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Venerable beads

Effective emi suppression in pcb design. By Alan Keenan.

There can be many causes of electromagnetic interference (emi) on a typical pcb, including higher order harmonics from system clocks, switching noise from on board dc/dc converters and high frequency emi from fast rise times of high speed data signals.

Traditional solutions have used a variety of components, including decoupling capacitors and series inductors. However, the use of these components in themselves can potentially lead to other problems.

With capacitors, the design engineer would use a shunt configuration to decouple high frequency emi to ground or to a ground plane. However, as this high frequency emi still exists in the ground or ground plane, there is the possibility that this energy could be recoupled back onto the pcb and give rise to conducted or radiated emi emissions.

A designer would typically use an inductor in series with the intended signal to block unwanted high frequency emi. However, as the inductor already achieves this by means of its inductive reactance, it merely blocks higher frequencies from passing through. What, then, happens to these high frequency emi signals?

There is a likelihood that they can bounce back and forward between the source of the emi and the inductor. As this energy has not been eliminated, it could therefore give rise to conducted emissions becoming radiated emissions and coupling onto other parts of the circuit.

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Graham Pitcher

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