Surface acoustic wave filter with parallel arm resonators having different resonant frequencies

A surface acoustic wave filter having a ladder type circuit configuration, includes series resonators provided as part of a series arm disposed between input and output terminals and parallel resonators included in a plurality of parallel arms, respectively, defined between the series arm and a reference potential. The series resonators and parallel resonators each include a one-port surface acoustic wave resonator, the parallel resonators are arranged alternately with the series resonators along a direction from the input terminal toward the output terminal, and the resonant frequency of at least one parallel resonator is different from the resonant frequencies of the other parallel resonators.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a surface acoustic wave filter including a
 plurality of one-port surface acoustic wave resonators (one-port SAW
 resonators) and, more particularly, to a surface acoustic wave filter
 having a ladder-type circuit configuration including a plurality of
 one-port SAW resonators connected to each other in a ladder-type
 configuration.
 2. Description of the Related Art
 A conventional band filter includes a surface acoustic wave filter having a
 ladder type circuit configuration of a plurality of one-port SAW
 resonators connected to each other in a ladder type configuration (for
 example, Japanese Examined Patent Publication No. 56-19765).
 The circuit of a surface acoustic wave filter of this type is partially
 shown in FIG. 10, a series arm is provided between an input terminal 51
 and an output terminal (not shown), and a series resonator S1 is included
 in the series arm. A parallel resonator P1 is connected between the series
 arm and a reference potential, and thereby, a parallel arm is formed.
 Although only one series resonator S1 and one parallel resonator P1 are
 shown in FIG. 10, a plurality of series arms containing a plurality of
 series resonators and a plurality of parallel arms containing parallel
 resonators are arranged to extend in the direction from the input terminal
 51 toward the output terminal.
 Ordinarily, the one-port SAW resonators each defining the above-mentioned
 series resonator S1 and the parallel resonator P1 have an electrode
 structure shown in FIG. 11. As seen in FIG. 11, the one-port SAW resonator
 has a structure in which one pair of interdigital electrodes 52a and 52b
 are provided on a piezoelectric substrate (not shown) whereby one
 interdigital transducer (IDT) 52 is produced. Grating type reflectors 53
 and 54 are arranged on the opposite sides of the IDT 52 in the surface
 acoustic wave propagation direction.
 FIG. 12 illustrates the typical filter characteristics of a surface
 acoustic wave device having a ladder type circuit configuration containing
 the above-described one-port SAW resonators defining the series resonator
 and the parallel resonator.
 For a band filter, it is necessary to increase the attenuation in the
 frequency range outside of the filter pass-band. Accordingly, for the
 purpose of increasing the attenuation in the frequency range outside of
 the filter pass band, a mirror image type connection structure shown in
 FIG. 13 is generally used.
 A mirror image type connection structure is a structure in which a
 connection structure including one series resonator and one parallel
 resonator is connected to a connection structure including one series
 resonator and one parallel resonator such that both connection structures
 define a mirror image of each other relative to the boundary between the
 connection structures. More particularly, the series and parallel
 resonators of the connection structure shown by a broken line A in FIG. 13
 are connected to portions of the connection structure surrounded by a
 broken line B adjacent to the broken line A such that both connection
 structures have a mirror image relationship relative to a boundary
 therebetween. Similarly, the connection structure surrounded by a broken
 line C is arranged to have a mirror image relationship with the connection
 structure surrounded by the broken line B relative to a boundary
 therebetween.
 More specifically, on the opposite sides of the boundary, the parallel
 resonators P1 and P2 are arranged in the boundary area of the connection
 structures surrounded by the broken lines A and B, and the series
 resonators S1 and S2 are arranged so as to be spaced far from the joining
 area of both of the connection structures, respectively.
 For a surface acoustic wave filter of the above described type, it is
 required that its reflection characteristic (VSWR) is low in the filter
 pass band. It is known that in order to reduce the reflection
 characteristic to a value of about 2.0, which is generally required, the
 difference between the resonant frequency of the series resonators S1, S2,
 and S3 and that of the parallel resonators P1, P2, and P3 is adjusted.
