2015 Exam

Question 1

Uniform Transmission Line

Answer 1

Geometry + material does NOT change in the direction of propagation

Question 2

Example of Uniform / Non-uniform T.L

Answer 2

Uniform - Co-axial cable

Non-uniform - PCB track

Question 3

Common Impedance Coupling (3)

Answer 3

• single conductor carrier two different signals
• As impedance of conductor is not zero, there will be a voltage drop proportional to the sum of the two signal currents
• Voltage appears in series w/ both signal causing unwanted coupling ​between them

Question 4

Example of Common Impedance Coupling

Answer 4

when two core screened cables are used to carry two signals using the screen as the common-earth

Question 5

How to avoid Ground Bounce (3)

Answer 5

• Using multiple earth pins
• Thicker conductors to reduce inductance
• Balanced signalling in which earth potental is NOT used as a ref.

Question 6

Adv. of multi-layer PCB over single layer (3)

Answer 6

• allows more connections
• allows shielding between tracks carrying large signals + tracks carrying sensitive circuits
• allows different DC voltages to exist w/o having to cut the image planes

Question 7

Design layout for 6 PCB circuit (4)

Answer 7

• Image planes @ top + bottom layers
• Image planes between signal layers
• Signal layers + image planes to be adj. for impedance control
• Achieve this @ expense of # possible tracks

Question 8

Important point if electrical lengths are equal (2)

Answer 8

If lengths are not the same:

• very high common mode currents induced at the time of transition

For a brief time between 1 wire has made transition + other hasn’t, the wire will carry the same current which has to return via an ill-defined earth path which is undesirable