Designing for EMC Compliance

EMC RULES
LCR Electronics, Inc. - EMC Workshop 1995

1. Anticipate Potential Problem Areas

• Changing voltage or circuit conditions will generate EMI. 
• Identify potential sources of EMI.  Brush Motors, clocks, relays, triac circuits, switchers, etc.
• Identify if the noise source will be broad or Narrowband in nature. (Brush motor noise is broadband and  Clock signals are Narrowband).

2. Filter the Emissions

• Select EMI filtering with care. If necessary get assistance from an EMI filter expert. 
• Custom filters designed for your application will usually work out far better and more cost effective than off the shelf standards.
• Make sure the filters are installed in accordance with the manufacturers instructions.
• Use integrated filter circuits in place of discrete EMI components positioned in a haphazard manner.

3. Grounding

• Select a solid ground point.
• Use a single point low impedance ground to avoid any ground loops.
• DC ground paths are very often hign impedance RF grounds. 
• Use ground planes whenever possible.
• Keep ground runs as short as possible.
• Avoid using thin wires for ground.

4. Printed Circuit Board Design and Layout

• Use multi-layer pc boards wherever possible.
• Place supply rails on opposite sides of multi-layer board.
• Use decoupling capacitors.
• Decouple RF currents and RF ground circuits
• Use short tracks.
• Avoid sharp corners on traces.
• Avoid unterminated inputs and outputs.
• Use filtered inputs and outputs. 
• Avoid clock signal trace loops.

5. Component Layout

• Keep similar circuitry components grouped together to maximize isolation.
• Group high offending components in selected areas so if necessary they can be shielded off from the rest of the circuitry.
• Design for minimum possible clock speeds for logic circuits.
• Design for minimum switching speeds.
• Keep RF ground separate from power, logic or digital ground. 
• Keep ground traces as thick, large and short as possible. 
• Group components in such a way as to avoid intrasystem interference.  (Keep HF oscillators away from sensitive analog circuits and keep power circuits away from signal processing).

6. Power Supply Design and Layout

• Use Multi-layer printed circuit boards where possible.
• Use decoupling capacitors as close to the component pins as possible. 
• Use twisted pair wires wherever possible to reduce common mode interference.  The power supply should if possible be designed for a low impedance source.  Ensure a solid ground plane. 
• Ensure single point or star grounding.
• Use decoupling from supply rails to ground at input and output.
• Keep PCB traces as short as possible.
• Use separate analog, digital and power supply rails and grounds.
• Decouple signal grounds.
• Avoid right angle bends on pc traces and wires.
• Keep input and output wires on opposite sides, if possible.
• Keep filtered and unfiltered wires from each other.

LCR Electronics, Inc. - EMC Workshop 1995

For More Information Contact:
LCR Electronics, Inc
9 South Forest Ave, Norristown, PA 19401
Tel: 1-800-527-4368
FAX: 1-610-278-0935
Internet: engineering@lcr-inc.com

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