Advanced Diploma of Industrial Automation

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)

1) Common Impedance Noise: Common Impedance Noise might occur in the circuit due to the presence of impedance (an 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.

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.

In = 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.

Fig. 2: Inductive Coupling

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

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)

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

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

1. Arcing faults
2. Contactor, breaker, or relay operations.

1. If an EMI filter includes both CM & DM then DM should have less than 30 MHz whereas 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 the entire frequency.
2. As most circuits carry high frequency thus it can be susceptible to RF. The best possible way to prevent conversion from CM – DM is to reduce the unwanted impedance. This would equalize the frequency as much as possible.

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 heart 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)

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. The maximum whole size should be 3mm.
2. Yes, it is adequate for cooling. However, further whole size can be increased if necessary.