Advanced Diploma of Industrial Automation

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Advanced Diploma of Industrial
Automation

DIA

Module 16: Practical Shielding EMC/EMI, Noise Reduction,

Earthing & Circuit Board Layout of Electronic Systems

Written Assessment

Version 3.1

Module 16: Practical Shielding EMC/EMI, Noise Reduction, Earthing & Circuit Board Layout of Electronic Systems

Written Assessment

Total Marks: 135 marks

Assessment Points:

  • Supply the required answers below in a blue font(not red or black).
  • You must submit this assessment along with the practical component.
  • Answer all the questions.

Module 16 Critical Questions:

  • Question 9 & Question 17.
  • These questions are mandatory to assess Module competency.
  • Questions must be answered completely correctly to be assessed as competent with a score for this module.
Q1 Interference is present in a signal cable between a PLC and a temperature sensor which is running next to a power cable in the tray.

  1. List and briefly describe the different types of noise coupling paths possible. (8 marks)
  2. List any 2 ways of minimizing the coupling issues? (2 marks)
(10 marks)
A1 Noise coupling or interference in a signal cable can occur in four possible ways, Common Impedance coupling, Inductive Coupling, Radiative Coupling, and Capacitive coupling.

1) Common Impedance Noise: Common Impedance Noise might occur in the circuit due to presence of impedance (can inductive, capacitive, or reactive) in the common return path of two circuits. This may result in the error of voltage measurement depending on the value of current in the circuit and amount of impedance.

impedance-coupling.gif

Fig. 1: Common Impedance Noise coupling

2) Inductive Coupling: These noises are produced by mutual induction between two nearby circuits. A time varying magnetic field is produced by the noise circuit (due to flow current) causes voltage induced in the signal circuit as given by the formula.

Vn = 2p fBACos , where symbols have their usual meaning.

A lumped equivalent circuit can be used to explain the same where this coupling is represented by a mutual inductance placed between them.

af64be30616.gif

Fig. 2: Inductive Coupling

3) Capacitive Coupling: It is caused due to disturbance due to electric field from noise circuit to signal circuit. This transfer of energy due to electric field is equivalent to a capacitance placed between them, which form the basis of conversion of the scenario into a lumped equivalent circuit as shown in the figure.

af64be30615.gif

Fig. 3: Capacitive coupling

4) Radiative Coupling:

This type of coupling occurs due to radiation from TV or Broadcast radio sources or any other communication channel.

Two ways to minimize noise coupling are:

i) By using different return paths for signals

ii) By using twisted cable in the circuit (Cao et al., 2018)

F1 Assessor Feedback (marks awarded)
Q2 The fundamental to an understanding of EMC are the concepts of differential mode, common mode and antenna mode radiated field coupling.

List any 5 differences between differential mode and common mode coupling.

(10 marks)
A2 In differential mode coupling, coupling occurs in two lines of a closed loop where current flows in opposite direction. However, common mode coupling occurs in two lines in the same direction and phase.

In differential mode coupling, disturbances are weakly coupled because of conductors being placed nearby. In Common mode coupling, current can be induced due to magnetic fields.

In differential mode coupling, the current is generally larger whereas in current mode coupling the current is very small.

In differential mode coupling, the area of the loop is very small compared to common code.

The solution of differential mode coupling is connection of impedance series in with the high side along with a shunt capacitor across the line. However, common mode coupling can be solved using a common-mode choke (Chen, Chen, Chiu, & Yeh, 2015)

F2 Assessor Feedback (marks awarded)
Q3 Describe in at least 4 sentences how an electronic circuit can create an electromagnetic (EM) wave. (4 marks)
A3 We know all electronic components work on electric current. A time varying electric field generates a time varying magnetic field and vice versa. Now, an electronic circuit has got both electric (capacitors) and magnetic (inductors) components. So, when an alternating electric current (which varies with time) is applied in an electronic circuit, it produces time varying electric and magnetic fields. These interactions between electric and magnetic fields cause the production of an electromagnetic wave.
F3 Assessor Feedback (marks awarded)
Q4 List any 2 primary sources of transients and spikes. (2 marks)
A4 Two sources of transients and spikes are:

  1. Arcing faults
  2. Contactor, breaker or relay operations.
F4 Assessor Feedback (marks awarded)
Q5 Describe 2 methods to prevent CM (common mode) to DM (differential mode) conversion in circuits that carry high-frequency signals (such as wideband data or video) or which could be susceptible to RF. (4 marks)
A5 Two methods are as follows:

  1. If an EMI filter includes both CM & DM then DM should have less than 30 MHz where as it can increase due to magnetic influx that can be controlled by mode choke. The CM should be 30-100MHz. Then both filters would require limited EMI for entire frequency.
  2. As most circuit carry high frequency thus it can be susceptible to RF. The best possible way to prevent conversion from CM – DM is reducing the unwanted impedance. This would equalize the frequency as much as possible.
F5 Assessor Feedback (marks awarded)
Q6 Explain the difference between EMI and EMC. (1 mark)
A6 EMI stands for Electromagnetic Interferences whereas EMC stands for Electromagnetic Compatibility. Electromagnetic interference refers to any disturbance, which is occurring in the circuit due to external sources. On the other hand, Electromagnetic Compatibility refers to devices ability to withstand any kind of electromagnetic interference( Cole, Linnebur, Preston, & Schofield, 2018).
F6 Assessor Feedback (marks awarded)
Q7 A micro-processor circuit with a clock frequency of 10MHz has been mounted in a metal enclosure. There are, however, some concerns about emissions leaking from the enclosure with the screws screw separation distance x. Close to the micro-processor circuit is a 100MHz receiver circuit which receives noisy signals. The receiver is sensitive at 10 times the clock frequency of the micro-processor circuit and one would have assumed that the 10th harmonic of a square wave would be significantly reduced.

  1. You first suspect that (as the other people thought) some RF might be leaking from the lid. You open the lid but the noise at the receiver does not increase a lot. In fact, taking the lid away does not make a big difference. Discuss what might be the problem and if it is possible to emit at 100MHz. Also recommend what the distance x between the screws should be. (5 marks)
  2. You are still confronted with a heat problem and need to make holes in the box for forced air cooling. What is the maximum whole size allowable if the people using the receiver inform you that 20dB attenuation should be enough? (1mark)
  3. If the hole size in 5.2 is not adequate for cooling, what can be implemented to increase the hole size without compromising the 20dB attenuation. (2 marks)
  1. marks)
A7
  1. If RF is not the problem then there might be some interference from other external sources.

The distance x should be 2.1 mm.

  1. Maximum whole size should be 3mm.
  2. Yes, it is adequate for cooling. However, further whole size can be increased if necessary.
F7 Assessor Feedback (marks awarded)
Q8 Although lightning and ESD are both pulsed noise, they differ in their treatment and threats. Describe why they are two distinct threats (list at least 3 differences). (6 marks)
A8 ESD or Electrostatic discharge is a phenomenon in which there is a sudden electricity flow between two charged objects whereas lightning are those ESDs, which occur naturally.

ESD are caused by the method of electrostatic induction (when a charged body is placed near an uncharged conductor, it induces an electric charge in it). Lightning is caused due to discharges occurring due to clouds..............


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