User's Manual

AVR2044

8339A-AVR-09/10

5.7.3 PCB Detail 3 – Bias DC Block, AC Ground

The RCB uses an integrated filter-balun, refer to [6]. The component provides pin 2

as a bias port towards the differential pins. To avoid a DC connection of the radio

transceiver circuitry, refer to [1], filter balun pin2 requires a DC blocking capacitor C1

to create an AC GND connection only.

This capacitor is to be placed as close as possible to the filter-balun to ensure a low

impedance AC connection. For the RCB128RFA1 the minimum distance was limited

by the minimum width of a solder mask separator in between the balun and the

capacitor pad. The actual value is related to the PCB manufacturer capabilities.

The grounded pad of the capacitor is surrounded by three through-holes in close

proximity to ensure lowest possible impedance.

The capacitor itself should have the size 0402 or smaller.

5.7.4 PCB Detail 4 – Analog GND Routing

It is recommended to design the ground trace as wide as possible to avoid parasitic

inductances.

ATmega128RFA1 analog ground pins are to be routed to the paddle underneath the

IC. The GND trace width is recommended to be similar to the pad width.

Each GND pin should be connected to the bottom plane with at least one through-

hole in direct proximity to the IC.

Soldering technology allows placing small through-holes (0.15 mm drill) within the

ground paddle underneath the Atmega128RFA1. Through-holes are filled with solder

during reflow soldering, however, solder paste loss is low due to the small drill size.

5.7.5 PCB Detail 5 – Digital GND Routing

The digital GND pins are not directly connected to the paddle. This is to avoid

dispersion of digital noise from IO pad cells or other digital processing units.

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.

5.7.6 PCB Detail 6 – GND 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’s recommended filling 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. Doing this, the PCB behaves like a coherent piece of

metal for an external EM field.

5.7.7 PCB Detail 7 – Crystal Guard Routing

The 16 MHz reference crystal PCB design requires special attention to avoid the

influence of external noise sources and to keep the radiation of 16 MHz harmonics

low.