User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08 Objective Specification Design-in
Page 101 of 188
In both cases, selecting an external or an internal antenna, observe these recommendations:
 Select an antenna providing optimal return loss (or V.S.W.R.) figure over all the operating frequencies
 Select an antenna providing optimal efficiency figure over all the operating frequencies
 Select an antenna providing appropriate gain figure (i.e. combined antenna directivity and efficiency figure)
so that the electromagnetic field radiation intensity do not exceed the regulatory limits specified in some
countries (e.g. by FCC in the United States, as reported in section 4.2.2)
For the additional specific guidelines for SARA-G350 ATEX modules integration in potentially explosive
atmospheres applications, see section 2.14.
2.4.1.2 Guidelines for antenna RF interface design
Guidelines for ANT pin RF connection design
Proper transition between the ANT pin and the application board PCB must be provided, implementing the
following design-in guidelines for the layout of the application PCB close to the pad designed for the ANT pin:
 On a multi layer board, the whole layer stack below the RF connection should be free of digital lines
 Increase GND keep-out (i.e. clearance, a void area) around the ANT pad, on the top layer of the application
PCB, to at least 250 µm up to adjacent pads metal definition and up to 400 µm on the area below the
module, to reduce parasitic capacitance to ground, as described in the left picture in Figure 48
 Add GND keep-out (i.e. clearance, a void area) on the buried metal layer below the ANT pad if the top-layer
to buried layer dielectric thickness is below 200 µm, to reduce parasitic capacitance to ground, as described
in the right picture in Figure 48
Min.
250 µm
Min. 400 µm
GND
ANT
GND clearance
on very close buried layer
below ANT pad
GND clearance
on top layer
around ANT pad
Figure 48: GND keep-out area on the top layer around ANT pad and on the very close buried layer below ANT pad
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.