User's Manual
Table Of Contents
- 1 Introduction
- 2 Disclaimer
- 3 Overview
- 4 Functional description
- 5 PCB layout description
- 6 Mechanical description
- 7 Electrical characteristics
- 8 Abbreviations
- References
- EVALUATION BOARD/KIT IMPORTANT NOTICE
- 9 Table of contents
Atmel AVR2092
15
8427A-AVR-10/11
Figure 5-6. Board layout – GND and shield.
Besides the function to provide supply ground to the individual parts, the ground
plane has to be considered as a counterpart for the antenna. Such an antenna base
plate is required to achieve full antenna performance. It has to be a continuous,
sustained metal plate for that purpose. The shield, covering the electronic section will
help to form this antenna base plate.
For that reason, any unused surface should be filled with a copper plane and
connected to the other ground side using sufficient through hole contacts. Larger
copper areas should also be connected to the other side layer with a grid of vias. This
will form kind of a RF sealing for the FR4 material. Any wave propagation in between
the copper layers across the RF4 will become impossible. This way, for an external
electromagnetic field, the board will behave like a coherent piece of metal.
When a trace is cutting the plane on one side, the design should contain vias along
this trace to bridge the interrupted ground on the other side. Place vias especially
close to corners and necks to connect lose polygon ends.
The pads where the shield is mounted also need some attention. The shield has to be
integrated in the ground planes. Vias, in a short distance to the pads, will ensure low
impedance integration and also close the FR4 substrate as mentioned above.
5.6 PCB detail 6 – transceiver RF tuning
The REB232ED implements a tuning structure to optimize the transceiver matching.
A transmission line in combination with a capacitor is used to vary the load
impedance. The capacitance value and the position of the capacitor can be changed
to tune the system. To vary the position along that line, the tuning capacitor can be
assembled on to the footprint of C23, C26 or C27.
To measure the tuning result, U1 has to be removed and a piece of rigid 50 cable
can be soldered to its pin 5. The measurement using X2 is not impossible but much
harder to calibrate and a way to control U1 has also to be found.