Surface wave resonator

An end-face reflecting-type surface wave resonator using the SH-type surface wave achieves two types of resonance characteristics by use of only a single device. An interdigital transducer having a pair of comb-like electrodes are formed on a piezoelectric substrate having two oppositely-facing end faces. The interdigital transducer has two peaks of main lobes and are assigned with weights so that the two peaks of main lobes substantially coincide with frequency positions of attenuation poles disposed across a main lobe of a virtual normal-type interdigital transducer having the same number of pairs of electrodes as the weighted interdigital transducer. Thus, two types of resonance characteristics can be obtained.

BACKGROUND OF THE INVENTION 
The present invention relates generally to an end-face reflecting-type 
surface wave resonator using an SH-type surface wave, such as the BGS 
wave, the Love wave, or the like. More particularly, the invention relates 
to an end-face reflecting-type surface wave resonator in which a plurality 
of resonance units are formed into a single device. 
At the video intermediate frequency stage of a television receiver or a 
video cassette recorder, in order to prevent interference with adjacent 
channels, it is necessary to sufficiently attenuate signals at the 
adjacent-channel video signal frequency fap (31.9 MHz according to the 
European system) and the adjacent-channel sound signal frequency fas 
(40.4 MHz according to the European system). FIG. 1 is a 
characteristic diagram of a typical filter indicating attenuation versus 
frequency at the video intermediate frequency stage. FIG. 1 reveals that a 
great level of attenuation is conventionally provided at the 
adjacent-channel video signal frequency fap and the adjacent-channel sound 
signal frequency fas. 
In order to guarantee a great level of attenuation at both the 
adjacent-channel video signal frequency fap and the adjacent-channel sound 
signal frequency fas, two trap devices are conventionally employed, such 
as a trap device having a great level of attenuation at the frequency fap 
and another trap device having a great level of attenuation at the 
frequency fas. Each trap device is formed of an LC resonance circuit, a 
piezo-resonator, or the like. 
As a piezo-resonator for the above-described use, attention is being 
focussed on a piezo-resonator utilizing an SH-type surface wave, such as 
the BGS wave. FIG. 2 illustrates an example of an end-face reflecting-type 
surface wave resonator using the BGS wave. 
The end-face reflecting-type surface wave resonator generally designated by 
1 has a piezoelectric substrate 2 formed in a quadrilateral planar shape. 
The piezoelectric substrate 2 is formed of a piezoelectric material, such 
as lead zircotitanate piezoelectric ceramics, LiNbO.sub.3 piezoelectric 
single crystal, LiTaO.sub.3 piezoelectric single crystal, etc; the 
substrate 2 formed of piezoelectric ceramics having been polarized in the 
direction indicated by the arrow P shown in FIG. 2. Disposed on the top 
surface 2a of the substrate 2 are a pair of comb-like electrodes 3 and 4 
which form an interdigital transducer (hereinafter simply referred to as 
"the IDT"). The comb-like electrodes 3 and 4 have a plurality of electrode 
fingers 3a through 3c and 4a through 4c, respectively. 
In the surface wave resonator 1 constructed as described above, an AC 
voltage is applied to the resonator 1 from the comb-like electrodes 3 and 
4 to excite the BGS wave which then propagates in the direction indicated 
by the arrow X and is further reflected between the end faces 2b and 2c. 
In this resonator 1, the frequency determined by the IDT is matched to the 
frequency determined by the distance between the end faces, thereby 
obtaining effective resonance characteristics. 
However, the end-face reflecting-type surface wave resonator described 
above, as well as conventional LC resonance circuits and other types of 
piezo-resonators, have only a single resonance. Accordingly, two surface 
wave resonators are required and must be connected to each other in order 
to achieve trap characteristics at both the adjacent-channel video signal 
frequency fap and the adjacent-channel sound signal frequency fas. 
On the other hand, a single surface-acoustic wave resonator using the 
Rayleigh wave exhibiting two types of resonance characteristics is 
disclosed (for example, in Kokusai Electric Co., Ltd., Technical Report 
No. 16, pages 1-7, 1992). More specifically, in a surface wave resonator 
filter using the Rayleigh wave, a double mode resonator using a 0th-order 
longitudinal mode (dominant mode) and a second-order longitudinal mode is 
known, exhibiting two types of resonance characteristics. 
However, in this type of resonator, at least two sets of IDTs and 
reflectors are required to obtain the two types of resonance 
characteristics. Also, the resonance characteristics of the double mode 
resonator are determined by the reflection coefficient versus the 
frequency characteristics of the reflectors. However, because the 
resonator has a narrow frequency range having a large reflection 
coefficient, there is only a very small difference, such as approximately 
1 MHz, between two resonance points in a region in which good resonance 
characteristics are obtained. This makes it difficult to provide the two 
required trap resonators merely by use of such a double mode resonator, 
since the two trap resonators have very different frequencies at which the 
attenuation characteristics shown in FIG. 1 are exhibited. 
