Electromagnetic Compatibility: Multi-Layer PCB Designing (Part 2 of 5)

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Reducing cross-talk: The 3W rule

Fig. 8: The 3W rule
Fig. 8: The 3W rule

Cross-talk is capacitive coupling of signals from one circuit track to another. This can occur when a high-frequency track is running parallel to a susceptible track. Cross-talk flux and associated capacitive coupling can be reduced by 70 per cent if the two tracks are separated by a distance of 3W, where W is the width of the traces (measured from trace centre). This will cause track flux to terminate on the ground plane below rather than on adjacent tracks (shown as dotted lines in Fig. 8).

Guard traces in Multi-Layer PCB Design

Traces carrying high-frequency signals like clocks can cause coupling of clock harmonics on nearby traces, causing a jittering clock that can lead to problems elsewhere in the circuit.

These problems can be reduced by providing guard traces (Fig. 9) on either sides of such a track. Distance between the guard trace and the trace to be protected follows 3W rule.

Guard trace is connected to ground at both ends. If it is long, it should be connected to the ground plane at multiple points l/20 apart, where l is the minimum wavelength of the signal/clock.

Shunt trace in Multi-Layer PCB Design

Fig. 9: Guard and shunt traces
Fig. 9: Guard and shunt traces

This is a trace located immediately below the critical clock trace. It need not be connected to ground. Best isolation can be obtained by providing both guard and shunt traces, the former providing isolation by terminating magnetic field lines (shown as dotted lines in Fig. 9) and the latter providing isolation by terminating electric field lines.

Image planes

An image plane is a plane physically adjacent (immediately below) to the signal routing plane.

It provides a low-impedance path for RF currents and reduces emissions since RF currents use the plane instead of air. Power supply and ground planes normally behave as image planes, although separate image planes can also be provided.

Via inductance

Fig. 10: Equivalent circuit of a via
Fig. 10: Equivalent circuit of a via

Each via in a board introduces additional inductance of 1hH and capacitance up to 0.5pF. Fig.

10 shows an equivalent circuit of a via; it consists of a series inductor and a shunt capacitor and, hence, acts as a low-pass filter. It can cause signal delays and affect the high-frequency performance of a board. Hence, vias should be reduced to a minimum.

Power and ground planes

A continuous sheet or foil of copper for ground (and power) can provide an infinite number of parallel paths and, hence, provide least-possible inductance. At higher frequency, skin effect also starts to play its role, causing an increase in resistive part of impedance.

A plane can provide a very large surface area and least-possible resistance. A ground plane therefore is the single-most cost-effective solution to reduce ground path impedance and solve most EMC problems.

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1 COMMENT

  1. “Now, the speed of movement of electrons in a PCB trace is less that the speed through air (which is the speed of light) since the field surrounding the electron has to travel through the PCB material that creates a drag.”

    I can’t tell if you’re comparing (A) speed of electron through PCB trace to speed of electron through air or (B) speed of electron through PCB trace to speed of photon through air. Photons are the quanta for electromagnetic fields, so either you’re saying A and its wrong, or B and it was phrased poorly.

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