User's Manual

14
AVR2044
8339A-AVR-09/10
Figure 5-5. RCB128RFA1 Bottom Layer Design Details
5.7.1 PCB Detail 1 – Balanced RF Fan Out
The radio transceiver RF ports require a small serial inductance in series with the
balun or antenna pins. A reasonable inductance value is 1…2 nH. With the given
1.5 mm RF4 substrate, it is not possible to design a differential 100 Ohm transmission
line. Thus, traces between filter-balun and single chip are kept at a reasonable small
width of 0.2mm. With this approach transmission lines are well routable and create
the required inductance at the same time.
5.7.2 PCB Detail 2 – Balun Ground Connection
The filter balun requires a solid ground connection, refer to [6]. Since the filter balun
has to drive a single ended line towards the SMA connector, each current injected
into this line creates a counterpart current into the ground plane. A parasitic
inductance to ground is therefore directly inserted into the signal path and increases
the insertion loss.
Further, the integrated harmonic low-pass filter has to perform well at much higher
frequencies to reduce harmonics. Any parasitic inductance causes limited harmonic
filter performance.
Lowest inductance is achieved with large copper areas on top and bottom planes.
Both planes are to be sewed together with sufficient through-holes, especially in close
proximity of GND pins of critical RF components. Resulting through-hole inductances
are to be considered as parallel connected, resulting in lowest possible overall
inductance.