BEng. (Hons) Electronics Engineering. Examinations for / Semester 2

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BEng. (Hons) Electronics Engineering Cohort: BTEL/11/FT Examinations for 2013-2014/ Semester 2 MODULE: ELECTROMAGNETIC COMPATIBILITY MODULE CODE: TELC3110 Duration: 2 ½ Hours Instructions to

1 BEng. (Hons) Electronics Engineering Cohort: BTEL/11/FT Examinations for / Semester 2 MODULE: ELECTROMAGNETIC COMPATIBILITY MODULE CODE: TELC3110 Duration: 2 ½ Hours Instructions to Candidates: 1. Answer ALL 4 (four) questions. 2. Questions may be answered in any order but your answers must show the question number clearly. 3. Always start a new question on a fresh page. 4. All questions carry equal marks. 5. Total marks 100. This question paper contains 4 questions and 7 pages. Page 1 of 7

2 ANSWER ANY 4 (FOUR) QUESTIONS QUESTION 1: (25 MARKS) a) On May 4th, 1982, during the Falkland Islands War, the H.M.S. Sheffield's search radar was turned off due to electromagnetic interference problems with the ship's satellite communication system. This disabled the ship's anti-missile defense systems and enabled an Exocet missile to hit the ship resulting in the loss of the ship and 20 lives. i) What is the definition of electromagnetic compatibility (EMC) according to the IEC? ii) Briefly explain how the problem encountered by the H. M. S Sheffield is related to EMC? iii) A large proportion of EMC problems can be avoided at the design stage of electronic systems. Briefly explain how. b) Two aspects of EMC are emission and immunity. Briefly discuss both and explain how they relate to the live and let live adage. (2 + 2 marks) c) Briefly explain what you understand by the term interference in terms of the following elements: intentional emitters, intentional receptors, unintentional emitters and unintentional receptors? Page 2 of 7

3 d) EMC problems are categorized into inter-system EMC and intra-system EMC. Differentiate between the two by providing an example for each case. Also explain which types of coupling are more likely to be encountered inter-system EMC and intra-system EMC respectively. (4 + 2 marks) e) Three techniques to reduce electromagnetic interference are to: Suppress the source producing the EMI Make the coupling path as inefficient as possible Harden the receptor (making it less susceptible) Provide an example of each of the above techniques. (3 x 2 marks) QUESTION 2: (25 MARKS) a) EMC requirements on electronic equipment can be categorized as: those imposed on products marketed in the United States Those imposed on products marketed outside the United States border The FCC is responsible for the first category while the CISPR is responsible for the second. i) What is the range of frequencies considered by the FCC to be radio frequencies? (1 mark) ii) Both the FCC and CISPR breaks digital devices into class A and class B devices. Briefly describe the two classes. iii) For which class is the FCC and CISPR tests more stringent and why? (1 + 1 marks) Page 3 of 7

4 b) Compliance for radiated emissions can be tested either at an open field test site or a semi-anechoic chamber. i) Briefly explain how the semi-anechoic chamber is equivalent to the open field test site. ii) Briefly explain the procedure for measuring the radiated emission limits for CISPR class A or class B devices. c) Consider Table 2.1 and 2.2. Table 2.1: FCC Radiated Emission Limits for Class B Digital Devices Table 2.2: CISPR radiated limits for class B devices i) Rewrite the two tables, filling in the missing values. (2 + 2 marks) Page 4 of 7

5 ii) Measurements are usually taken in terms of quasi-peak (QP) value for measuring radiated emissions below 1 GHz. Briefly explain what is meant by a quasi-peak value, and why is it more important than peak value for measurements under 1 GHz. iii) Assume you are required to make a comparison FCC and CISPR radiated emission limits for class B devices measured at a distance of 3 meters. Which table would need to be modified and why? (1 + 1 marks) iv) Following iii) above, sketch a graph of FCC Class B and the CISPR Class B limits measured at a distance of 3 meters. List the range of frequencies for which the FCC is more stringent and the range of frequencies for which the CISPR is more stringent. (4 + 2 marks) QUESTION 3: (25 MARKS) a) Most electronic circuits nowadays operate at high frequency. Hence the importance of studying the behavior of circuit elements when frequency increases to ensure that operation works as designed. i) What happens to the resistance of conductors when the frequency increases? Briefly explain why. (1 + 3 marks) ii) For a round conductor, the current density at any distance from its surface is given by d J J se, where s J is the current density at the surface of the conductor, d is the distance from the surface and where is the 2 angular frequency of operation, is the permeability of the surrounding medium and is Page 5 of 7

6 the conductivity of the conductor. Starting with the definition of the skin depth,, show that the latter is given by: 1 (3.1) f (3 + 2 marks) iii) Given that the conductivity of copper is 5.8 x 10 7 S/m, calculate the skin depth of a copper conductor at an operating frequency of 1 KHz and 1 GHz (assume that H/m). What do you observe? Briefly explain. ( marks) b) If the per-unit resistance at dc (of a round wire) is given by: R dc 1 2 r (3.2) i) Show that at high frequencies, the resistance can be derived as: R hf 2 1 r (3.3) ii) Using the results of equation (3.3), show that: R hf 1 r 2 r 2 R dc (3.4) Page 6 of 7

7 iii) Sketch a graph of per-unit length resistance versus the ratio r. When the frequency is high enough, the resistance will be proportional to such proportionality is equivalent to an increase of 10 db/decade. f. Show that (3 + 2 marks) QUESTION 4: (25 MARKS) Two common elements that is used in electrical circuits are: the inductor and the capacitor. a) Briefly illustrate how a capacitor could be used to protect an electrical device from being affected by a high frequency noise current? b) Sketch the bode plots of an ideal inductor and capacitor. (2 + 2 marks) c) Draw the simplified equivalent circuit of a practical inductor and a practical capacitor and briefly describe the various elements of the models. (4 + 4 marks) d) Sketch the bode plots of the practical inductor and practical capacitor. Indicate on the sketches the frequencies at which the elements are resistive, capacitive and inductive respectively (4 + 4 marks) e) Briefly explain what would happen if the frequency of operation was beyond the resonant frequency of both the practical inductor and capacitor of part a). ***END OF QUESTION PAPER*** Page 7 of 7

ASSOCIATE DEGREE IN ENGINEERING RESIT EXAMINATIONS SEMESTER 1. "Electrical Eng Science"

ASSOCIATE DEGREE IN ENGINEERING RESIT EXAMINATIONS SEMESTER 1 COURSE NAME: "Electrical Eng Science" CODE: GROUP: "[ADET 2]" DATE: December 2010 TIME: DURATION: 9:00 am "Two hours" INSTRUCTIONS: 1. This