SARA-N2 / N3 series - System integration manual
UBX-17005143 - R13
Design-in
Page 49 of 95
C1-Public
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 itself, to match the characteristic impedance of the
ANT
pin of the
module.
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 antenna mismatch, the RF
termination must provide optimal return loss (or V.S.W.R.) figure over all the operating frequencies,
as summarized in
If an external antenna is used, the antenna connector represents the RF termination on the PCB:
•
Use a suitable 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 (see
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, 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 first inner layer to reduce parasitic capacitance to ground
If an integrated antenna is used, the RF termination is represented by the integrated antenna itself:
•
Use an antenna designed by an antenna manufacturer, providing the best possible return loss.
•
Provide a ground plane large enough according to the related integrated antenna requirements:
the ground plane of the application PCB can be reduced 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 antenna design-in guidelines related to the custom application.
Additionally, these recommendations regarding the antenna system must be followed:
•
Do not include antenna within closed metal case.
•
Do not place the antenna in close vicinity to end users, 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
electromagnetic compatibility issues.
•
Take care of interaction between co-located RF systems since the cellular transmitted RF power
may interact or disturb the performance of companion systems.
•
The antenna shall provide optimal efficiency figure over all the operating frequencies.
•
The antenna shall provide appropriate gain figure (i.e. combined antenna directivity and efficiency
figure) so that the electromagnetic field radiation intensity does not exceed the regulatory limits
specified in some countries (e.g. by FCC in the United States).
•
Consider including extra footprints for a “pi” network in between the cellular module and the
antenna, for further improvement in the antenna matching circuit to reach optimal antenna
performance.