One of the measures that have been considered to overcome the 
above-described drawback and to obtain two types of resonance 
characteristics may be to form two IDTs on a piezoelectric substrate so as 
to constitute two resonance units, in an end-face reflecting-type surface 
wave resonator discussed above. However, in the above type of resonator 
using an SH-type surface wave, such as the BGS wave, the relationship 
between the wavelength .lambda. of the IDT and the distance L between the 
oppositely-facing two end faces of the piezoelectric substrate between 
which the surface wave is reflected can be expressed by 
L=(.lambda./2).times.n (wherein n is an integer), as schematically 
illustrated in FIG. 3. A great difference between the adjacent-channel 
video signal frequency fap and the adjacent-channel sound signal frequency 
fas causes a significant difference between the respective wavelengths 
.lambda.. For example, when it is assumed that the speed of sound in the 
PZT substrate is 2400 m/s, according to the system, the wavelength 
.lambda.ap for the adjacent-channel video signal frequency fap is 75.2 
.mu.m, while the wavelength .lambda.as for the adjacent-channel sound 
signal frequency fas is 59.4 .mu.m. Accordingly, the distance L between 
the above-described oppositely-facing end faces of one resonance unit 
having a resonance point at the adjacent-channel video signal frequency 
fap differs from that of the other resonance unit having a resonance point 
at the adjacent-channel sound signal frequency fas. Thus, it is very 
difficult to form two resonance units having different resonant 
frequencies on the same substrate of an end-face reflecting-type surface 
wave resonator using the SH-type surface wave. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
end-face reflecting-type surface wave resonator using the SH-type surface 
wave wherein two resonance units having different resonant frequencies are 
formed as a single device. 
In order to achieve the above object, there is provided an end-face 
reflecting-type surface wave resonator comprising: a piezoelectric 
substrate having two oppositely-facing end faces for reflecting an SH-type 
surface wave; and an interdigital transducer disposed on the piezoelectric 
substrate and assigned with weights according to a cross-width weighting 
method or an electrode-finger withdrawal method. The frequency 
characteristics of the weighted interdigital transducer have two main 
lobes, wherein the weights are assigned to the interdigital transducer in 
such a manner that the peaks of the two main lobes substantially coincide 
with frequency positions of attenuation poles, respectively, disposed 
across a main lobe of frequency characteristics of a virtual normal-type 
interdigital transducer having the same number of pairs of electrode 
fingers as that of the weighted interdigital transducer or having a number 
of pairs of electrode fingers determined by the same distance between the 
two oppositely-facing end faces as that of the weighted interdigital 
transducer. The assigned weights obtain first and second types of 
resonance characteristics in the positions of the attenuation poles of the 
virtual normal-type interdigital transducer. 
More specifically, in the surface wave resonator of the present invention, 
weights are assigned to an IDT, so that two types of resonance 
characteristics can be implemented by use of only a single device, 
utilizing two suppressed attenuation poles disposed across a main lobe of 
a conventional normal IDT. In this specification, using the maximum point 
of the excitation strength of the surface wave resonator as a reference, 
the frequency range in which the attenuation magnitude of the excitation 
strength relative to the reference is 10 dB or smaller is referred to as 
"the main lobe", while the frequency range in which the attenuation 
magnitude of the excitation strength is greater than 10 dB is referred to 
as "the sub-lobe". 
The above-described SH-type surface waves broadly include not only the BGS 
waves, but also surface waves whose displacements are perpendicular to the 
propagation direction of the surface wave, such as the Love wave. 
The surface wave resonator described herein may preferably be for use in a 
trap circuit at the video intermediate frequency stage of a television 
receiver or a video cassette recorder. More specifically, the positions of 
the above-described respective attenuation poles may be determined in such 
a manner that the first type of the two resonance characteristics 
corresponds to the adjacent-channel video signal frequency (fap), and the 
second type of resonance characteristics has a frequency higher than the 
frequency of the first type of resonance characteristics and is associated 
with the adjacent-channel sound signal frequency (fas). It is thus 
possible to construct a trap circuit at both the adjacent-channel video 
signal frequency and the adjacent-channel sound signal frequency by use of 
only a single device.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
A description will now be given of an embodiment of an end-face 
reflecting-type surface wave resonator using the BGS wave. As discussed 
above, the surface wave resonator of the present invention is constructed 
so as to obtain frequency characteristics in which the two peaks of the 
main lobes are located at the frequencies corresponding to the attenuation 
poles of the normal IDT. The best known mode of carrying out the invention 
will now be explained by way of a surface wave resonator used as a device 
that forms a trap circuit disposed at the video intermediate frequency 
stage of a television receiver or the like. Two types of resonance 
characteristics exhibited by the surface wave resonator are set to 
correspond to the adjacent-channel video signal frequency fap and the 
adjacent-channel sound signal frequency fas, respectively. 
