AT02876: Atmel REB212BSMA Hardware User Manual [APPLICATION NOTE]
Atmel-42097B-WIRELESS-AT02876-REB212BSMA-Hardware-User-Manual_ApplicationNote_062014
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5.2 Crystal Routing
The reference crystal PCB area requires optimization to minimize external interference and to keep any radiation
of 16MHz harmonics low.
Since the board design incorporates a shield, the crystal housing has been tied hard to ground. This method will
minimize the influence of external impairments such as burst and surge.
To prevent crosstalk, the crystal is encircled by a ring of ground traces and vias. Special care has to be taken in
the area between the IRQ line and the crystal. Depending on the configuration, the interrupt may be activated
during a frame receive. Crosstalk into the crystal lines would increase the phase noise and therefore reduce the
signal to noise ratio.
Figure 5-2.
Board Layout – Crystal Routing
The reference crystal and its load capacitors C37/38 form the resonator circuit. These capacitors are to be placed
close to the crystal. The ground connection in between the capacitors should be a solid copper area right
underneath the crystal, including the housing contacts.
5.3 Transceiver Analog GND Routing
With the Atmel AT86RF212B, consider pins 3, 6, 27, 30, 31, and 32 as analog ground pins. Pin 7 is an exception
and can be connected to the central paddle like the other analog ground pins.
Analog ground pins are to be routed to the paddle underneath the IC. The trace width has to be similar to the pad
width when connecting the pads, and increase, if possible, in some distance from the pad. This measure will
decrease the parasitic inductance as far as possible.
Each ground pin should be connected to the bottom plane with at least one via. Move the vias as close to the IC
as possible. It is always desired to integrate the single-pin ground connections into polygon structures after a
short distance. Top, bottom, and, on multilayer boards, the inner ground planes, should be tied together with a
grid of vias. When ground loops are smaller than one tenth of the wavelength, it is safe to consider this as a solid
piece of metal.
