LEXI-R422 - System integration manual
UBX-23007449 - R02
Design-in
Page 49 of 108
C1-Public
2.4
Antenna interfaces
LEXI-R422 modules provide an RF interface for connecting the external antenna: the
ANT
pin
represents the RF input/output for RF signals transmission and reception.
The
ANT
pin has a nominal characteristic impedance of 50
and must be connected to the related
external antenna system through a 50
transmission line to allow clean transmission / reception of
RF signals.
2.4.1
Cellular antenna RF interface (ANT)
2.4.1.1
Guidelines for ANT pin RF connection design
A clean transition between the
ANT
pad and the application PCB must be provided, implementing the
following design-in guidelines for the layout of the application PCB close to the
ANT
pad:
•
On a multilayer PCB, the whole layer stack below the RF connections should be free of digital lines.
•
Increase GND keep-out (i.e. clearance, a void area) at least up to 500 µm around the
ANT
pad on
the top layer of the application PCB, to reduce parasitic capacitance to ground, as described in the
left picture in
•
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
GND clearance
on top layer
around RF pad
GND clearance
on buried layer very close to top layer
below RF pad
RF pad
0.5 mm
Figure 29: GND keep-out area on top layer around RF pad and on very close buried layer below RF pad (ANT)
☞
See section
for the description of the antenna trace design implemented on the u-blox host
printed circuit board used for conformity assessment of LEXI-R422 surface-mounted modules for
regulatory type approvals such as FCC United States, ISED Canada, RED Europe, etc.
2.4.1.2
Guidelines for RF transmission lines 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.
and
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.
