SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R11 Design-in Page 82 of 157
35 µm
35 µm
1510 µm
L2 Copper
L1 Copper
FR-4 dielectric
1200 µm 400 µm400 µm
Figure 33: 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 stack-up, then 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 Avago / Broadcom AppCAD
(https://www.broadcom.com/appcad) 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 32 and Figure 33)
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 32, 1510 µm in Figure 33)
the dielectric constant of the dielectric material (e.g. dielectric constant of the FR-4 dielectric material
in Figure 32 and Figure 33)
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 32, 400 µm in Figure 33)
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.
Additionally to the 50 impedance, the following guidelines are recommended for transmission lines
design:
Minimize the transmission line length: the insertion loss should be minimized as much as possible, in
the order of a few tenths of a dB,
Add GND keep-out (i.e. clearance, a void area) on buried metal layers below any pad of component
present on the RF transmission lines, if top-layer to buried layer dielectric thickness is below 200 µm, to
reduce parasitic capacitance to ground,
The transmission lines width and spacing to GND must be uniform and routed as smoothly as possible:
avoid abrupt changes of width and spacing to GND,
Add GND stitching vias around transmission lines, as described in Figure 34,
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