SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 118 of 217
2.3.3.2
Guidelines for EXT32K and 32K_OUT layout design
The
EXT32K
input pin and the
32K_OUT
output pin require accurate layout design: avoid injecting noise on
these pins as it may affect the stability of the RTC timing reference.
2.4
Antenna interface
The
ANT
pin, provided by all the SARA-G3 and SARA-U2 series modules, represents the RF input/output used to
transmit and receive the 2G/3G RF cellular signals: the antenna must be connected to this pin. The
ANT
pin has
a nominal characteristic impedance of 50
and must be connected to the antenna through a 50
transmission
line to allow transmission and reception of radio frequency (RF) signals in the 2G and 3G operating bands.
2.4.1
Antenna RF interface (ANT)
2.4.1.1
General guidelines for antenna selection and design
The cellular antenna is the most critical component to be evaluated: care must be taken about it at the start of
the design development, when the physical dimensions of the application board are under analysis/decision,
since the RF compliance of the device integrating a SARA-G3 and SARA-U2 series module with all the applicable
required certification schemes depends from antenna radiating performance.
Cellular antennas are typically available as:
External antenna (e.g. linear monopole):
o
External antenna usage basically does not imply physical restrictions on the design of the PCB where the
SARA-G3 and SARA-U2 series module is mounted.
o
The radiation performance mainly depends on the antenna: select the antenna with optimal radiating
performance in the operating bands.
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.
o
Select an RF cable with minimum insertion loss: additional insertion loss due to low quality or long cable
reduces radiation performance.
o
Select a suitable 50
connector providing proper PCB-to-RF-cable transition: it is recommended to
strictly follow the layout and cable termination guidelines provided by the connector manufacturer.
Integrated antenna (PCB antennas such as patches or ceramic SMT elements):
o
Internal integrated antenna implies physical restriction to the design of the PCB: the ground plane can
be reduced down to a minimum size that must be similar to the quarter of the wavelength of the
minimum frequency that must be radiated. As numerical example:
Frequency = 824 MHz
Wavelength = 36.4 cm
Minimum GND plane size = 9.1 cm
o
The radiation performance depends on the whole PCB and antenna system design, including product
mechanical design and usage: select the antenna with optimal radiating performance in the operating
bands according to the mechanical specifications of the PCB and the whole product.
o
Select a complete custom antenna designed by an antenna manufacturer if the required ground plane
dimensions are very small (e.g. less than 6.5 cm long and 4 cm wide): the antenna design process should
begin at the start of the whole product design process.
o
Select an integrated antenna solution provided by an antenna manufacturer if the required ground
plane dimensions are large enough according to the related integrated antenna solution specifications:
the antenna selection and the definition of its placement in the product layout should begin at the start
of the product design process.
