NORA-W36 series - System integration manual
UBX-22021120 - R01
Design-in
Page 17 of 48
C1-Public
Typically, the RF plug is available as a cable assembly. Different types of cable assembly are available;
the end product designer should select the cable assembly best suited to the application. The key
characteristics of a suitable cable assembly include:
•
RF plug type: select U.FL or equivalent
•
Nominal impedance: 50
•
Cable thickness: Typically, from 0.8 mm to 1.37 mm. Select thicker cables to minimize insertion
loss.
•
Cable length: Standard length is typically 100 mm or 200 mm; custom lengths may be available on
request. Select shorter cables to minimize insertion loss.
•
RF connector on the other side of the cable: For example, another U.FL (for board-to-board
connection) or SMA (for panel mounting).
Consider that SMT connectors are typically rated for a limited number of insertion cycles.
Additionally, the RF coaxial cable may be relatively fragile compared to other types of cables. To
increase application ruggedness, connect U.FL connector to a more robust connector such as SMA
fixed on panel.
☞
A de-facto standard for SMA connectors implies the usage of reverse polarity connectors
(RP-SMA) on Wi-Fi and Bluetooth
®
end products to increase the difficulty for the end user to
replace the antenna with higher gain versions and exceed regulatory limits.
The following recommendations apply for the proper layout of the connector:
•
Strictly follow the connector manufacturer’s recommended
layout:
o
SMA Pin-Through-Hole connectors require GND keep-out (that is, clearance, a void area) on all
the layers around the central pin up to annular pads of the four GND posts.
o
U.FL. surface mounted connectors require no conductive traces (that is, clearance, a void area)
in the area below the connector between the GND land pads.
If the connector’s RF pad size is wider than the micro strip, remove the GND layer beneath the RF
connector to minimize the stray capacitance thus keeping the RF line 50
Ω
. For example, the active
pad of the U.FL. connector must have a GND keep-out (that is, clearance, a void area) at least on the
first inner layer to reduce parasitic capacitance to ground.
3.2.2.2
Integrated antenna design
If integrated antennas are used, the transmission line is terminated by the integrated antennas
themselves. Follow the guidelines mentioned below:
•
The antenna design process should begin at the start of the whole product design process. Self-
made PCBs and antenna assembly are useful in estimating overall efficiency and radiation path of
the intended design.
•
Use antennas designed by an antenna manufacturer providing the best possible return loss (or
VSWR).
•
Provide a ground plane large enough according to the related integrated antenna requirements.
The ground plane of the application PCB may be reduced to a minimum size that must be similar
to one quarter of wavelength of the minimum frequency that has to be radiated; however overall
antenna efficiency may benefit from larger ground planes.
•
Proper placement of the antenna and its surroundings is also critical for antenna performance.
Avoid placing the antenna close to conductive or RF-absorbing parts such as metal objects, ferrite
sheets and so on as they may absorb part of the radiated power or shift the resonant frequency of
the antenna or affect the antenna radiation pattern.
•
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.
