Surface wave resonator having a plurality of resonance frequencies

A free edge reflective-type surface wave resonator for reflecting SH-type surface waves between two confronting free edges of a piezoelectric substrate includes a piezoelectric substrate having two confronting free edges and an interdigital transducer having a plurality of electrode fingers which are weighted by a cross width weighting method or an electrode thinning method. The weighting of the electrode fingers is performed so that an attenuation pole of a main lobe of the weighted IDT is located outside of a frequency of an attenuation pole of a normal non-weighted IDT which has the same number of electrode finger pairs as the weighted IDT. The weighting of the electrodes is such that the main lobe of the weighted IDT and a plurality of resonance characteristics determined by the attenuation poles of the normal unweighted IDT are obtained.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a free edge reflective-type surface wave 
resonator using SH-type surface waves such as BGS waves, Love waves or the 
like, and particularly to a free edge reflective-type surface wave 
resonator in which plural resonance units are fabricated as a single 
unitary element. 
2. Description of Related Art 
A television receiver, a video tape recorder, and other similar devices 
have been required to sufficiently attenuate signals at an adjacent 
channel picture carrier frequency f.sub.ap (31.9 MHz in European 
system) and the adjacent channel sound carrier frequency f.sub.as (40.4 
MHz in European system) at a picture intermediate frequency stage 
thereof in order to avoid any obstruction between adjacent channels. FIG. 
1 is a graph showing an attenuation-frequency characteristic at the 
picture intermediate frequency stage. It is apparent from FIG. 1 that the 
attenuation is relatively large at the adjacent channel picture carrier 
frequency f.sub.ap and the adjacent channel sound carrier frequency 
f.sub.as. 
In order to keep the attenuation large at the adjacent channel picture 
carrier frequency f.sub.ap and the adjacent channel sound carrier 
frequency f.sub.ap as described above, two conventional traps have been 
used. One of the conventional traps has a large attenuation at the 
adjacent channel picture carrier frequency f.sub.ap and the other 
conventional trap has a large attenuation at the adjacent channel picture 
carrier frequency f.sub.as. Each of the conventional traps is constructed 
by an LC resonance circuit, a piezoelectric resonator, etc. 
With respect to the piezoelectric resonator which is used for the above 
purpose, much attention has been paid to a piezoelectric resonator using 
SH-type surface waves such as BGS waves. FIG. 2 is a perspective view 
showing a free edge reflective-type surface wave resonator using BGS 
waves. 
A free edge reflective-type surface wave resonator 1 is constructed by a 
piezoelectric substrate 2 having a substantially rectangular flat surface. 
The piezoelectric substrate 2 is formed of piezoelectric material such as 
piezoelectric ceramic of lead titanate zirconate, LiNbO.sub.3 
piezoelectric monocrystal, LiTaO.sub.3 monocrystal or the like. When the 
substrate 2 is formed of piezoelectric ceramic, the substrate 2 is 
subjected to a polarization treatment in a direction as indicated by an 
arrow P in FIG. 2. A pair of comb-shaped electrodes 3 and 4 are provided 
on the upper surface 2a of the piezoelectric substrate 2 to form an 
interdigital transducer (hereinafter referred to as "IDT"). Each of the 
comb-shaped electrodes 3 and 4 has plural electrode fingers 3a to 3c and 
4a to 4c, respectively. 
By applying an alternating voltage through the comb-shaped electrodes 3 and 
4, BGS waves are excited on the free edge reflective-type surface wave 
resonator 1, and the BGS waves propagate in a direction as indicated by an 
arrow X in FIG. 2. The BGS waves are reflected between the free edges 2b 
and 2c of the piezoelectric substrate 2. The free edge reflective-type 
surface wave resonator 1 can obtain an effective resonance characteristic 
by setting the frequency determined by the IDT and the frequency 
determined by the dimension between the free edges to be substantially 
equal to each other. 
However, the free edge reflective-type surface resonator only has a single 
resonance characteristic like an LC resonance circuit and other types of 
piezoelectric resonators. Accordingly, two surface wave resonators must be 
prepared and connected to each other to achieve a trap characteristic at 
the two frequency positions (the adjacent channel picture carrier 
frequency f.sub.ap and the adjacent channel sound carrier frequency 
f.sub.as). 
An elastic surface wave resonator using Rayleigh waves has been known as 
another type surface wave resonator. This elastic surface wave resonator 
has two resonance characteristics although it uses a single resonator, as 
disclosed in "INTERNATIONAL ELECTRICAL TECHNOLOGY REPORT", No. 16, pp. 
