Unidirectional Surface Acoustic Wave Filter

The present invention relates to a surface acoustic wave filter, and more particularly to an internal reflection type unidirectional surface acoustic wave filter having floating electrodes.

As means for converting a transversal surface acoustic wave filter to a low-loss version, there has hitherto been proposed a unidirectional surface acoustic wave filter. Japanese Patent Publication (b2) No. 20929 of 1991 shows that unidirectivity can be obtained by inserting an open or short-circuit type floating electrode between positive and negative finger electrodes of an interdigital transducer (IDT) and displacing the position of this floating electrode from a center position between the positive and negative finger electrodes.

For example, as illustrated in FIG. 1, when a short-circuit type floating electrode 12 is disposed between positive and negative finger electrodes 10 and 11 of the interdigital transducer in such a way as to be displaced from a center position therebetween, a surface acoustic wave is strongly excited in an arrow-indicated direction in FIG. 1, with the result that unidirectivity occurs.

In this conventional internal reflection type unidirectional surface acoustic wave filter having open or short-circuit type floating electrodes, when using electrodes weighted by an apodization or withdrawal weighted electrodes in order to suppress the outside-band sidelobe levels, a structure of finger electrodes that includes the short-circuit type floating electrodes is as illustrated in FIGS. 2 and 3.

In a unidirectional surface acoustic wave filter with an IDT finger electrode width of .lambda./12 (.lambda.: wavelength) and short-circuit type floating electrodes 12, such as that illustrated in FIG. 3, when using the withdrawal weighted electrodes, the following phenomenon occurs at portions where the positive and negative finger electrodes 10 and 11 do not cross each other, i.e., in case where adjacent finger electrodes are the same positive finger electrode 10 or negative finger electrode 11. That is, although these finger electrodes 10, 11 or floating electrodes 12 have no relevancy to the excitation of surface acoustic waves, they disturb the phase thereof by acoustic reflection thereof, with the result that the inside-passband characteristic exhibits ripples as illustrated in FIG. 4.

FIG. 4 illustrates an amplitude characteristic 41 and a group delay characteristic 42 obtained from a filter prepared by disposing the electrode structure of FIG. 3 with respect to each of input and output transducers on a quartz substrate. In this graphic diagram, the center frequency is 201 MHz (megahertz'), the thickness of the aluminium electrode is 200 nm (nanometers) and the impedance is 90 ohms at the input side and 150 ohms at the output side. The axis of abscissas represents the frequency and is graduated in 100 kHz (kilohertz') per graduation. The axis of ordinates represents the insertion loss of the amplitude characteristic and the time delay of the group delay characteristic and is graduated in 1 dB (decibel) and 1 .mu.s (microsecond), respectively, per graduation.

In the conventional internal reflection type unidirectional surface acoustic wave filter, there is the problem that when using the withdrawal or apodization weighted electrodes, ripples occur inside the passband and so the filtering characteristic deteriorates.

The unidirectional surface acoustic wave filter is theoretically discussed in a paper contributed by K. Yamanouchi and another to the ELECTRONICS LETTERS, Vol. 20, No. 24 (Nov. 22, 1984), pages 989 to 990, under the title of "NEW LOW-LOSS SAW FILTER USING INTERNAL FLOATING ELECTRODE REFLECTION TYPES OF SINGLE-PHASE UNIDIRECTIONAL TRANSDUCER" and a paper contributed by T. Kodama and three others to the IEEE 1986 ULTRASONICS SYMPOSIUM, pages 59 to 64, under the title of "DESIGN OF LOW-LOSS SAW FILTERS EMPLOYING DISTRIBUTED ACOUSTIC REFLECTION TRANSDUCERS".

An object of the present invention is to improve the inside-passband filtering characteristic of an internal reflection type unidirectional surface acoustic wave filter using weighted electrodes.

To attain the above object, the present invention adopts the following means.

(1) A unidirectional surface acoustic wave filter comprising input and output interdigital transducers disposed on a piezoelectric substrate, the interdigital transducers being subjected to weighting of their finger electrodes and having floating electrodes, wherein the floating electrodes of each interdigital transducer are constructed in such a way as to cancel waves reflected at portions where the finger electrodes of the interdigital transducers do not electrically cross each other.

(2) A unidirectional surface acoustic wave filter comprising input and output interdigital transducers disposed on a piezoelectric substrate, the interdigital transducers being subjected to weighting of their finger electrodes by a withdrawal technique and having floating electrodes, wherein the floating electrodes of each interdigital transducer are disposed in such positions that the floating electrodes located at portions where the finger electrodes of the interdigital transducers do not electrically cross each other cancel waves reflected thereby.

