONAL CERTIFICATECOMMUNICATION-ELECTRONICS N424 JULY 2018This marking guideline consists of 6 pages.Copyright reservedPlease turn over

MARKING GUIDELINE-2COMMUNICATION-ELECTRONIC N4T380(E)(J24)TQUESTION 11.11.1.1Current law states that in a parallel circuit, the algebraic sum of thecurrent meeting at a junction of a circuit is equal to the algebraicsum of the current, flowing away from the same junction of acircuit, i.e. the algebraic sum of the current is zero.1.1.2Voltage law states that in a series circuit, the algebraic sum of allvoltage drops measured across each resistor is equal to thealgebraic sum of all supply voltages. i.e. the algebraic sum of allthe voltages in a circuit (voltage drops and supply voltages) in aclosed loop is zero.(2 3)1.2The equivalent impedance at the terminal AB of both circuits are identical1.3A phasor is defined as any physical quantity that possesses both magnitudeand direction. It is represented by means of a straight line.(6)(2)(4)[12]QUESTION 22.12.1.1 13 ΩIT Copyright reserved3, 85 A(5)Please turn over

MARKING GUIDELINE2.1.2-3COMMUNICATION-ELECTRONIC N4T380(E)(J24)TVR I R 3, 85 x 5 19, 25 VVL I X L 3, 85 x 12 46, 2 VI R 2 I XL2IZ i.e VZ VR 2 VL2 19,25 2 46.2 2Hence IZ 50 VIRRor COS IZZ19,255 or5013COS COS 1519,25or COS 11350 67,38⁰ or 67,38⁰2.1.3(6) pf 0, 385 log(2)2.1.4(2)Copyright reservedPlease turn over

MARKING GUIDELINE-4COMMUNICATION-ELECTRONIC N4T380(E)(J24)T2.2(5)[20]QUESTION 33.13.23.33.4A composite filter is a combination of the prototype and derived sectionsconnected in tandem and designed to give the required input impedance andsharpness of cut-off frequency. The derived section is designed to give therequired sharpness of cut-off frequency. The prototype section compensatesfor the falling off of attenuation of the derived section beyond the frequency ofinfinite attenuation.(5)Piezoelectric effect is a process whereby the crystal is placed undermechanical strain, such as compression, expansion, bending and/or twisting;the mechanical energy expended is converted to minute electrical energy.(5)It must provide the correct input impedance provide the correct output impedance provide the correct attenuation be independent of frequency(4)3.4.1In RC integrator circuit, the output voltage is taken from across thecapacitor and the wave of the output form is dependent upon thetime constant and the frequency and the shape of the inputwaveform. The RC product is many times greater than the durationand the interval of the pulses to be integrated is usually sufficient toprovide good integration.3.4.2In a RC-differentiator circuit, the output voltage is taken fromacross the resistor and the wave of the output form is dependentupon the time constant and the frequency and the shape of theinput waveform. The RC product should be much smaller than theduration of the interval of the pulses to be differentiated is usuallysufficient to provide good differentiation.(2 4)Copyright reserved(8)[22]Please turn over

MARKING GUIDELINE-5COMMUNICATION-ELECTRONIC N4T380(E)(J24)TQUESTION 44.14.24.34.4Phase modulation is a process whereby the phase of the carrier wave ismade to vary in accordance with the instantaneous value of the modulatingsignal.(3)During transmission of the double side band, at the receiving end of the circuitthe receiver accepts both side bands and the carrier wave and uses thecarrier wave to demodulate the side bands to reproduce the originalmodulating frequencies(4) Only a restricted number of receivers are involved and so more expensiveand sophisticated single side band equipment may be used It is economical because there is a considerable saving of power. There is a saving in bandwidth because only one sideband is transmitted.(3 2)(6)Carrier waves are used in telecommunication to convey either video or audiosignals(3)[16]QUESTION 55.1Demodulation is a process of extracting or recovering the information from amodulated signal.(3)5.2During the positive half cycles of the input signal, current flows from L 2through L 4 , R3 , R1 , D1 and back to L 2 . During the negative half cycles of theinput signal, current flows from L3 through D 2 , R 2 R3 , L 4 , and back to L3 .The audio output voltage is developed across R3 , C 5 filters off any voltagevariations present on the FM signal. No limiter required due to the action ofC5 .Copyright reserved(7)[10]Please turn over

MARKING GUIDELINE-6COMMUNICATION-ELECTRONIC N4T380(E)(J24)TQUESTION 66.1FM Transmitter(2)6.2 Microphone: It converts mechanical sound waves to electrical currents foramplification by the audio amplifier. Audio Frequency Amplifier: It amplifies the signal from the microphone tothe required value for superimposing the radio frequency by the modulator Reactance modulator: It converts the audio frequency into a reactivecomponent which is directly proportional to the amplitude and frequency ofthe intelligence. RF Oscillator: It generates the RF which has constant amplitude that is tobe modulated and transmitted. Frequency Multiplier: It multiplies the oscillator's output frequency to therequired carrier frequency for transmission Intermediate Power Amplifier: It isolates the oscillator from load variationsin other parts of the transmitter and provides stability to the oscillator. RF Power Amplifier: It sufficiently amplifies the RF modulated carrier waveto a value enough to drive the antenna circuits. Antenna: It converts electrical energy into electromagnetic energy which isradiated into free space.(8)[10]QUESTION 77.1It is the ratio of power fed to an antenna compared to the field strength powerradiated from the antenna.7.2It is the ratio of the actual radiation resistance to the total resistant thatdissipated power.7.3It is the regular separation between the two half power points on the radiationpatterns and applier to the narrow beat7.4The ratio of power radiated by the antenna to the square of the current at thefeed point7.5Is the direction in space of the electric vector of the electromagnetic waveradiated from an antenna and is parallel to the antenna(5 2)[10]TOTAL:100Copyright reserved

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1.1.2 Voltage law states that in a series circuit, the algebraic sum of all voltage drops measured across each resistor is equal to the algebraic sum of all supply voltages. i.e. the algebraic sum of all the voltages in a circuit (voltage drops and supply voltages) in a closed loop is zero. (2 3) (6)