1-7, 1992, for example. That is, there is known a double mode resonator 
using a zero-order longitudinal mode (fundamental mode) and a second-order 
longitudinal mode in a surface wave resonator filter using Rayleigh waves, 
and two resonance characteristics are obtained by this resonator. However, 
the double mode resonator needs two or more IDTs and reflectors. Further, 
in the double mode resonator, the resonance characteristics are determined 
on the basis of the frequency characteristic of reflection coefficient of 
the reflector. Furthermore, a frequency range where a large reflection 
coefficient is obtained is narrow. Therefore, the difference between the 
two resonance points is a very small value, for example, about 1 MHz in a 
range where an excellent resonance characteristic is obtained. Thus, it is 
impossible to construct a double mode resonator including two trap 
resonators which have such a characteristic as shown in FIG. 1 and which 
have a large frequency gap therebetween. 
Therefore, it may be considered that if two IDTs are provided on the 
piezoelectric substrate to form two resonance units in the free edge 
reflective-type surface wave resonator using BGS waves as described above, 
two resonance characteristics can be obtained. However, in the free edge 
reflective-type surface wave resonator using SH-type surface waves such as 
BGS waves, schematically shown in FIG. 3, the following relationship is 
satisfied between the wavelength .lambda. of the IDT and the distance L of 
the two confronting free edges where the surface wave of the piezoelectric 
substrate is reflected: 
EQU L=(.lambda./2).times.N(N represents an integer) 
Since the frequency difference between f.sub.ap and f.sub.as is large, the 
wavelengths .lambda. of the respective IDTs differ greatly from each 
other. For example, assuming the sound velocity of the PZT substrate to be 
equal to 2400 m/s, .lambda.ap for f.sub.ap =75.2 .mu.m and .lambda. as for 
f.sub.as =59.4 .mu.m in case of the system. Accordingly, the value of 
the distance L between the two confronting free edges is different between 
the resonance unit having a resonance point at the adjacent channel 
picture carrier frequency f.sub.ap and the resonance unit having a 
resonance point at the adjacent channel sound carrier frequency f.sub.as. 
That is, with respect to the free edge reflective-type surface wave 
resonator using SH-type surface waves, it is extremely difficult to form 
two resonance units having different resonance frequencies on the same 
substrate. 
SUMMARY OF THE INVENTION 
The preferred embodiments of present invention provide a free edge 
reflective-type surface wave resonator using SH-type surface waves in 
which at least two resonance units having different resonance frequencies 
are fabricated as a single unitary element. 
More specifically, according to the preferred embodiments of the present 
invention, a free edge reflective-type surface wave resonator for 
reflecting SH-type surface waves between two confronting free edges of a 
piezoelectric substrate, includes a piezoelectric substrate having two 
confronting free edges, and an IDT provided on the piezoelectric substrate 
including a plurality of electrode fingers wherein electrode fingers are 
weighted by an apodized method or a withdraw method, wherein the weighting 
is performed so that an attenuation pole of a main lobe of the weighted 
IDT is located outside of a frequency position of an attenuation pole of a 
"normal" IDT (wherein "normal" refers to an unweighted IDT having equal 
cross widths between electrode fingers) having the same number of 
electrode finger pairs as the weighted IDT, and the electrode fingers of 
the weighted IDT being weighted such that a resonance characteristic 
determined by the main lobe of the weighted IDT and a plurality of 
resonance characteristics determined by the attenuation poles of the 
normal IDT are obtained. 
That is, in the surface wave resonator of the preferred embodiments of the 
present invention, two or more resonance characteristics are obtained with 
only a single unitary element by using the attenuation pole of the normal 
IDT which has been suppressed in conventional resonators. The SH-type 
surface wave in this specification is broadly defined as containing not 
only BGS waves, but also surface waves like Love waves having a component 
whose displacement is substantially perpendicular to the propagation 
direction of the surface waves. 
Further, the surface resonator of the preferred embodiments of the present 
invention is preferably used to constitute a trap circuit at the picture 
intermediate frequency stage of a television receiver or a video tape 
recorder. That is, the positions of the respective attenuation poles as 
described above are set so that one of the plural resonance 
characteristics is set as the adjacent channel picture carrier frequency 
(f.sub.ap) and another resonance characteristic having a higher frequency 
is set as the adjacent channel video signal frequency (f.sub.as), whereby 
a trap circuit having traps at the adjacent channel picture carrier 
frequency and the adjacent channel sound carrier frequency can be 
constructed by a single unitary element. 
