Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 150

Reflectors are now applied to the crystal in a characteristic sequence along the
propagation path of the surface wave. At each of the reflectors a small part of the surface
wave is reflected and runs back along the crystal in the direction of the interdigital
transducer. Thus a number of pulses are generated from a single interrogation pulse.
In the interdigital transducer the incoming acoustic pulses are converted back into
high-frequency voltage pulses and are emitted from the antenna of the transponder as
the transponder's response signal. Due to the low propagation speed of the surface
wave the first response pulses arrive at the reader after a delay of a few microseconds.
After this time delay the interference reflections from the vicinity of the reader have
long since decayed and can no longer interfere with the transponder's response pulse.
Interference reflections from a radius of 100 m around the reader have decayed after
around 0.66 µ s (propagation time for 2 × 100 m). A surface wave on a quartz substrate
( v = 3158 m/s) covers 2 mm in this time and thus just reaches the first reflectors on the
substrate. This type of surface wave transponder is therefore also known as 'reflective
delay lines' (Figure 4.96).
Surface wave transponders are completely linear and thus respond with a defined
phase in relation to the interrogation pulse (see Figure 4.97). Furthermore, the phase
angle φ 2-1 and the differential propagation time τ 2-1 between the reflected individual
signals is constant. This gives rise to the possibility of improving the range of a surface
wave transponder by taking the mean of weak transponder response signals from many
interrogation pulses. Since a read operation requires only a few microseconds, several
hundreds of thousands of read cycles can be performed per second.
The range of a surface wave transponder system can be determined using the radar
equation (see Section 4.2.4.1). The influence of coherent averaging is taken into account
as 'integration time' t I (Reindl et al. , 1998a).

P T · G T · G R · t i · λ 4
k ·

d = ( 4 . 118 )

4

S
N ·

T 0 · F · IL

A

Request signal

RF response

Time

Environmental echoes Sensor

echoes

Figure 4.96 Sensor echoes from the surface wave transponder do not arrive until environmental
echoes have decayed (reproduced by permission of Siemens AG, ZT KM, Munich)