 See, for example, THE INSTITUTE OF ELECTRONICS, INFORMATION AND
 COMMUNICATION ENGINEERS TECHNICAL REPORT, JAPAN US95-25, EMD95-21, 33
 (1995-07), p39-p46.
 More particularly, it is known that in the surface acoustic wave filter
 having a ladder type circuit configuration shown in FIG. 13, VSWR can be
 reduced by changing the difference between the resonant frequency of the
 series resonators S1, S2, and S3 and that of the parallel resonators P1,
 P2, and P3.
 In the above-described prior art, it is described that VSWR is reduced in a
 filter pass band by changing the difference between the resonant frequency
 of the series resonators S1, S2, and S3 and that of the parallel
 resonators P1, P2, and P3. In this prior art, it is described that VSWR is
 varied as shown in FIG. 14 when the above resonant frequency difference is
 changed.
 More particularly, in the case that the difference between the resonant
 frequency of the series resonator and that of the parallel resonator is
 small, VSWR is large on the lower frequency side, as shown by an arrow C
 in FIG. 14. On the contrary, in the case that the above frequency
 difference is small, VSWR is large on the higher frequency side as shown
 by an arrow D in FIG. 14.
 Thus, there is a problem with the prior art described above in that if VSWR
 on the lower frequency band side is reduced, VSWR is increased and
 degraded on the higher frequency band side.
 SUMMARY OF THE INVENTION
 To overcome the problems described above, preferred embodiments of the
 present invention provide a surface acoustic wave filter having a ladder
 type circuit configuration containing a plurality of series resonators and
 parallel resonators in which VSWR can be reduced in a wide frequency range
 in filter pass band, and thereby, the insertion loss is significantly
 reduced.
 The surface acoustic wave filter having a ladder type circuit configuration
 includes series resonators provided in a series arm located between input
 and output terminals and parallel resonators included in plural parallel
 arms, respectively, located between the series arm and a reference
 potential. The series resonators and the parallel resonators each include
 a one-port surface acoustic wave resonator, the parallel resonators are
 arranged alternately with the series resonators in a direction extending
 from the input terminal toward the output terminal, and the resonant
 frequency of at least one of the parallel resonators is different from the
 resonant frequencies of the other parallel resonators. Thus, the different
 resonant levels in the band can be adjusted, depending on the way the
 resonant frequencies of the parallel resonators are differentiated, and
 thereby, VSWR in the band can be significantly reduced, and the insertion
 loss is greatly improved in a wide frequency range in the band.
 The electrode finger pitches of the interdigital transducers of the
 plurality of parallel resonators are preferably different from each other
 so that the resonant frequency of the at least one parallel resonator is
 different from the resonant frequencies of the other parallel resonators.
 Accordingly, when the series resonators and the parallel resonators are
 arranged to define a ladder type circuit on one piezoelectric substrate,
 the surface acoustic wave filter with the VSWR characteristic improved in
 the pass band can be easily provided by changing the electrode pattern of
 an IDT of one of the parallel resonators.
 In a preferred embodiment of the present invention, first, second, and
 third parallel arms are arranged along the direction extending from the
 input terminal toward the output terminal in that order, and the resonant
 frequency of the parallel resonator included in the second parallel arm is
 different from the resonant frequencies of the parallel resonators
 included in the first and third parallel arms, respectively. In the
 surface acoustic wave filter having the ladder type circuit configuration
 containing a relatively small number of the parallel resonators, the
 reduction of VSWR in the pass band, particularly on the lower frequency
 side, can be achieved, and the surface acoustic wave filter having
 excellent insertion characteristics in the pass band can be provided.
 The resonant frequency of the parallel resonator included in the second
 parallel arm is preferably lower than the resonant frequency of each
 parallel resonator included in the first and third parallel arms, and more
 preferably, the resonant frequency f1 of the parallel resonator included
 in the second parallel arm and the resonant frequency f2 of at least one
 of the parallel resonators included in the first and third parallel arms
 preferably are arranged so as to satisfy the formula (1):
 f2&gt;f1&gt;0.99.times.f2. In this case, the VSWR characteristic in the band,
 particularly on the lower frequency side, can be improved. In this case,
 the electrode finger pitch L1 of the parallel resonator included in the
 second parallel arm and the electrode finger pitch L2 of the parallel
 resonators included in the first and/or third parallel arm are preferably
 arranged to satisfy the formula (2): L2&lt;L1&lt;1.01.times.L2.