As described above, the resonance frequency f.sub.r of the free edge 
reflective-type surface wave resonator using BGS waves is determined as 
follows: 
f.sub.r =v/.lambda. (.lambda. represents the wavelength of surface wave 
excited by the IDT, and v represents the sound velocity of the 
piezoelectric substrate). 
Further, the dimension L between the two confronting free edges of the 
piezoelectric substrate is preferably determined so as to satisfy the 
equation: 
EQU L=(.lambda./2).times.n(n represents an integer). 
Accordingly, when a resonator constituting a trap for the adjacent channel 
picture carrier frequency and a resonator constituting a trap for the 
adjacent channel sound carriers frequency, which are used at the picture 
intermediate frequency stage, are constructed by a free edge 
reflective-type surface wave resonator, the value of the dimension L is 
different between the above two resonators because they have greatly 
different wavelengths .lambda.. Therefore, it has been hitherto considered 
as being difficult to form two resonators on the same chip or substrate. 
On the other hand, according to the preferred embodiments of the present 
invention, the weighting is performed so that the attenuation pole of the 
main lobe of the weighted IDT is located outside of the frequency position 
of the attenuation pole of the normal unweighted IDT having the same 
number of electrode finger pairs. Accordingly, plural resonance 
characteristics are obtained, which are the resonance characteristic 
determined by the substrate dimension L as described above, that is, the 
resonance characteristic determined by the attenuation pole of the normal 
IDT and the resonance characteristic determined by the main lobe of the 
weighted IDT. That is, as is apparent from the foregoing description, by 
arranging the resonance characteristic determined by the main lobe of the 
weighted IDT and the resonance characteristic determined by the substrate 
dimension L, the free edge reflective-type surface wave resonator having 
plural resonance characteristics can be constructed despite the fact that 
it has a single-element unitary structure. 
As described above, according to the preferred embodiments of the present 
invention, a free edge reflective-type surface wave resonator which 
achieves plural resonance characteristics despite its single-element 
unitary structure can be provided by weighting the IDT as described above, 
and an optimum surface wave resonator can be provided for use in devices 
requiring two resonance characteristics such as a trap circuit for a 
picture intermediate frequency stage. Since two resonance characteristics 
are achieved and provided by a single unitary element, the circuit 
construction of a trap circuit and other devices can be simplified and 
cumbersome connection work of several circuit elements can be omitted. In 
addition, the size of the trap circuit and similar devices can be 
effectively reduced. 
For the purpose of illustrating the invention, there is shown in the 
drawings several preferred embodiments which are presently preferred, it 
being understood that the present invention is not limited to the precise 
structural arrangements and instrumentalities shown in the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferred embodiments according to the present invention will be described 
with reference to the accompanying drawings. 
FIG. 4 is a diagram showing the frequency spectrum of a normal unweighted 
IDT in a free edge reflective-type surface wave resonator using BGS waves. 
The frequency spectrum of the IDT is described in "Lecture Papers of Japan 
Acoustic Society", pp. 351-352, issued on March, 1976. The axis of the 
abscissa of FIG. 4 represents the frequency, and 2N, 2N-1, etc. on the 
axis of abscissa represent resonance points of a 2N-order mode, 
(2N-1)-order mode, etc. The axis of the ordinate represents an attenuation 
amount when the peak of a main lobe is set to 0 dB. 
As is apparent from FIG. 4, when a normal IDT is used, the resonance mode 
of 2N-order (N represents an integer) can be excited by forming an IDT 
having N pairs of electrode fingers on the surface of the piezoelectric 
substrate. Further, odd-number modes out of the other high-order modes, 
that is, modes of (2N-1)-order, (2N-3)-order, (2N+1)-order and 
(2N+3)-order are not excited due to symmetry of the electrode. With 
respect to the even-number modes, as is apparent from FIG. 4, the 
resonance frequency is coincident with the attenuation pole of the 
frequency characteristic of the IDT having N pairs of electrode fingers so 
that these modes are not excited. That is, the even-number modes of 
(2N-2)-order, (2N-4)-order, (2N+2)-order, (2N+4)-order, etc. are not 
excited. Accordingly, only the 2N-order mode is excited in the free edge 
reflective-type surface wave resonator on which the normal IDT is formed 
as shown in FIG. 2, and thus a resonator having relatively little spurious 
can be formed. However, as described above, only a single resonance 
characteristic can be obtained by the surface wave resonator shown in FIG. 
2. 
On the other hand, according to the preferred embodiments of the present 
invention, plural resonance characteristics can be obtained by weighting 
the IDT as described. A weighting method will be hereunder described with 
reference to FIG. 5. 
