Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 152

100

10 1 1

10

433 MHz, IL = 35 dB

2.45 GHz, IL = 40 dB

1

0.1

10 − 5 10 − 4 10 − 3 10 − 2 10 − 1 10 1

1

Measurement time t(s)

Figure 4.98 Calculation of the system range of a surface wave transponder system in relation
to the integration time t i at different frequencies (reproduced by permission of Siemens AG, ZT
KM, Munich)

The sensitivity S to a certain influence quantity y is dependent here upon substrate
material and crystal section. For example, the influence of temperature T upon prop-
agation speed v for a surface wave on quartz is zero. Surface wave transponders are
therefore particularly temperature stable on this material. On other substrate materials
the propagation speed v varies with the temperature T .
The temperature dependency is described by the sensitivity S T (also called the
temperature coefficient Tk ). The influence of temperature on the propagation speed v ,
the mid-frequency f 0 and the propagation time τ can be calculated as follows (Reindl
and Magori, 1995):

− S T · (T

v(T ) = v(T 0 ) · [1 − T 0 ) ] (4.120)

S T · f 0 (T ) = f 0 (T 0 ) · [1 − (T − T 0 ) ] (4.121)

τ(T) = τ(T 0 ) · [1 + S T · (T − T 0 ) ] (4.122)

4.3.4.1 Reflective delay lines

If only the differential propagation times or the differential phases between the indi-
vidual reflected pulses are evaluated, the sensor signal is independent of the distance
between the reader and the transponder. The differential propagation time τ 2-1 ,and
the differential phase θ 2-1 between two received response pulses is obtained from the
distance L 2-1 between the two reflectors, the velocity v of the surface wave and the
frequency f of the interrogation pulse.

2 · L 2 − 1
v

τ 2 − 1 = (4.123)