LEXI-R422 - System integration manual
UBX-23007449 - R02
Design-in
Page 52 of 108
C1-Public
•
Cut out the GND layer under the RF connector and close to any buried vias, to remove stray
capacitance and thus keep the RF line at 50
, e.g. the active pad of U.FL connector needs to have
a GND keep-out (i.e. clearance, a void area) at least on the first inner layer to reduce parasitic
capacitance to ground.
Figure 33: U.FL surface mounted connector mounting pattern layout
If an integrated antenna is used, the integrated antenna represents the RF termination. The following
guidelines should be followed:
•
Use an antenna designed by an antenna manufacturer providing the best possible return loss.
•
Provide a ground plane large enough according to the relative integrated antenna requirements:
the ground plane of the application PCB can be reduced down to a minimum size that must be
similar to one quarter of wavelength of the minimum frequency that needs to be radiated; as
numerical example,
Frequency = 617 MHz
→
Wavelength
48 cm
→
Minimum GND plane size
12 cm
•
It is highly recommended to strictly follow the detailed and specific guidelines provided by the
antenna manufacturer regarding correct installation and deployment of the antenna system,
including the PCB layout and matching circuitry.
•
Further to the custom PCB and product restrictions, the antenna may require a tuning to comply
with all the applicable required certification schemes; it is recommended to consult the antenna
manufacturer for antenna matching design-in guidelines relative to the custom application.
Additionally, these recommendations regarding the antenna system placement must be followed:
•
Do not place the antennas within a closed metal case.
•
Do not place the cellular antenna in close vicinity to the end user since the emitted radiation in
human tissue is restricted by regulatory requirements.
•
Place the antennas as far as possible from VCC supply line and related parts (see also
from high-speed digital lines (as USB) and from any possible noise source.
•
Place the antenna far from sensitive analog systems or employ countermeasures to reduce EMC
or EMI issues.
•
Be aware of interaction between co-located RF systems since the LTE transmitted power may
interact or affect the performance of companion systems as a GNSS receiver (see section
for further details and design-in guidelines regarding cellular / GNSS RF coexistence).
☞
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.
2.4.1.4
General guidelines for antenna selection and design
The antenna is the most critical component to be evaluated. Designers must take care of the antenna
from all perspective at the very start of the design phase when the physical dimensions of the
application board are under analysis/decision, since the RF compliance of the device integrating the
LEXI-R422 modules with all the applicable requirements depends on antenna radiating performance.
Cellular antennas are typically available as:
