RF layout guidelines

AN4169

6/14

Doc ID 023704 Rev 1

2.3

PCB transmission line

unlike all other interconnections, do not resonate however long they are. Transmission lines

can easily be made on PCBs by controlling materials and dimension and providing accurate

termination resistances at source and/or load. They may be also extended off the PCB with

appropriate controlled-impedance cables and connectors.

Comparing the length of a PCB track conductor with the wavelength of the highest

frequencies of concern in the relevant medium give us what is called “electrical length” of

the track. When a conductor is “electrically long”, transmission lines need to be used to

maintain the frequency response or to prevent excessive distortion of the wave shape.

Due to the very high frequency of the signal in the RF PCBs, a transmission line technique

must be used. Different types of transmission line structure were studied. The more simple

one commonly used in the RF PCBs is the microstrip trace. Figure 3 illustrates an example

of a PCB microstrip, where a trace on the top side of the board is isolated by the PCB

dielectric material from the ground plane layer. From a knowledge of the physical properties

of the PCB it is possible to construct a transmission line trace with the desired characteristic

impedance.

A different structure is shown in Figure 4. In this case, called grounded coplanar line, the

trace is isolated from the ground plane layer by the PCB dielectric, but also the track is near

to the ground plane in the same layer between an air gap.

A lot of formulas were developed to define the characteristic impedance of the two different

structures, but a presentation of these formulas are beyond the scope of this application

note.

The grounded coplanar structure guarantees a reduced sensitivity to the PCB thickness

variation. This also reduces radiation and coupling effects. Use 50

Ω grounded coplanar

lines to connect the SMA connector to the RF matching/filtering network and/or the RF

switch wherever possible.

Figure 3.

PCB microstrip structure

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AM14764V1