Digital Signal Processing Reference
In-Depth Information
0
20
−
40
−
60
−
−
80
−
100
−
120
−
140
0.1
1
10
100
Distance r (m)
Reader with dipole antenna
Reader with yagi antenna (G
=
3)
Figure 4.90
Damping of a signal on the way to and from the transponder
4.3 Surface Waves
4.3.1 The creation of a surface wave
If a voltage is applied to the electrodes of a
piezoelectric crystal
such as
quartz
(SiO
2
),
lithium niobate
(LiNbO
3
)or
lithium tantalate
(LiTaO
3
) mechanical distortions arise in
the
crystal lattice
as a result of the
piezoeffect
. This effect is used to generate
surface
acoustic waves
on the crystal. To achieve this, electrode structures made of approxi-
mately 0.1
µ
m thick aluminium are applied to the polished surface of a piezoelectric
single crystal in the form of an electroacoustic converter. When an alternating voltage is
applied to the electroacoustic converter, surface acoustic waves — so-called
Rayleigh
waves
— propagate on the surface of the crystal (Meinke and Gundlach, 1992). The
deflections in the crystal lattice decrease exponentially as the depth increases.
The majority of the induced acoustic power is thus concentrated within a thin layer
with a depth of approximately one wavelength
λ
on the surface of the crystal. The
propagation of a surface acoustic wave on the highly polished surface of a substrate is
almost undamped and dispersion free. The propagation speed
v
is approximately 3000
to 4000 m/s, i.e. only around 1/100 000 of the speed of light
c
.
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