HF-Protection of Electronic Circuits Thomas Losert

2 Overview Noise Sources (External, Internal) Basics about Shielding Examples Outlook

3 Overview Noise Sources (External, Internal) Basics about Shielding Examples Outlook

4 External Noise Sources External noise is received mostly through I/O- connectors or electromagnetic waves Shield I/O connectors (or even the cables) and/or use filters for blocking noise Use a star-topology for ground, i.e. connect all ground lines in a common point

5 Internal Noise Sources Internal noise is caused mostly by signals with high frequency (e.g. Oscillator of Microcontroller) or high currents (e.g. switching power supply) Minimize length of circuit paths that could act as antennas for noise signals (e.g., ground-planes) Maximize Distance to sensitive signals (e.g. measurement signals) Subdivide the system in (nested) „System Zones“ and use filters for blocking noise at the boundaries Eliminate noise sources (e.g., no floating input pins)

6 Overview Noise Sources (External, Internal) Basics about Shielding Examples Outlook

7 Basics about Shielding Due to the „skin effekt“ for higher frequencies a thinner shield is sufficient (e.g., conductive foil, tape, or paint) For higher frequencies (shorter wavelengths) the tolerable gap dimension in the shield decreases Noise should be eliminated at the source if possible (e.g., decoupling capacitors) Noise sensitive circuits should be separated from noise sources (e.g., ground planes, conductive material, or distance)

8 Overview Noise Sources (External, Internal) Basics about Shielding Examples Outlook

9 Example: Decoupling Capacitors

10 Example: Ground Planes Current can only flow in loops Larger loops (bigger „antennas“) emit more noise Dedicated ground-planes help minimizing the effective area of the loop Ground-planes should not be used for routing of other signals Adding paths to the ground-plane require a detailed analysis of every track on the board (otherwise the ground-plane could be wasted)

11 Example: Shielded Circuit Metal case protects against undesired electro-magnetic waves feed-through capacitor for decoupling the power supply

12 Example: Subdivision of a System Further shielding could be achieved with compartments of conductive material For shielding of ventilation openings lattice- like panels are available

13 Example: Path to Ground Separate high noise/current lines and analog/sensitive signals

14 Overview Noise Sources (External, Internal) Basics about Shielding Examples Outlook

15 Outlook On Saturday, May 1 st, 2004 same time, same place „PCB Design“

16 Vielen Dank für Ihre Aufmerksamkeit ! Noch Fragen?