AN4169

RF layout guidelines

Doc ID 023704 Rev 1

5/14

Modern PCB manufacturing achieves very accurate etching and so this is not considered as

a performance variable. Z-axis expansion depends on the FR4 material and does not alter

much between vendor material.

2.2

Two or multi-layer board design

A different choice regarding the number of the layers can be made. A two-layer or an N-layer

(where N may be 3 or 4) application board can be used to develop the RF part of an

application.

The usual distribution for a four-layer board should be:

1.

TOP layer, used for the RF signals;

2.

GROUND layer, used only as ground plane;

3.

POWER layer, used for DC power plane;

4.

BOTTOM layer, used for the low frequency and digital signals.

The advantage of N-layer designs over two-layer designs is that the former allows for

distributed RF decoupling of a DC power plane by a ground plane, as illustrated in Figure 2.

Placing a distributed ground plane between the RF plane and the power plane enables an

evenly distributed RF decoupling layer. In addition, the power plane provides a low

impedance trace at radio frequency.

The main advantage of the multi-layer solution is the possibility to put the ground layer very

near to the RF plane to reduce the parasitic effect of the ground plane. All the RF devices

need to have a strong ground plane and it is possible to obtain this by reducing how much

the parasitic effect is possible by reducing the distance between the TOP layer and the

GROUND layer.

A two-layer design typically requires a little more care with the PCB routing, but can be

successfully implemented for applications with a low number of tracks. The power supply

trace on the component is made quite thick to present as low an impedance trace as

possible. Large areas of ground on this side of the board provide a low impedance path for

the decoupling.

A two-layer PCB is cheaper to manufacture than a three or four-layer PCB. However, to

implement microstrip or stripline transmission lines, the PCB thickness should not exceed

0.8 mm - 1.00 mm, since the width of the transmission line trace becomes rather large.

Figure 2.

Three-layer application board structure

AM14761V1