TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 75 of 143
Guidelines for RF termination design
RF terminations must provide a characteristic impedance of 50
as well as the RF transmission lines up to the RF
terminations themselves, to match the characteristic impedance of the
ANT1
/
ANT2
ports of the modules.
However, real antennas do not have a perfect 50
load on all the supported frequency bands. Therefore, to
reduce as much as possible any performance degradation due to antennas mismatch, the RF terminations must
provide optimal return loss (or VSWR) figures over all the operating frequencies, as summarized in Table 8 and
Table 9.
If external antennas are used, the antenna connectors represent the RF termination on the PCB:
Use suitable 50
connectors providing a correct 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 the annular pads of the four GND posts, as shown in Figure 35.
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 RF connectors and close to buried vias, in order to remove stray capacitance
and thus keep the RF line 50
, e.g. the active pad of U.FL connectors 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.
If integrated antennas are used, the RF terminations are represented by the integrated antennas themselves. The
following guidelines should be followed:
Use antennas designed by an antenna manufacturer, providing the best possible return loss (or VSWR).
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
a wavelength of the minimum frequency that must be radiated. As a 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, antennas 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 antennas within a closed metal case.
Do not place the antennas in close vicinity to the end user since the emitted radiation in human tissue is
limited by regulatory requirements.
Place the antennas far from sensitive analog systems or employ countermeasures to reduce EMC issues.
Take care of interaction between co-located RF systems since the LTE/3G/2G transmitted power may interact
or disturb the performance of companion systems.
Place the two LTE antennas providing low Envelope Correlation Coefficient (ECC) between the primary
(
ANT1
) and secondary (
ANT2
) antenna: the antenna 3D radiation patterns should have lobes in different
directions. The ECC between the primary and secondary antennas needs to be low enough to comply with
the radiated performance requirements specified by related certification schemes, as indicated in Table 10.
Place the two LTE antennas providing enough high isolation (see Table 10) between the primary (
ANT1
) and
secondary (
ANT2
) antennas. The isolation depends on the distance between antennas (separation of at least
a quarter wavelength required for good isolation), antenna type (using antennas with different polarization
improves isolation), and the antenna 3D radiation patterns (uncorrelated patterns improve isolation).