A solid line A of FIG. 5 represents the frequency spectrum of the IDT when 
the IDT is subjected to a cross width weighting according to the preferred 
embodiments of the present invention, and a broken line B of FIG. 5 
represents the frequency spectrum of a normal unweighted IDT having the 
same number of electrode finger pairs as the IDT weight by an apodized 
method. 2N, 2N-1, etc. represent the positions of resonance points of the 
2N-order mode, the (2N-1)-order mode, etc. The attenuation pole of the 
main lobe of the IDT which is subjected to the weighting by an apodized 
method as described above is located outside of the attenuation poles 
2N+2, 2N-2 of the main lobe of the normal IDT. 
In the frequency spectrum A which is weighted by an apodized method, the 
attenuation amount is relatively small in the (2N-2)-order and 
(2N+2)-order modes, and thus it is apparent that the excitation can be 
achieved in these modes. In the preferred embodiments of the present 
invention, plural resonance characteristics are achieved by using the 
resonance characteristics which are closest to the excited even-number 
modes. 
FIG. 6 shows an example of the weighting of the IDT having the frequency 
spectrum indicated by the solid line A in FIG. 5. The free edge 
reflective-type surface wave resonator 11 shown in FIG. 6 has a pair of 
comb-shaped electrodes 13 and 14 provided on a piezoelectric substrate 12. 
An IDT is constructed by these comb-shaped electrodes 13 and 14. As is 
apparent from FIG. 6, the IDT electrode is subjected to weighting by an 
apodized method. The degree of the weighting is selected so as to provide 
the frequency spectrum indicated by the solid line A in FIG. 5 as 
described above. However, the manner of the weighting to achieve the 
frequency spectrum indicated by the solid line A is not limited to that of 
FIG. 6. That is, the IDT having the frequency spectrum indicated by the 
solid line A of FIG. 5 may be formed by another electrode having a 
different shape from that of the comb-shaped electrodes 13 and 14 shown in 
FIG. 6. The weighting may be performed by a withdraw method. This 
weighting method has been well known, and it is described at pages 66 to 
68 of "ELASTIC SURFACE WAVE ENGINEERING" supervised by Inuo Shibayama 
(Electrical Information Communication Society Corporation), for example. 
The free edge reflective-type surface wave resonator 11 shown in FIG. 6 has 
a similar construction as the free edge reflective-type surface wave 
resonator shown in FIG. 2, except that the IDT is weighted in the manner 
as described above. Therefore, the same description of the same or 
corresponding elements as that of FIG. 2 is applied, and thus the detailed 
description thereof is omitted. 
FIG. 7 is a graph showing an impedance-frequency characteristic of the free 
edge reflective-type surface wave resonator 11 using the cross-width 
weighted IDT as indicated by the solid line A of FIG. 5. As is apparent 
from FIG. 7, three resonance characteristics as represented by FR1, FR2 
and FR3 are obtained. The FR1 represents the resonance point due to 
"spurious" corresponding to the (2N-2)-order mode in the frequency 
spectrum of the normal IDT which is determined by the substrate dimension 
as described above, the FR2 represents the resonance point of a resonance 
characteristic of the 2N-order mode of the main lobe of the IDT weighted 
by an apodized method and the FR3 represents the resonance point of a 
resonance characteristic of the (2N+2)-order mode. As is apparent from 
FIG. 7, three resonance characteristics can be obtained. Two traps can be 
constructed by using any two resonance points, for example, the resonance 
points FR1 and FR2. 
For example, if the free edge reflective-type surface wave resonator 11 is 
constructed so that the FR1 corresponds to the adjacent channel picture 
carrier frequency at the picture intermediate stage and the FR2 
corresponds to the adjacent channel sound carrier frequency at the picture 
intermediate stage, the attenuation-frequency characteristic shown in FIG. 
8 can be achieved. 
That is, the wavelength of the IDT is determined in accordance with the 
adjacent channel sound carrier frequency, and the number of electrode 
finger pairs is determined in accordance with the adjacent channel picture 
carrier signal frequency. Further, the IDT is weighted so that the 
attenuation poles of the weighted IDT are located outside of the 
attenuation poles of the normal IDT having the same number of pairs of 
electrode fingers, whereby a resonator having two resonance 
characteristics and being formed of a single unitary element can be 
provided. 
While preferred embodiments of the present invention have been disclosed, 
various modes of carrying out the principles described herein are 
contemplated as being within the scope of the following claims. Therefore, 
it is understood that the scope of the present invention is not to be 
limited except as otherwise set forth in the claims.