SARA-R4 series - System Integration Manual
UBX-16029218 - R06
Design-in
Page 53 of 102
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 the
ANT
port.
However, real antennas do not have perfect 50
load on all the supported frequency bands. Therefore, to
reduce as much as possible performance degradation due to antennas mismatch, the RF termination must
provide optimal return loss (or V.S.W.R.) figure over all the operating frequencies, as summarized in Table 7.
If an external antenna is used, the antenna connector represents the RF termination on the PCB:
Use suitable a 50
connector providing proper PCB-to-RF-cable transition.
Strictly follow the connector manufacturer’s recommended layout, for example:
o
SMA Pin-Through-Hole connectors require GND keep-out (i.e. clearance, a void area) on all the layers
around the central pin up to annular pads of the four GND posts, as shown in Figure 23
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.
Cut out the GND layer under the RF connector and close to buried vias, to remove stray capacitance and
thus keep the RF line 50
, e.g. the active pad of UFL connector needs to have a GND keep-out (i.e.
clearance, a void area) at least on first inner layer to reduce parasitic capacitance to ground.
If an integrated antenna is used, the RF terminations are represented by the integrated antenna. The following
guidelines should be followed:
Use an antenna designed by an antenna manufacturer, providing the best possible return loss (or V.S.W.R.).
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 has to be radiated. As numerical example,
Frequency = 750 MHz
Wavelength = 40 cm
Minimum GND plane size = 10 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 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 the
design-in guidelines for the antenna matching relative to the custom application.
Additionally, these recommendations regarding the antenna system placement must be followed:
Do not place the antenna within closed metal case.
Do not place the antenna in close vicinity to end user since the emitted radiation in human tissue is limited
by regulatory requirements.
Place the antenna far from sensitive analog systems or employ countermeasures to reduce EMC issues.
Take care of interaction between co-located RF systems since the LTE transmitted power may interact or
disturb the performance of companion systems.
