Signal Integrity Notes

• Introduction to Signal Integrity
  • Signal Integrity Definition
    • Three main considerations:
    • Four families of signal Integrity Problem
    • Three factors to Signal quality with one Net
    • Two situations that Cross-talk happens
  • Routing Techniques
    • Three basic aspects of traces to consider :
    • Four basic techniques for GNDing and vias placement :
    • Practical techniques for Gounding

Introduction to Signal Integrity#

Signal Integrity Definition#

Three main considerations:#

• Timing

• Noise

• Electromagnetic interference (Consider Singal integrity when the clock reaches approximately 100Mhz)

Four families of signal Integrity Problem#

1. Signal quality of one net, including losses on the line

2. Cross-talk between two or more nets, including ground and power balance

3. Rail collapse in the power and ground distribution

4. Electromagnetic interference and radiation from the entire system

Definition of a Net: A net comprises all the metal connected in a system.

Three factors to Signal quality with one Net#

• 1 and 2: Frequency-dependent losses in the line from the conductor and the dielectric cause higher-frequency signal components to be attenuated more than the lower-frequency components. The end result is an increase in the rise time of the signal as it propagates. When this rise time degradation approaches the period of a single bit, the digital information will be distorted. This effect is called inter-symbol interference (ISI)
• Skew in the single net (time delay between two or more nets can be adjusted by matching the length of two nets)

Two situations that Cross-talk happens#

• When the interconnects are uniform transmission lines: Anything that changes the return path from a wide uniform plane will increase the amount of coupled noise between two transmission lines.
• When they are not uniform transmission lines:

Routing Techniques#

Three basic aspects of traces to consider :#

1. Trace width / cross section: wider or narrower for impedence matching.

2. Trace length:

   • Length match for phase control (differential pairs, PS ground plane underneath should uninterrupted)

   • bigger loop area for less induced currents interference.

3. Trace distribution: concentrated on one region for less EMI.

   • Seperate digital and analog components part (seperate grounding), sometimes called split ground.
   • High-Speed components should be placed near the ground plane / away from the board edges.

Four basic techniques for GNDing and vias placement :#

1. Star grounding topology to prevent ground loops.
2. Power-Ground Couping
3. Shielding using metal shield or guard traces arround sensitive components

Practical techniques for Gounding#

• Grounding Near Connectors: Place ground ports adjacent to I/O connectors to provide a direct return path, minimizing loop area.
• Ground Pours: Use in unused board areas, connected via vias to prevent antenna effects from floating metal.