User's Manual

Atmel AVR2080
11
8479A-AVR-01/12
5.1 PCB detail 1 – balanced RF pin fan out
Figure 5-2. Board layout – SE2431L layout.
The SE2431L (U1) and associated circuitry follow a standard Skyworks Solutions
recommended layout to achieve specified RF performance. The SE2431L requires a
central PCB ground pad which is completely relieved of solder resist and has a grid of
15 ground vias [9]. This i
s essential to achieve good RF performance and adequate
thermal conduction, especially in transmit mode. The solder paste mask has limited
coverage for assembly purposes.
The RF tracks to SE2431L TR, ANT1 and ANT2 pins, and tracking to the antennas,
are all 50 microstrip.
The 10pF decoupling capacitors C38 and C39 are placed close to the respective
power supply pins. Grounded pins on the SE2431L are routed directly to the central
ground pad.
5.2 PCB detail 2 – crystal routing
The reference crystal PCB area requires optimization to minimize external
interference and to keep any radiation of 16MHz harmonics low.
The reference crystal and load capacitors C34/35 form the resonator circuit. These
capacitors are to be placed close to the crystal. The ground connection in between
the capacitors should be the crystal housing contact, resulting in a compact, robust
and stable resonator.
The resonator block is enclosed within ground traces around it and a plane on the
bottom side. Do not connect the resonator directly to the plane beneath the block.
The only ground connection for the resonator block should be a trace in parallel with
the two crystal lines that connects to TRX pin 27 or the paddle.
Based on recent experiments, the bottom ground connection shall be routed directly
to the paddle or pin 27. The loop is not required. In addition, the open space
underneath the crystal can be filled with copper. A small keep out trace next to the
bottom ground connection can help to keep this connection separate and prevent the
layout tool from flooding across this trace.