Atmel AVR10004: RCB256RFR2 – Hardware User Manual [APPLICATION NOTE]
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A direct connection causes a small voltage drop for digital noise due to the limited impedance of the paddle through
holes, resulting in an increased noise floor transferred to the analog domain.
Digital ground pins should be connected to the top layer ground fill, and from there with vias to the ground plane below.
4.7.7
PCB detail 6 – ground plane
Besides acting as an electrical ground plane, the PCB area creates a counterpart pole for the antenna. Such an
antenna base plate is considered as a continuous metal plane.
Therefore it is recommended to fill any unused PCB area with copper electrically connected to GND. Both PCB sides
are to be connected using individual or, if possible, grids of through holes. By doing this, the PCB behaves like a
coherent piece of metal for an external electromagnetic field.
4.7.8
PCB detail 7 – crystal guard routing
The 16MHz radio reference crystal PCB design requires special attention to avoid the influence of external noise
sources and to keep the radiation of 16MHz harmonics low.
Any crosstalk from digital lines into the crystal signals increases the phase noise and reduces the radio transceiver
performance.
A grounded guard trace is placed around the crystal area to protect the crystal against digital noise.
To investigate the impact of digital noise on the reference crystal, it is recommended to perform packet error rate tests
with potential digital noise sources enabled and disabled. The influence of disturbances such as MCLK or SPI activity
during transmit or receive can be evaluated by comparing the measurement results.
4.7.9
PCB detail 8 – RF section shielding
A shield covering the Atmel Atmega256RFR2 and related parts is used to protect the IC from external noise and strong
interferers. The shield is not required to suppress any radiation generated by the IC.
4.7.10 board ground plane design
A PCB ground plane with openings that are small compared to the RF wavelength can be considered as continuous.
Signal lines required for normal operation create electrically long slots within the ground plane. A PCB design should
accommodate ground sections on the opposite PCB side to short the slots. This design technique helps create a solid
antenna ground in spite of the limitations of a cost-effective two-layer board.
