The Architecture of Electronic Data Carriers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 297

Table 10.5 Doppler frequencies at different speeds

f d (Hz) V (m/s) V (km/h)

0 0 0

10 0.612 1.123

20 1.224 4.406

50 3.061 9.183

100 6.122 18.36

200 12.24 36.72

500 30.61 110.2

1000 61.22 220.39

2000 122.4 440.6

and the direction of movement of the 'target'. This leads to a second, expanded
Doppler equation:

f TX · 2 v
c

f d = · cos α (10.1)

f d · c
2 f TX · cos α

v = (10.2)

The Doppler frequency f d is the difference between the transmitted frequency f TX and
the received frequency f RX . The relative speed of the object is v ·

cos α , c is the speed

10 8 m/s.
A transmission frequency of 2.45 GHz yields the Doppler frequencies shown in
Table 10.5 at different speeds.
To measure the distance d of a transponder, we analyse the travelling time t d of a
microwave pulse reflected by a transponder:

of light, 3 ×

1
2 · t d · c

d = ( 10 . 3 )

The measurement of the speed or distance of a transponder is still possible if the
transponder is already a long way outside the normal interrogation zone of the reader,
because this operation does not require communication between reader and transponder.

10.4.3 Sensor effect in surface wave transponders

Surface wave transponders are excellently suited to the measurement of temperature
or mechanical quantities such as stretching, compression, bending or acceleration .
The influence of these quantities leads to changes in the velocity v of the surface
wave on the piezocrystal (Figure 10.38). This leads to a linear change of the phase
difference between the response pulses of the transponder. Since only the differences
of phase position between the response pulses are evaluated, the measuring result is
fully independent of the distance between transponder and reader.
A precise explanation of the physical relationships can be found in Section 4.3.4.