(3) A unidirectional surface acoustic wave filter comprising input and output interdigital transducers disposed on a piezoelectric substrate, the interdigital transducers being subjected to weighting of their finger electrodes by an apodization technique and having floating electrodes, wherein the finger electrodes of each interdigital transducer are disposed in such a manner that portions of the finger electrodes which do not electrically cross each other in a direction perpendicular to the propagation direction of surface acoustic waves cancel the waves reflected thereby.

(4) A unidirectional surface acoustic wave filter comprising input and output interdigital transducers disposed on a piezoelectric substrate, the interdigital transducers each having finger electrodes with a width of .lambda./12 (.lambda.: wavelength) and being subjected to weighting of their finger electrodes by a withdrawal technique and having short-circuit type floating electrodes, wherein the intercentral distances between the floating electrodes and the finger electrodes located at portions where the finger electrodes of each interdigital transducer do not electrically cross each other are each (3/12) .lambda..

(5) A unidirectional surface acoustic wave filter comprising input and output interdigital transducers disposed on a piezoelectric substrate, the interdigital transducers each having finger electrodes with a width of .lambda./12 and being subjected to weighting of their finger electrodes by an apodization technique and having short-circuit type floating electrodes, wherein the intercentral distances between dummy electrodes located at portions where the finger electrodes of each interdigital transducer do not electrically cross each other are each (3/12) .lambda..

FIG. 1 is a plan view illustrating a first example of a conventional unidirectional surface acoustic wave filter;

FIG. 2 is a plan view illustrating a second example of a conventional unidirectional surface acoustic wave filter;

FIG. 3 is a plan view illustrating a third example of a conventional unidirectional surface acoustic wave filter;

FIG. 4 is a graph illustrating an amplitude characteristic and group delay characteristic obtained by the third example of the conventional unidirectional surface acoustic wave filter;

FIG. 5 is a plan view illustrating a first embodiment of the present invention;

FIG. 6 is a graph illustrating an amplitude characteristic and group delay characteristic obtained by an example of the first embodiment of the present invention;

FIG. 7 is a plan view illustrating a second embodiment of the present invention;

FIG. 8 is a graph illustrating an amplitude characteristic and group delay characteristic obtained by an example of the second embodiment of the present invention; and

FIG. 9 is a plan view illustrating a third embodiment of the present invention.

Three embodiments of the present invention will now be sequentially explained with reference to FIGS. 5 through 9.

First, a first embodiment of the present invention will be explained with reference to FIG. 5.

FIG. 5 is a view illustrating a structure of the first embodiment of the present invention. An internal reflection type unidirectional surface acoustic wave filter having positive and negative finger electrodes 10 and 11 of input and output interdigital transducers 100 and 200 and having open or short-circuit type floating electrodes 12 therebetween is subjected to weighting of its electrodes by the withdrawal technique and is structured such that at portions where the positive and negative finger electrodes 10 and 11 do not electrically cross each other (a voltage does not occur), i.e., in case where adjacent finger electrodes are the same positive or negative finger electrode 10 or 11, the floating electrodes 13 located therebetween are not shifted from the center positions therebetween.

The floating electrode 12 is displaced from the center position between the positive and negative finger electrodes 10 and 11. As a result, the waves reflected in the rightward direction and those reflected in the leftward direction have their phases shifted from each other. The waves reflected in the leftward direction are made the same in phase as the waves excited by the positive and negative finger electrodes 10 and 11 while, on the other hand, the waves reflected in the rightward direction are made opposite in phase with respect thereto. By this arrangement, the floating electrode 12 causes the surface acoustic wave energy to have unidirectivity. On the other hand, the floating electrode 13 located at the portion where no electrical cross is present between the positive finger electrode 10 and the negative finger electrode 11 operates such that the waves reflected in the leftward and rightward directions are the same in phase and so does not cause the occurrence of the unidirectivity. That is, this floating electrode 13 produces equal reflected waves in the both leftward and rightward directions. However, by disposing the floating electrode 13 at an appropriate position, the internal reflection is canceled.

Next, an example of the first embodiment of the present invention will be explained with reference to FIGS. 5 and 6.

