User Manual

9
November ’00 Chapter 2. Electrical Description
2.1 General
The RF module is the interface between the TIRIS transponder and the data pro-
cessing unit of a TIRIS Reader. It contains circuitry to:
- send a charge-up signal to a TIRIS transponder
- program a read/write transponder
- to receive the signal from the transponder and to prepare the received signal
for digital decoding.
Figure 2 shows the block diagram of the RF module and unless otherwise noted the
following description refers to this schematic.
2.2 Transmitter
The transmitter control logic generates the transmitter output frequency from a
crystal-controlled oscillator. This signal is amplified by the push-pull transmitter
power stage which is connected to the antenna terminal A1. The supply current
I_VSP of the transmitter output stage depends on the quality factor of the antenna
being used and the supply voltage VSP. The Mini-RFM’s antenna circuit is shown in
Figure 3.
The transmitter control signal TXCT- activates the transmitter as long as it is “low”.
In receive mode when TXCT- is “high” the antenna terminal A1 has a low impedance
path to the ground.
The transmitter power control signal TPC controls the intensity of the transmitter
charge pulse by changing the duty cycle of the power stage control signal. The level
of the RF power output can be reduced by setting the TPC signal to “low”. TPC con-
trols the power setting of the RF signal which is used to make-up the “power-burst”.
Both input signals TXCT- and TPC have internal pull-up resistors.
Because of the limited heatsink possibilities of the transmitter power transistors the
duty cycle of the power pulse must not be allowed to exceed that shown in Figure 6.
CAUTION:
Because of the necessarily low impedance of the output
stage, a short-circuit from antenna terminal A1 either to the
ground or to the supply voltage can damage the output
stage if the supply current is not limited.