Digital Signal Processing Reference
In-Depth Information
1
4
−
20
−
40
1234
2
−
60
3
−
80
−
100
0
2
4
6
8
10
Time (
µ
s)
Figure 4.97
Surface wave transponders operate at a defined phase in relation to the inter-
rogation pulse. Left, interrogation pulse, consisting of four individual pulses; right, the phase
position of the response pulse, shown in a clockface diagram, is precisely defined (reproduced
by permission of Siemens AG, ZT KM, Munich)
The relationship between the number of read cycles and the range of the system is
shown in Figure 4.98 for two different frequency ranges. The calculation is based upon
the system parameters listed in Table 4.9, which are typical of surface wave systems.
4.3.4 The sensor effect
The velocity
v
of a surface wave on the substrate, and thus also the propagation time
τ
and the mid-frequency
f
0
of a surface wave component, can be influenced by a range of
physical variables (Reindl and Magori, 1995). In addition to temperature, mechanical
forces such as static elongation, compression, shear, bending and acceleration have
a particular influence upon the surface wave velocity
v
. This facilitates the remote
interrogation of mechanical forces by surface wave sensors (Reindl and Magori, 1995).
In general, the sensitivity
S
of the quantity
x
to a variation of the influence quantity
y
can be defined as:
1
x
·
∂x
∂y
S
y
=
(
4
.
119
)
Table 4.9
System parameters for the range calculation shown in Figure 4.97
Value
At 433MHz At 2.45GHz
P
S
: transmission power
+
14 dBm
G
T
: gain of transmission antenna
0 dB
G
R
: gain of transponder antenna
−
3 dBl
0 dBl
Wavelength
λ
70 cm
12 cm
F: Noise number of the receiver (reader)
12 dB
S/N: Required signal/noise distance for error-free data detection
20 dB
IL: Insertion loss: This is the additional damping of the
electromagnetic response signal on the return path in the
form of a surface wave
35 dB
40 dB
T
0
: Noise temperature of the receiving antenna
300 K
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