Advanced Diploma of Industrial Automation (DIA) Assignment for Module 15 MODULE NAME: Shielding EMC/EMI, Noise Reduction, Earthing & Circuit Board layout of Electronic Systems Rev 6 Your Name:...

Power System Protection
DIA11 & MODULE 15
P-15
Advanced Diploma of Industrial Automation
(DIA)
Assignment for Module 15
MODULE NAME: Shielding EMC/EMI, Noise Reduction, Earthing & Circuit Board layout of Electronic Systems
Rev 6
Your Name: ................................................
ITH MODEL ANSWERS
General Instructions to candidates
1. The best marks can be earned by giving concise
ief answers that addresses the questions as put to you.
2. The time indicated is tentative and is not meant to restrict your answers
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Total Marks 135
Time: 240 minutes
Question 1
Interference is present in a signal cable between a PLC and a temperature sensor which is running next to a power cable in the cable tray. Describe the different types of noise coupling paths possible and how you can solve or minimize the coupling.
[10]
Answe
The various coupling paths available might be:
· Electromagnetic (Radiated)
· Inductive (Magnetic) –
AC cu
ent flowing in a conductor creates a magnetic field that will couple with a nea
y conductor and induce a voltage in it. Voltage induced depends on the mutual inductance between the circuits which inturn depends on the areas of the source and victim cu
ent loops, their orientation and separation distance, and the presence of any magnetic screening.
· Capacitive (Electrostatic) –
Changing voltage on one conductor creates an electric field that may couple with a nea
y conductor and induce a voltage on it. Voltage induced depends on the coupling capacitance. The value of this coupling capacitance is a function of the distance between the conductors, their effective areas and the presence of any electric screening material.
The above mentioned coupling mechanisms depends on the following factors:
• Space separation
• Shielding materials
• Absorptive materials
• Filters and circuits
• Power lines
The means of reducing interference at coupling path is to remove or attenuate the coupling path by:
Isolation, Re-routing, Shielding, Spacing, Filtering etc.
Question 2
The fundamental to an understanding of EMC are the concepts of differential mode, common mode and antenna mode radiated field coupling. Describe the differences between differential mode and common mode coupling. How does it apply to both emissions and interference?
[10]
Answe
Differential mode coupling –
Differential simply means the difference between things of the same kind. Differential mode is the normal voltage and cu
ent between the signal and its return lines (or for that matter, it can be the positive and negative points in a circuit). Differential (in this case) means the difference between the two lines. Consider two items of equipment interconnected by a cable as shown in the figure below. The cable ca
ies signal cu
ents in differential mode (go and return) down the two wires in close proximity. A radiated field can couple to this system and induce differential mode inteference between the two wires. Similarly, the differential cu
ent will induce a radiated field of its own. The ground plane plays no role in this coupling. The differential mode cu
ents that are governed by Kirchoff’s cu
ent law can be easily predicted.
Common mode coupling –
Common mode comes into the picture when there is another ground
(reference) plane with respect to which voltages can exist and cu
ents can flow. Common (here) means common between the two lines and a reference common to them. In common mode the two lines are seen as one. The cable also ca
ies cu
ents in common mode, i.e., all flowing in the same direction of each wire as shown in the figure below. These cu
ents very often have nothing at all to do with the signal cu
ents. They may be induced by an external field coupling to the loop formed by the cable, the ground plane and the various impedances connecting the equipment to ground, then may cause internal differential cu
ents (to which the equipment is susceptible). Unconventionally, they may be generated by internal noise
voltages between the ground reference point and the cable connection, and are responsible for radiated emissions. The stray capacitances and inductances associated with the wiring and enclosure of each unit are an integral part of the common mode coupling circuit and play a major role in determining the amplitude and spectral distribution of the common mode cu
ents. These stray impedances are incidental rather than designed in to the equipment. They don't appear on any circuit diagram and are
difficult to control or predict. Common mode cu
ents contributes to the radiated emmissions. The interference cu
ent generated in common mode from ground noise developed across the equipment may flow along the conductors or along the shield of the shielded cable.
Question 3
Describe how an electronic circuit can create an electromagnetic (EM) wave.
[4]
Answe
Most electronic devices consist of elements that are capable of antenna-like behaviour (i.e., they tend to radiate) such as cables, PCB tracks, internal wiring and mechanical structures. These elements can transfer energy in the form of either electric, magnetic or electromagnetic fields (EM waves) that couple with the circuits.
Question 4
What are the primary sources of radiated emissions in most equipment?
[2]
Answe
In most equipment, the primary sources for radiated emmisions are cu
ents flowing in circuits such as clocks, oscillators, etc. that are mounted on the PCB. Some of the energy that is directly radiated from the PCB is modeled as a small loop antenna ca
ying the interference cu
ent.
Question 5
Describe two methods to prevent CM to DM conversion in circuits that ca
y high-frequency signals (such as wideband data or video) or which could be susceptible to RF.
[4]
Answe
To prevent CM to DM conversion in circuits that ca
y high-frequency signals (such as wideband data or video) or which could be susceptible to RF are designed in such a way that the stray impedances of each conductor are balanced as near as possible. Alternatively, a common-mode choke is used that swamps the imbalance of the strays and reduces the magnitude of common mode cu
ent ICM.
Question 6
Explain the difference between EMI and EMC.
[2]
Answe
EMI is electromagnetic interference
· Electromagnetic interference (EMI) is distu
ance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The distu
ance may inte
upt, obstruct, or otherwise degrade or limit the effective performance of the...