Ublox SARA-N4 Series, System Integration Manual

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SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R11 Design-in Page 82 of 157
35 µ m
35 µ m
1510 µ m
L2 Copper
L1 Copper
FR-4 dielect ric
120 0
µ
m 40 0
µ
m 40 0
µ
m
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
µ
m 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
µ
m in Figure 32 , 1510
µ
m 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
µ
m in Figure 32 , 400
µ
m 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
µ
m, 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 ,