The aforesaid surface wave resonator can be acquired by the following 
procedure. After the number N of pairs of electrode fingers of the IDT has 
been determined, weights are assigned to the IDT having N pairs of 
electrode fingers so that the peaks of the main lobes can be positioned at 
the frequencies corresponding to the frequencies fap and fas. This 
procedure will now be explained in detail. 
(1) Determination of the Number N of Pairs of Electrode Fingers of the IDT 
A description will first be given of the resonant frequency characteristics 
of the normal IDT for use in the end-face reflecting-type surface wave 
resonator employing the BGS wave. FIG. 4 illustrates a resonant frequency 
spectrum of the normal IDT. This type of spectrum is known and is 
described in, for example, Proceedings of the Spring Meeting of ASG, pp. 
351-352, issued in May 1976. Referring to FIG. 4, the horizontal axis 
indicates the frequency, 2N, 2N-1, etc., designating the positions of the 
resonance points in the 2N-order mode and the 2N-1-order mode, 
respectively. On the other hand, the vertical axis represents the 
attenuation magnitude relative to the peak of the main lobe, which is 0 
dB. 
FIG. 4 clearly shows that the 2N-order resonance mode (wherein N is an 
integer) can be excited by forming the normal IDT provided with the N 
pairs of electrode fingers on the surface of the piezoelectric substrate. 
Among the other high-order modes, odd-number modes, such as 2N-1, 2N-3, 
2N+1 and 2N+3-order modes, cannot be excited due to the symmetrical 
properties of the electrodes. On the other hand, even-number modes, such 
as 2N-2, 2N-4, 2N+2 and 2N+4-order modes, are not excited because, as 
shown in FIG. 4, the resonant frequencies of such even-number modes 
respectively coincide with the attenuation poles of the frequency 
characteristics of the IDT having the N pairs of electrode fingers. Thus, 
in the above type of surface wave resonator provided with the normal IDT, 
such as illustrated in FIG. 2, only the 2N-order mode is excited, thereby 
achieving a resonator with less spurious response. It should be noted, 
however, that only a single-type of resonance characteristic can be 
obtained by use of the surface resonator shown in FIG. 2. 
In order to overcome the above-described drawback, according to the present 
invention, the number N of pairs of electrode fingers are determined so 
that the attenuation poles 2N-2 and 2N+2 of the main lobe of the 
conventional normal IDT illustrated in FIG. 4 can coincide with the 
adjacent-channel video signal frequency fap and the adjacent-channel sound 
signal frequency fas, respectively. More specifically, when the frequency 
position f.sub.0 in the 2N-order mode of the peak of the main lobe is set 
at the intermediate frequency between the adjacent-channel video signal 
frequency fap and the adjacent-channel sound signal frequency fas, the 
number N of pairs of electrode fingers can be found from the formula 
expressed by 2.multidot.f.sub.0 /N.apprxeq.fas-fap. For example, according 
to the video intermediate frequency (VIF) circuit at the frequency fap of 
31.9 MHz and at the frequency fas of 40.4 MHz employed in Europe, the 
frequency f.sub.2 of one attenuation pole 2N+2 of the main lobe of the 
normal IDT matches the frequency fas, while the frequency f.sub.1 of the 
other attenuation pole 2N-2 coincides with the frequency fap, causing the 
center frequency f0 of the peak of the main lobe to be f.sub.0 
=(31.9+40.4)/2=36.15 MHz. Accordingly, the number N of pairs of electrode 
fingers can be obtained from the expression of 
N=2.times.36.15/(40.4-31.9).apprxeq.8.5. The pair number N further 
determines the distance between the end faces of the piezoelectric 
substrate of the surface wave resonator. 
(2) Assigning Weights to the IDT 
FIG. 5 illustrates a characteristic diagram indicating the frequency 
spectrum B of the normal IDT having the N pairs of electrode fingers 
obtained by use of the above-described method and also indicating the 
frequency spectrum A achieved by the below-described weighting method. As 
indicated by the dotted line B, the frequency corresponding to one 
attenuation pole of the main lobe of the normal IDT matches the frequency 
fap (=31.9 MHz), while the frequency associated with the other attenuation 
pole coincides with the frequency fas (=40.40 MHz). The normal IDT is not 
excited either in the position of the frequency fap or the fas. 
