Connection interferance

Question 1

Define “Interference”

Answer 1

The unwanted coupling of signals from outside the system under consideration.

Question 2

Define “Signal”

Answer 2

Any waveform that is generated systematically somewhere.

Question 3

Define “Crosstalk”

Answer 3

Unwanted coupling of signals from within the system under consideration

Question 4

Define “Noise”

Answer 4

Random process which may originate internally or externally of the system under consideration.

Question 5

Define “Coupling mechanism”

Answer 5

The unwanted circuit element equivalents that can explain the interference problem

Question 6

Define “Parasitic coupling mechanism”

Answer 6

> Unwanted but inevitable noise
Example: Capacitor dielectric leakage can cause a resistance which forms an ‘equivalent series resistor’ (ESR). ESR is dependent on frequency.

Question 7

Define “Stray coupling mechanism”

Answer 7

> Unwanted electrical element which exists between circuit components.
Two wires will both have capacitance and mutual inductance between them.

Question 8

What is capacitive coupling?

Answer 8

A capacitance that occurs between any two stray points, not between the plates of a manufactured capacitor.

Question 9

What is the biggest example of capacitive coupling?

Answer 9

Electrical mains power cables near sensitive electronics or between two PCB traces

Question 10

How can we plan to combat capacitive coupling?

Answer 10

> We can estimate the possible size of the capacitance and use this to add an approximate equivalent capacitor to the circuit.
We can then change the design to include this capacitance

Question 11

What is the equation for the capacitance of a capacitor?

Answer 11

C = εA / d
C = Capacitance
ε = Permittivity of the medium
A = Area of the plate
d = Distance between plates

Question 12

What is the equation for capacitive coupling?

Answer 12

C = πε / ln⁡(d / a) 
C = Capacitance
ε = Permittivity of the medium
A = Area of the plate
d = Distance between plates

Question 13

What are the units of capacitive coupling?

Answer 13

Farads / meter

Question 14

When do we use the equation for the capacitance of a capacitor?

Answer 14

When d &laquo_space;A

Question 15

When do we use the equation for capacitive coupling?

Answer 15

When d&raquo_space; A

Question 16

What can happen with capacitive coupling and high resistances?

Answer 16

> This can accidentally form an RC network and filter frequencies.
This also load resistors in the parallel circuit when the parallel circuit is not powered
[Picture48]

Question 17

What is the problem with bread boards?

Answer 17

Prototyping boards have a small stray capacitance between each strip ≈ 3pF and 30-40pF across the long bus strips.

Question 18

What can help to stop capacitive coupling? (2)

Answer 18

> A grounded screen (or shield) can be placed around the circuits that might be affected.
For PCB traces a grounded guard trace could be placed between the two traces where cross talk might be a problem.

Question 19

What is mutual inductance?

Answer 19

> Where two current-carrying conductors are in proximity, there will be a mutual inductance between them
This will form an induced voltage source on the victim circuit.

Question 20

Will mains supply cables cause mutual inductance?

Answer 20

Mains supply cables are less likely to cause inductive coupling because the magnetic fields around the live and neutral conductors tend to cancel each other out.

Question 21

Can you use screening to block inductive coupling? What is the best solution to blocking it?

Answer 21

Screening is not effective for inductive coupling as it is magnetic in nature. The best defence is to separate the source of inductive coupling.

Question 22

What is the equation for mutual inductance?

Answer 22

e.m.f = M(di / dt)
M = The mutual inductance

Question 23

Define “balanced lines”

Answer 23

Two conductors which have the same impedance with respect to a common, external ground.

Question 24

What is XLR?

Answer 24

This is a form of balanced lines used for audio applications

Question 25

What are the conductors in XLR?

Answer 25

> 2 balanced lines

> 1 Grounded shield.

Question 26

What are two method of converting unbalanced lines to balanced ones?

Answer 26

> Using a differential amplifier. Two inputs will therefore have the same impedance.
Use a transformer to couple signals from an unbalanced system of conductors to a balanced system (Called baluns [balanced to unbalanced transformer])

Question 27

What are ground loops?

Answer 27

Caused by connecting signals from one item of equipment to another and, in doing so, making a duplicate ground connection because both items of equipment have the mandatory protective earth connections

Question 28

Why are ground loops an issue in the situation [Picture49]?

Answer 28

No current flows in the protective conductor but is free to pick up interference. This produces a potential difference between a point on the ground cable and ground. Problems occur when the source of interference creates a voltage drop within the loop.

Question 29

What are the solutions to ground loops?

Answer 29

> Break the loop
Double insulation
Using battery-powered equipment.
Isolated electrically floating inputs or outputs

Question 30

What happens if you break the ground loop?

Answer 30

> Bad solution: Disconnecting the signal cable ground. Requires the return signal to flow through the mains earth conductor.
Very bad solution: Disconnect the mains protective ground on one of the items of equipment.

Question 31

What is the best way to break the ground loop?

Answer 31

[Picture50]
> To use isolated electrically floating inputs or outputs
> Locally ground system that is not directly connected to the mains power supply ground.

Question 32

What happens when you share a ground path with lots of devices?

Answer 32

> There will be IR voltage drops along this path due to the resistance of the wire.
This can be a problem for sensitive circuits

Question 33

What is the solution for sensitive circuits on a ground path shared with lots of devices?

Answer 33

> Separate ground paths for sensitive circuits routed directly to low impedance ground point near to the supply circuit

Question 34

What happens when sensitive circuits share a ground path with digital circuits?

Answer 34

> Digital circuits can draw significant current pulses which must be returned via the ground path.
For sensitive analogue circuits we can use separate ground paths for analogue and digital circuits.