LEXI-R422 - System integration manual
UBX-23007449 - R02
Design-in
Page 51 of 108
C1-Public
•
Avoid stubs on the transmission lines.
•
Avoid signal routing in parallel to transmission lines or crossing the transmission lines on buried
metal layer.
•
Do not route microstrip lines below discrete component or other mechanics placed on top layer.
Two examples of a suitable RF circuit design for
ANT
pin
are illustrated in
, where the cellular
antenna detection circuit is not implemented (if the cellular antenna detection function is required by
the application, follow the guidelines for circuit and layout implementation detailed in section
•
In the first example shown on the left, the
ANT
pin is directly connected to an SMA connector by
means of a suitable 50
transmission line, designed with the appropriate layout.
•
In the second example shown on the right, the
ANT
pin is connected to an SMA connector by means
of a suitable 50
transmission line, designed with the appropriate layout, with an additional high
pass filter to improve the ESD immunity at the antenna port. The filter consists of a suitable series
capacitor and shunt inductor, for example the Murata GRM1555C1H150JB01 15 pF capacitor
and the Murata LQG15HN39NJ02 39 nH inductor with SRF ~1 GHz.
LEXI module
U.FL
connector
High-pass filter
to improve
ESD immunity
-
LEXI module
U.FL
connector
Figure 32: Example of circuit and layout for ANT RF circuits on the application board
☞
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.
2.4.1.3
Guidelines for RF termination design
The RF termination must provide a characteristic impedance of 50
as well as the RF transmission
line up to the RF termination, to match the characteristic impedance of
ANT
port.
However, real antennas do not have a perfect 50
load on all the supported frequency bands. So to
reduce as much as possible any performance degradation due to antenna mismatching, the RF
termination must provide optimal return loss (or VSWR) figures over all the operating frequencies, as
summarized in
If an external antenna is used, the antenna connector represents the RF termination on the PCB:
•
Use a suitable 50
connector providing a clean PCB-to-RF-cable transition.
•
Strictly follow the connector manufacturer’s recommended layout, for example:
o
SMA Pin-Through-Hole connectors require a GND keep-out (i.e. clearance, a void area) on all the
layers around the central pin up to the annular pads of the four GND posts.
o
U.FL surface mounted connectors require no conductive traces (i.e. clearance, a void area) in
the area below the connector between the GND land pads, as shown in
