31/38

EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

Figure 18. PCB Board Oscillator Layout Examples

The reduction of inductance can be obtained by making the lengths and surfaces of the track

smaller. This can be obtained by placing the track loops closer on the same PCB layer or on

top of one another (Figure 19.). The resulting loop area is small and the electromagnetic fields

reduce one another.

The ratio in order of magnitude relating to the inductance value and the area defined by the

wire loop is around 10nH/cm2. Typical examples of low inductivity wires are coaxial, twisted

pair cables or multiple layer PCBs with one ground and one supply layers. The current density

in the track can also be smaller due to track enlargement or the paralleling of several small ca-

pacitances mounted in the current flow.

In critical cases, the distance between the MCU and the PCB, and therefore the surfaces of

the loops between an MCU and its environment, has also to be minimized. This can be

achieved by removing any socket between the MCU package and the PCB, by replacing a ce-

ramic MCU package by a plastic one or by using Surface Mounting instead of Dual In Line

packages.

Note: Board vias are inductances. Try to avoid them. If needed, use multi vias.