User Manual

11
November ’00 Chapter 2. Electrical Description
2.3 Receiver
The receiver comprises three parts: the RF part, the interface and the logic.
The selective amplifier in the RF Part of the receiver:
- amplifies the RF signal received from the antenna circuit, then
- demodulates the FSK signal from the TIRIS transponder, and
- generates an analog voltage (RSSI) that provides an indication of the received
signal strength.
The demodulated FSK signal, carrier signal and analog signal strength voltage are
all connected to the receiver interface. The demodulated data signal and the carrier
signal are converted to logic signals, and connected to the receiver logic for further
processing. The signal strength indicator voltage is converted into RXSS- which is
fed directly to the module connector ST1. The function of the RXSS- signal is ex-
plained in more detail in section 5.3.
The receiver logic generates the receiver data signal RXDT and the receiver clock
signal RXCK to allow a simple data processing. The NRZ data stream of RXDT con-
tains the identification data, the protection data and the framing bits. The clock signal
RXCK is used as time reference for the data stream of RXDT. The RXCK signal
changes from “low” to “high” level in the middle of each data bit.
2.4 Antenna Circuit
Figure 3 shows a simplified schematic of the antenna circuit. The antenna coil L_ANT
is not part of the module but together with the antenna circuit’s built-in capacitors it
forms a resonance circuit. The resonant current through the antenna coil generates
the magnetic field which charges (and programs) the transponder.
For the system to transmit and receive correctly, the antenna must be precisely tuned
to the transmitter output frequency f_TXO to compensate for the (allowed) tolerances
of the antenna coil L_Ant and the antenna capacitors C_A1 and C_A2. To enable this
tuning, six tuning capacitors C_AT1 to C_AT6 have been added to the antenna cir-
cuit. Their values are binary weighted in normalized steps of 1, 2, 4, 8 16 and 32.
C_AT1 has the smallest value corresponding to the normalized value 1. C_AT2 has
the double capacitance of C_AT1, so that C_AT2 corresponds to the normalized val-
ue 2 and so on. With this capacitance array and six jumpers, 64 different capacitance
values can be tuned. Each of the five tuning pins has its adjacent antenna ground pin
for a simple short circuit with jumpers.
The antenna terminal A3, which is not used in normal applications and the antenna
ground pins of ST2 have same potential as GNDP but should be used only for an-
tenna purposes and not as a convenient ground connection.
The damping circuit is part of the antenna circuit and damps the antenna circuit by
reducing the quality factor of the built-in antenna circuit capacitor connected to the
terminal A2. The quality factor is reduced during the receive mode when the TXCT-
signal is “high”.
Information about the tuning of the antenna circuit is given in section 5.6.