u-blox LARA-R2 series System Integration Manual page 87

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 board.
Figure 40 and Figure 41 provide two examples of proper 50  coplanar waveguide designs. The first example of
RF transmission line can be implemented in case of 4-layer PCB stack-up herein described, and the second
example of RF transmission line can be implemented in case of 2-layer PCB stack-up herein described.
L1 Copper
FR-4 dielectric
L2 Copper
FR-4 dielectric
L3 Copper
FR-4 dielectric
L4 Copper
Figure 40: Example of 50  coplanar waveguide transmission line design for the described 4-layer board layup
L1 Copper
FR-4 dielectric
L2 Copper
Figure 41: Example of 50  coplanar waveguide transmission line design for the described 2-layer board layup
If the two examples do not match the application PCB layup, 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 example of Figure 40 and Figure 41)

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 40, 1510 µm in Figure 41)
UBX-16010573 - R02
500 um 380 um 500 um
400 um 1200 um 400 um
Objective Specification
LARA-R2 series - System Integration Manual
35 um
270 um
35 um
760 um
35 um
270 um
35 um
35 um
1510 um
35 um
Design-in
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