User's Manual

20
Atmel AVR2092
8427A-AVR-10/11
The antenna test board, as specified by the manufacturers, has a ground plane size
of 20mm x 40mm, an antenna placed in a 12mm x 20mm FR4 area, and an FR4
substrate height of 0.8mm. This is the expected environment where the antenna
performance should be equivalent to the datasheet values.
On the REB232ED, the environment differs considerably because the FR4 height is
1.5mm, the antenna is placed in a triangular corner and the ground plane geometry is
different, too. To compensate for the larger substrate height, a 2mm not-plated drill
hole is placed underneath the antenna ceramic core (see Figure 5-11, detail 1).
The P
CB ground is designed with a 45 degree angle along the red line (Figure 5-11,
detail 2), form
ing an optimum antenna ground reference.
The antenna tuning requires two more elements, a series capacitor at the antenna
feed point (Figure 5-11, de
tail 3) and a capacitor that can be moved in position along
the feed line (Figure 5-11, detail
4). The series capacitor must be placed at the
antenna feed point. Under normal conditions, only one capacitor is required within the
detail 4 section of the PCB. By choosing the correct footprint, the capacitor can be
moved along the line. The impedance transformation across the transmission line
depends on the distance between the antenna feed point and the capacitor in detail 4,
resulting in the tuning effect.
The tuning procedure is explained in Section 5.7.3.
5.7.3 Antenna tuning
The first
step for the measurements is a board rework to access the RF line with a
50 coaxial cable. In the case of the REB232ED, the balun, B1, was removed and a
small, rigid 50 cable with an SMA connector was connected to the balun pin 1 pad.
The ground planes next to this feed point where used to create a solid ground
connection for this cable.
This feed point will allow the measurement of both antennas because of the on-board
RF switch. The RF switch was controlled by applying the correct voltage levels from a
lab power supply.
Figure 5-12. Antenna feed line short for extended length calibration.