Hardware
Design notes for the RF pad version
XBee Wi-Fi RF Module User Guide
30
between the XBee Wi-Fi RF Module and the antenna violates the device's certification. The RF trace
should have a controlled impedance of 50 Ω.
For the transmission line, we recommend either a microstrip or coplanar waveguide trace on the PCB.
We provide a microstrip example below, because it is simpler to design and generally requires less
area on the host PCB than coplanar waveguide.
We do not recommend using a stripline RF trace because that requires routing the RF trace to an
inner PCB layer, and via transitions can introduce matching and performance problems.
It is essential to follow good design practices when implementing the RF trace on a PCB. The following
figures show a layout example of a host PCB that connects an RF Pad module to a right angle,
through-hole RPSMA jack.
n
The top two layers of the PCB have a controlled thickness dielectric material in between. The
second layer has a ground plane which runs underneath the entire RF pad area. This ground
plane is a distance
d
, the thickness of the dielectric, below the top layer.
n
The top layer has an RF trace running from pin 36 of the module to the RF pin of the RPSMA
connector. The RF trace's width determines the impedance of the transmission line with
relation to the ground plane. Many online tools can estimate this value, although you should
consult the PCB manufacturer for the exact width. Assuming
d
= 0.025 in, and that the
dielectric has a relative permittivity of 4.4, the width in this example will be approximately
0.045 in for a 50 Ω trace. This trace width is a good fit with the module footprint's 0.060 in pad
width.
n
We do not recommend using a trace wider than the pad width, and using a very narrow trace
(under 0.010 in) can cause unwanted RF loss. You can minimize the length of the trace by
placing the RPSMA jack close to the module. All of the grounds on the jack and the module are
connected to the ground planes directly or through closely placed vias. Space any ground fill on
the top layer at least twice the distance
d
(in this case, at least 0.050 in) from the microstrip to
minimize their interaction.
This figure shows PCB layer 1 of the RF pad layout.