User Manual

LARA-R2 series - System Integration Manual
UBX-16010573 - R02 Objective Specification Design-in
Page 85 of 148
2.4 Antenna interface
LARA-R2 series modules provide two RF interfaces for connecting the external antennas:
 The ANT1 pin represents the primary RF input/output for LTE/3G/2G RF signals transmission and reception.
 The ANT2 pin represents the secondary RF input for LTE Rx diversity RF signals reception.
Both the ANT1 and the ANT2 pins have a nominal characteristic impedance of 50  and must be connected to
the related antenna through a 50  transmission line to allow proper transmission / reception of RF signals.
Two antennas (one connected to ANT1 pin and one connected to ANT2 pin) must be used to support the
LTE Rx diversity radio technology. This is a required feature for LTE category 1 User Equipments (up to
10.2 Mb/s Down-Link data rate) according to 3GPP specifications.
2.4.1 Antenna RF interface (ANT1 / ANT2)
2.4.1.1 General guidelines for antenna selection and design
The antenna is the most critical component to be evaluated. Designers must take care of the antennas 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 LARA-R2 series modules with all the
applicable required certification schemes depends on antennas radiating performance.
Cellular antennas are typically available in the types of linear monopole or PCB antennas such as patches or
ceramic SMT elements.
 External antennas (e.g. linear monopole)
o External antennas basically do not imply physical restriction to the design of the PCB where the LARA-R2
series module is mounted.
o The radiation performance mainly depends on the antennas. It is required to select antennas with
optimal radiating performance in the operating bands.
o RF cables should be carefully selected to have minimum insertion losses. Additional insertion loss will be
introduced by low quality or long cable. Large insertion loss reduces both transmit and receive radiation
performance.
o A high quality 50  RF connector provides proper PCB-to-RF-cable transition. It is recommended to
strictly follow the layout and cable termination guidelines provided by the connector manufacturer.
o If antenna detection functionality is required, select an antenna assembly provided with a proper built-in
diagnostic circuit with a resistor connected to ground: see guidelines in section 2.4.2.
 Integrated antennas (e.g. patch-like antennas):
o Internal integrated antennas imply physical restriction to the design of the PCB:
Integrated antenna excites RF currents on its counterpoise, typically the PCB ground plane of the device
that becomes part of the antenna: its dimension defines the minimum frequency that can be radiated.
Therefore, the ground plane can be reduced down to a minimum size that should be similar to the
quarter of the wavelength of the minimum frequency that has to be radiated, given that the orientation
of the ground plane relative to the antenna element must be considered.
The isolation between the primary and the secondary antennas has to be as high as possible and the
correlation between the 3D radiation patterns of the two antennas has to be as low as possible. In
general, a separation of at least a quarter wavelength between the two antennas is required to achieve
a good isolation and low pattern correlation.
As numerical example, the physical restriction to the PCB design can be considered as following:
Frequency = 750 MHz  Wavelength = 40 cm  Minimum GND plane size = 10 cm