SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R11 Design-in Page 81 of 157
Min.
250 µm
Min. 400 µm
GND
ANT
GND clearance
on buried layer very close to top layer
below ANT pad
GND clearance
on top layer
around ANT pad
Figure 31: GND keep-out area on top layer around ANT pad and on very close buried layer below ANT pad
Guidelines for RF transmission line design
Any RF transmission line, such as the ones from the ANT pad 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 32 and Figure 33 provide two examples of suitable 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.
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 µm500 µm
Figure 32: Example of 50 coplanar waveguide transmission line design for the described 4-layer board layup
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