Digital Signal Processing Reference
In-Depth Information
the finger structure at the busbar as a result of the piezoelectric effect (Meinke and
Gundlach, 1992).
The distance between two fingers of the same polarity is termed the electrical period
q
of the interdigital transducer. The maximum electroacoustic interaction is obtained at
the frequency
f
0
, the mid-frequency of the transducer. At this frequency the wavelength
λ
0
of the surface acoustic wave precisely corresponds with the electrical period
q
of the interdigital transducer, so that all wave trains are superimposed in-phase and
transmission is maximized (Reindl and Magori; 1995).
v
f
0
=
λ
0
=
q
(
4
.
115
)
The relationship between the electrical and mechanical power density of a sur-
face wave is described by the material-dependent piezoelectric coupling coefficient
k
2
. Around
k
−
2
overlaps of the transducer are required to convert the entire electrical
power applied to the interdigital transducer into the acoustic power of a surface wave.
The bandwidth
B
of a transducer can be influenced by the length of the converter
and is:
B
=
2
f
0
/N(N
=
number of fingers
)
(
4
.
116
)
4.3.2 Reflection of a surface wave
If a surface wave meets a mechanical or electrical discontinuity on the surface a small
part of the surface wave is reflected. The transition between free and metallised surface
represents such a discontinuity, therefore a periodic arrangement of
N
reflector strips
can be used as a reflector. If the reflector period
p
(see Figure 4.93) is equal to half a
wavelength
λ
0
, then all reflections are superimposed in-phase. The degree of reflection
Figure 4.93
Scanning electron microscope photograph of several surface wave packets on a
piezoelectric crystal. The interdigital transducer itself can be seen to the bottom left of the
picture. An electric alternating voltage at the electrodes of the interdigital transducer generates a
surface wave in the crystal lattice as a result of the piezoelectric effect. Conversely, an incoming
surface wave generates an electric alternating voltage of the same frequency at the electrodes
of the transducer (reproduced by permission of Siemens AG, ZT KM, Munich)
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