TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 73 of 143
Guidelines for RF transmission line design
Any RF transmission line, such as the ones from the
ANT1
and
ANT2
pads up to the related antenna connector
or up to the related internal antenna pad, must be designed so that the characteristic impedance is as close as
possible to 50
.
RF transmission lines can be designed as a micro strip (consists of a conducting strip separated from a ground
plane by a dielectric material) or a strip line (consists of a flat strip of metal which is sandwiched between two
parallel ground planes within a dielectric material). The micro strip, implemented as a coplanar waveguide, is the
most common configuration for printed circuit boards.
Figure 33 and Figure 34 provide two examples of suitable 50
coplanar waveguide designs. The first example
of an RF transmission line can be implemented for a 4-layer PCB stack-up herein illustrated, and the second
example of an RF transmission line can be implemented for a 2-layer PCB stack-up herein illustrated.
35 µm
35 µm
35 µm
35 µm
270 µm
270 µm
760 µm
L1 Copper
L3 Copper
L2 Copper
L4 Copper
FR-4 dielectric
FR-4 dielectric
FR-4 dielectric
380 µm 500 µm
500 µm
Figure 33: Example of a 50
coplanar waveguide transmission line design for the described 4-layer board layup
35 µm
35 µm
1510 µm
L2 Copper
L1 Copper
FR-4 dielectric
1200 µm 400 µm
400 µm
Figure 34: Example of a 50
coplanar waveguide transmission line design for the described 2-layer board layup
If the two examples do not match the application PCB stack-up, the 50
characteristic impedance calculation
can be made using the HFSS commercial finite element method solver for electromagnetic structures from Ansys
Corporation, or using freeware tools like AppCAD from Agilent (www.agilent.com) or TXLine from Applied
Wave Research (www.mwoffice.com), taking care of the approximation formulas used by the tools for the
impedance computation.
To achieve a 50
characteristic impedance, the width of the transmission line must be chosen depending on:
the thickness of the transmission line itself (e.g. 35 µm in the examples of Figure 33 and Figure 34)
the thickness of the dielectric material between the top layer (where the transmission line is routed) and the
inner closer layer implementing the ground plane (e.g. 270 µm in Figure 33, 1510 µm in Figure 34)
