EMC #42. How to Stop EMI Radiation in PCB Design. Differential Vs Common Mode (High-Speed Layout)

EMC playlist. Watch these video to understand more on EMC.

EMC Part 17. Different Techniques to Reduce the Differential Mode Radiation from a Closed Loop Path.

EMC Part 18. How to Reduce Common Mode Radiation in Far Field in Order to Pass the EMC Certification

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Two Types of PCB EMI Radiation: Differential-mode (DM) & Common-mode (CM) Radiation.

Why PCBs cause EMI radiation?

In a PCB, 2 types of EMI radiation may occur.

Differential-Mode (DM) Radiation

Common-Mode (CM) Radiation

Differential-Mode Radiation is the result of current flowing around loops formed by the conductors of the circuit. In a PCB, RF currents are generated by a source driver and transferred to a load through a trace. RF return currents must return to their source through a ground return path. As a result, a closed loop is developed. Such a loop may be modelled as a small loop antenna, giving rise to radiation.

Differential-mode radiation depends on:

The magnitude of the current, I

The frequency of harmonic content of the current, f

The loop area, A

Common-Mode Radiation is the result of undesired voltage drop in the circuit causing some part of the circuit to be at a common-mode potential above the ‘true’ ground plane. Often this is the result of voltage drops in the digital logic ground system. When external cables are then connected to the system, they are driven at this common-mode potential, forming antennas which radiate electric fields.

Common-mode radiation can be modeled as a short (less than /4) monopole antenna (the cable / PCB trace) driven by a voltage (the ground voltage). Common-mode radiation is harder to control and normally determines the overall radiated emission performance of the product.

Name 2 important objectives to follow in laying out a PCB in order to minimize emissions and susceptibility of the circuits on the PCB.

Keep the loop areas of critical signals (clocks, etc) and their current return paths very small.

Use an image plane (ground plane) for the return current path. The image plane will provide a low impedance path for RF currents to return to their source (flux return) and hence reducing EMI emissions through flux cancellation. In addition, use a guard trace to provide an additional “alternate low impedance return path” to the image plane return path.