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.
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?.
Differential mode - differential simply means the difference between things of the same
kind. Differential mode is the normal voltage and current 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.
Common mode - common mode comes into the picture when there is another ground
(reference) plane with respect to which voltages can exist and currents 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.
Question 3
Describe how an electronic circuit can create an electromagnetic (EM) wave
Question 4
What are the primary sources of radiated emissions in most equipment?
Question 5
Describe two methods to prevent CM to DM conversion in circuits that carry high-frequency signals (such as wideband data or video) or which could be susceptible to RF.
Question 6
Explain the difference between EMI and EMC.
EMI is electromagnetic interference -Noise from a device or the problem
EMC is elctromagnetic compatabilty is the the solution the standard driven ability of a device, equipment or
a system to function satisfactorily in its electromagnetic environment without introducing
intolerable electromagnetic disturbance to anything in that environment.
Question 7
You inherit a micro-processor circuit design with a processor running at 10MHz. Close to this area is a 100MHz receiver that is receiving noisy signals. Your predecessor knew that the processor circuit was going to be used close to a receiver and enclosed the circuit in a metal enclosure. There are, however, some concerns about the lid of the enclosure because the screws are separated at a distance x and some emissions may be leaking from here. You investigate the circuit and see that there was a heat problem so that the PCB is mounted close to metal enclosure. A non-conductive heat pad was used and the PCB assembly is a distance h from the enclosure floor. You are concerned about the 100MHz. The receiver is sensitive at 10 times this frequency and one would have thought that the 10th harmonic of a square wave would be significantly reduced.
7.1 You first suspect that (as the other guys 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 recommend what the distance x between the screws should be.
7.2 You are still confronted with a heat problem and need to make holes in the box for forced air cooling. What is the maximum hole size allowable if the people using the receiver inform you that a 20dB "drop" should do the job.
7.3 If the hole size in 5.2 is not adequate, what can be implemented to increase the hole size without compromising the 20dB "drop".
Question 8
Although lightning and ESD are both pulsed noise, they differ in their treatment and threats. Describe why they are two distinct threats.
Question 9
What is the difference between Earthing and Grounding in EMC and what is the primary EMC function of a ground system.
Question 10
Discuss the three ways that grounding systems for a circuit reference can be configured.
Question 11
Describe why ground loops exists and how it can be avoided.
Question 12
The major source of radiation in digital circuits is the processor clock(s) and its harmonics. All the energy in these signals is accumulated at a few specific frequencies, with the result that the clock signal levels are 10 to 20 dB higher than the rest of the digital circuit radiation.
12.1 Since the commercial radiated emissions standards do not distinguish between narrowband and broadband how can you reduce the narrowband emissions ?
12.2 List the high frequency broadband sources.
12.3 If circuit constraints allow, list the methods that can be used to slow down clock rates in order to minimize harmonic generation.
12.4 How can we decrease the level, measured in a constant 120 kHz bandwidth falls, by between 10 to 20 dB without any extra effort made in layouts and without slowing the clock rise times?
Question 13
Why is it difficult or almost impossible to shield against magnetic fields at low frequencies?
impossible with purely conductive materials. This is because the reflection loss to an impinging magnetic field (RH) depends on the mismatch of the field impedance to the barrier impedance.
Question 14
Generally, analog circuits are not as susceptible to transient upset as digital ones, but may be more susceptible to demodulation of RF energy. Describe how to restrict the operating bandwidth to the minimum acceptable level in order to reduce the level of interfering signal.
Question 15
Describe the advantages and disadvantages of surface mount devices (SMDs).
For high accuracy and reliability, SM components should be mounted and soldered by machine rather than by hand. Besides, for some of the special SM components used by DACT, the component manufacturers require large order quantities. So while SM components offer technical advantages over leaded components, it requires a production of a certain scale to make it feasible to use them.
Question 16
Describe the four basic types of SPDs Surge protections devices Spark arresters See chapter 7
Question 17
List the two main issues with PSUs
Question 18
Describe the applications for different type of screened cables.
Question 19
What are two main principles of cable routing?
Question 20
What are the advantages of using screened connectors?
Question 21
What are the thumb rules for various available conditions for unwanted signals?
Question 22
How does the performance of any filter depends on the impedance seen at its terminal?
Question 23
Describe the difference between differential and common mode coupling from PCB's.
Question 24
What is the first and foremost step in PCB layout.
Question 25
How can you distinguish between CM (common mode) and DM (differential mode) currents when using a current probe.