Weights are thus assigned to the IDT so as to obtain the frequency spectrum 
A having two main lobes. In the frequency spectrum A of the weighted IDT, 
the peaks of the main lobes are respectively positioned at the attenuation 
poles (the frequencies of the 2N-2 and 2N+2-order modes) of the normal IDT 
and are excited at the respective frequencies, as shown in FIG. 7, which 
will be explained later. FIG. 7 also reveals that a great level of 
attenuation is exhibited at the resonance point of the 2N-order mode, and 
the frequency corresponding to this mode is not excited. Namely, in this 
weighted IDT, two types of resonance characteristics can be obtained in 
which the wave is excited at the frequency positions (fap and fas) 
associated with the two attenuation poles of the main lobe of the normal 
IDT. 
One example of a technique of assigning weights to the IDT exhibiting the 
desired frequency spectrum indicated by the solid line A is shown in FIG. 
6. An end-face reflecting-type surface wave resonator generally indicated 
by 11 illustrated in FIG. 6 has a pair of comb-like electrodes 13 and 14 
formed on a piezoelectric substrate 12, the electrodes 13 and 14 
constituting one IDT. The electrodes 13 and 14 have 8.5 pairs of electrode 
fingers and are weighted by changing the cross width according to the 
apodization method. The degree of the weights is selected so that the 
above-described frequency spectrum indicated by the solid line A is 
exhibited. 
However, the weighting method is not restricted to the technique 
illustrated in FIG. 6. The frequency spectrum indicated by the solid line 
A may be achieved by, for example, forming the electrodes 13 and 14 in a 
shape other than the shape shown in FIG. 6. 
Alternatively, the IDT may be weighted according to the finger withdrawal 
method. Such weighting methods are known and disclosed in, for example, 
Surface Acoustic Wave Technique, supervised by Kimio Shibayama, The 
Institute of Electronics, Information and Communication Engineers, pages 
66 to 68, incorporated by reference. 
The surface wave resonator 11 shown in FIG. 6 is constructed in a manner 
similar to the resonator 1 illustrated in FIG. 2, except that the IDT is 
weighted in the above-described manner. An explanation of the construction 
of the resonator 11 will thus be omitted, while incorporating by reference 
the description given above in connection with FIG. 2. 
FIG. 7 illustrates the impedance versus frequency characteristics of the 
end-face reflecting-type surface wave resonator 11 using the IDT to which 
cross-width weights are assigned according to the aforesaid procedure. 
FIG. 7 shows that two types of resonance characteristics indicated by FR1 
and FR2 are obtained in this resonator. FR1 indicates a resonance point 
corresponding to the peak of one main lobe of the cross-width weighted IDT 
in the frequency position associated with the (2N-2)-order mode of the 
normal IDT, while FR2 designates a resonance point corresponding to the 
peak of the other main lobe of the weighted IDT in the frequency position 
associated with the (2N+2)-order mode of the normal IDT. Accordingly, two 
trap devices can be constructed by use of the above-described two 
resonance points. 
FIG. 8 is a characteristic diagram indicating the attenuation magnitude 
against frequency characteristics obtained by the application of the 
resonator 11 to a trap circuit. FIG. 9 is a circuit diagram of a measuring 
circuit used for measuring the characteristics shown in FIG. 8. Referring 
to FIG. 9, the circuit includes an AC supply 20, a voltmeter 21, resistors 
22 and 25 exhibiting a characteristic impedance of the measuring system, 
and resistors 23 and 24 for adjusting the voltage applied to the surface 
wave resonator 11 and the output voltage, respectively. Further, resistors 
28 and 29 located adjacent to the output and input terminals, 
respectively, are used to match the impedance. 
FIG. 8 shows that a great level of attenuation is achieved at the frequency 
FR1 corresponding to the adjacent-channel video signal frequency at the 
video intermediate stage and at the frequency FR2 associated with the 
adjacent-channel sound signal frequency at the same stage, thereby forming 
a trap device. 
The aforesaid embodiment has been explained in which the peaks of the two 
main lobes coincide with the positions corresponding to the (2N+2)-order 
mode and the (2N-2)-order mode, respectively, of the attenuation poles of 
the main lobe of the normal IDT. However, weights may be assigned to the 
IDT so that the peaks of the two main lobes can correspond to higher-order 
even-number modes, for example, 2N+4 and 2N-4, of the normal IDT. 
As will be clearly understood from the foregoing description, the present 
invention offers the following advantages. Weights are assigned to an IDT 
so as to form peaks of two main lobes, whereby two resonance 
characteristics can be obtained merely by a single device. It is thus 
possible to provide a surface wave resonator best suited for uses 
requiring two resonance characteristics, for example, a trap circuit at 
the video intermediate frequency stage. Further, since two resonance 
characteristics can be achieved by a single device, it is possible to 
simplify the configuration of a circuit, such as a trap circuit or the 
like, to eliminate the complicated and troublesome connecting operation, 
and also to effectively reduce the space required for the circuit. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. The present 
invention is not limited by the specific disclosure herein.