WORST

BETTER

BEST

1W min.

W

SN74CBT3244

SCDS001O – NOVEMBER 1992 – REVISED SEPTEMBER 2015

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11 Layout

11.1 Layout Guidelines

Reflections and matching are closely related to the loop antenna theory but are different enough to be discussed

separately from the theory. When a PCB trace turns a corner at a 90° angle, a reflection can occur. A reflection

occurs primarily because of the change of width of the trace. At the apex of the turn, the trace width increases to

1.414 times the width. This increase upsets the transmission-line characteristics, especially the distributed

capacitance and self-inductance of the trace, which results in the reflection. Not all PCB traces can be straight;

therefore, some traces must turn corners. Figure 6 shows progressively better techniques of rounding corners.

Only the last example (BEST) maintains constant trace width and minimizes reflections.

11.2 Layout Example

Figure 6. Trace Example

10

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