Readers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 306

The reader contains an instantaneous switching unit to switch between transmit-
ter and receiver mode. This function is normally performed by PIN diodes in radio
technology.
No special demands are made of the receiver in an SEQ system. Because the strong
signal of the transmitter is not present to cause interference during reception, the
SEQ receiver can be designed to maximise sensitivity. This means that the range of
the system as a whole can be increased to correspond with the energy range ,i.e.the
distance between reader and transponder at which there is just enough energy for the
operation of the transponder.

11.2.1.4 Microwave system for SAW transponders

A short electromagnetic pulse transmitted by the reader's antenna is received by the
antenna of the surface wave transponder and converted into a surface wave in a
piezoelectric crystal. A characteristic arrangement of partially reflective structures in
the propagation path of the surface wave gives rise to numerous pulses, which are
transmitted back from the transponder's antenna as a response signal (a much more
comprehensive description of this procedure can be found in Section 4.3).
Due to the propagation delay times in the piezoelectric crystal the coded signal
reflected by the transponder can easily be separated in the reader from all other elec-
tromagnetic reflections from the vicinity of the reader (see Section 4.3.3). The block
diagram of a reader for surface wave transponders is shown in Figure 11.8.
A stable frequency and phase oscillator with a surface wave resonator is used
as the high-frequency source. Using a rapid HF switch, short HF pulses of around
80 ns duration are generated from the oscillator signal, which are amplified to around
36 dBm (4W peak) by the connected power output stage, and transmitted by the
reader's antenna.
If a SAW transponder is located in the vicinity of the reader it reflects a sequence
of individual pulses after a propagation delay time of a few microseconds. The pulses

SAW
resonator

Clock

Antenna

90 0

° °

I

I

A/D
converter

Micro-
controller

Q Q

Figure 11.8 Block diagram of a reader for a surface wave transponder