In FIG. 5, assuming that the width of the finger electrodes of the interdigital transducers is .lambda./12 (.lambda.: wavelength), by displacing the short-circuit type floating electrode 12 from the center position between the positive and negative finger electrodes 10 and 11 by a distance of .lambda./12, the surface acoustic wave is strongly excited in an arrow-indicated direction in FIG. 5, with the result that unidirectivity occurs. On the other hand, the floating electrode 13 at the portion where no electrical cross is made between the positive and negative finger electrodes 10 and 11 is disposed in such a way that the floating electrode 13 is located at the center between the positive and negative finger electrodes 10 and 11, namely that the intercentral distance between itself and the positive or negative finger electrode 10 or 11 is (3/12) .lambda.. By this disposition, it follows that the positive finger electrode 10, negative finger electrode 11 and floating electrode 13 are disposed at intervals of .lambda./4. Whereby, the reflected waves of the respective finger electrodes cancel each other and so the internal reflection is canceled.

FIG. 6 illustrates an amplitude characteristic 21 and a group delay characteristic 22 obtained from a filter prepared by disposing the electrode structure of FIG. 5 with respect to each of input and output transducers on a quartz substrate. In this graphic diagram, the center frequency is 201 MHz (megahertz'), the thickness of the aluminium electrode is 200 nm (nanometers) and the impedance is 90 ohms at the input side and 150 ohms at the output side. The axis of abscissas represents the frequency and is graduated in 100 kHz (kilohertz') per graduation. The axis of ordinate represents the insertion loss of the amplitude characteristic and the time delay of the group delay characteristic and is graduated in 1 dB (decibel) and 1 .mu.s (microsecond), respectively, per graduation.

A second embodiment of the present invention will be explained with reference to FIG. 7.

FIG. 7 is a view illustrating a structure of the second embodiment of the present invention. As in the case of the first embodiment, the floating electrodes 13 are disposed in such positions that the internal reflection is canceled, and further, they are connected to busbars 14 and constitute single electrodes.

Next, an example of the second embodiment of the present invention will be explained with reference to FIGS. 7 and 8.

In FIG. 7, assuming that the width of the finger electrodes of the interdigital transducers is .lambda./12, by disposing the positive and negative finger electrodes 10 and 11 and the floating electrode 13 in such a way that the intercentral distances between the electrodes 10 or 11 and the electrode 13 are each (3/12) .lambda. as in the case of the example of the first embodiment, the internal reflection is canceled. The floating electrodes 13 are connected to the busbars and constitute single electrodes.

FIG. 8 illustrates an amplitude characteristic 31 and a group delay characteristic 32 obtained from a filter prepared by disposing the electrode structure of FIG. 7 with respect to each of input and output transducers on a quartz substrate. In this graphic diagram, the center frequency is 201 MHz (megahertz'), the thickness of the aluminium electrode is 200 nm (nanometers) and the impedance is 90 ohms at the input side and 150 ohms at the output side as in the case of the example of the first embodiment.

A third embodiment of the present invention will be explained with reference to FIG. 9.

FIG. 9 is a view illustrating a structure of the third embodiment of the present invention. An internal reflection type unidirectional surface acoustic wave filter having positive and negative finger electrodes 10 and 11 of interdigital transducers 300 and 400 and having open or short-circuit type floating electrodes therebetween is subjected to weighting of its electrodes by the apodization technique and is structured such that among dummy electrodes located at the portions where the positive and negative finger electrodes 10 and 11 do not cross each other, the dummy electrodes 15 located at the portions of the floating electrodes 12 are disposed by being displaced relatively thereto.

Conventionally, as illustrated in FIG. 2, the dummy electrodes 15 exist at a portion where the dummy electrodes 15 coincide with the floating electrode 12 in the X direction (the propagation direction of the surface acoustic waves) thereof. Therefore, at this portion, the internal reflection occurs. According to this third embodiment, the dummy electrodes 15 are disposed at the positions enabling the cancellation of this internal reflection.

Subsequently, an example of the third embodiment of the present invention will be explained with reference to FIG. 9.

In FIG. 9, assuming that the width of the finger electrodes of interdigital transducers be .lambda./12, by disposing the positive and negative finger electrodes 10 and 11 and the dummy electrodes 15 in such a way that the intercentral distances between the electrodes 10 or 11 and the dummy electrodes 15 are each (3/12) .lambda., the internal reflection will be canceled.

According to the present invention, by displacing the position of the floating electrodes located at the portion where no electrical cross is made between the positive and negative finger electrodes of the interdigital transducer, the internal reflection having no relevancy to the directivity is canceled with the result that the inside-band characteristic can be improved.