SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 120 of 217
Guidelines for RF transmission line design
The transmission line from the
ANT
pad up to antenna connector or up to the internal antenna pad must be
designed so that the characteristic impedance is as close as possible to 50
.
The transmission line 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 57 and Figure 58 provide two examples of suitable 50
coplanar waveguide designs. The first
transmission line can be implemented for a 4-layer PCB stack-up described herein, the second transmission line
can be implemented for a 2-layer PCB stack-up described herein.
35 um
35 um
35 um
35 um
270 um
270 um
760 um
L1 Copper
L3 Copper
L2 Copper
L4 Copper
FR-4 dielectric
FR-4 dielectric
FR-4 dielectric
380 um 500 um
500 um
Figure 57: Example of 50
coplanar waveguide transmission line design for the described 4-layer board layup
35 um
35 um
1510 um
L2 Copper
L1 Copper
FR-4 dielectric
1200 um 400 um
400 um
Figure 58: 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 57 and Figure 58)
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 57, 1510 µm in Figure 58)
the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material in
Figure 57 and Figure 58)
the gap from the transmission line to the adjacent ground plane on the same layer of the transmission line
(e.g. 500 µm in Figure 57, 400 µm in Figure 58)
If the distance between the transmission line and the adjacent GND area (on the same layer) does not exceed
5 times the track width of the micro strip, use the “Coplanar Waveguide” model for the 50
calculation.