 For the purpose of illustrating the invention, there is shown in the
 drawings several forms which are presently preferred, it being understood,
 however, that the invention is not limited to the precise arrangements and
 instrumentalities shown.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Hereinafter, preferred embodiments of the present invention are explained
 in detail with reference to the drawings.
 FIG. 1 is a circuit diagram showing a circuit configuration of a surface
 acoustic wave filter according to a first preferred embodiment of the
 present invention. In a surface acoustic wave filter 1, series resonators
 S1 and S2 are connected between an input terminal 2 and an output terminal
 3 so as to define a series arm. Between the series arm and a reference
 potential, a plurality of parallel arms are provided. That is, a first
 parallel resonator P1 is connected between a node 4, which lies between
 the input terminal 2 and the series resonator S1, and a reference
 potential, so as to define a first parallel arm. Further, a second
 parallel resonator P2 is connected between a node 5, located between the
 series resonators S1 and S2, and the reference potential, so as to define
 a second parallel arm. Moreover, a third parallel resonator P3 is
 connected between a node 6, located lies between the series resonator S2
 and the output terminal 3, and the reference potential, so as to define a
 third parallel arm.
 One of the novel features of the surface acoustic wave filter 1 of this
 preferred embodiment is that the resonant frequency of the second parallel
 resonator P2 is different from the resonant frequencies of the first and
 third parallel resonators P1 and P3, and thereby, VSWR is reduced in a
 wide frequency range in a pass band of the filter. This will be described
 below.
 In the prior art described above, it is described that the position of the
 frequency where VSWR is increased is varied by changing the difference
 between the resonant frequency of the series resonators and that of the
 parallel resonators. Probably, this is caused by different resonances
 occurring due to the impedance relationships between the series and the
 parallel resonators. The resonance levels are varied with the
 above-described frequency differences.
 However, as shown in FIG. 13, with the conventional surface acoustic wave
 filter including a plurality of connection structures each containing one
 series resonator and one parallel resonator and connected to each other in
 a mirror image relationship, it is difficult to combine all of these
 different types of resonance without problems. That is, for the surface
 acoustic wave filter shown in FIG. 13, the resonant frequencies of all of
 the series resonators S1 through S3, and moreover, the resonant
 frequencies of all of the parallel resonators P1 through P3 are set to be
 the same, since the filter has the mirror image connection-arrangement.
 Thus, it was very difficult to optimize all of the above-described
 resonance phenomena only by adjusting the frequency difference between the
 series resonators S1 through S3 and the parallel resonators P1 through P3.
 On the contrary, in the present preferred embodiment, by making the
 resonant frequencies of the plurality of parallel resonators P1 through P3
 different, the above-described different levels of resonance within the
 band are greatly improved, and thereby, VSWR can be greatly reduced in a
 wide frequency range in the band.
 In the surface acoustic wave filter 1 including the first, second, and
 third parallel resonators P1 through P3 arranged along the direction from
 the input terminal toward the output terminal in order of the first, the
 second and the third parallel resonators, as in the present preferred
 embodiment, if the resonant frequency f1 of the second parallel resonator
 P2 disposed at the middle position and the resonant frequency f2 of the
 first and third parallel resonators P1 and P3 satisfy f2&gt;f1, preferably,
 f2&gt;f1&gt;0.99.times.f2, VSWR on the lower frequency side in the band is
 greatly reduced, and thereby, the insertion loss is significantly
 improved. The reasons for such advantageous results will be described
 below.
 Regarding the surface acoustic wave filter having the same connection
 structure as shown in FIG. 1, that is, a connection structure obtained by
 arranging the series resonators S1 and S2 and the parallel resonators
 P1-P3 on a piezoelectric substrate, according to the following
 specifications, its VSWR was determined.
 Series resonators S1 and S2
 Length of an aperture: 40 .mu.m
 Number of pairs of electrode fingers: 100 pairs
 Number of electrode fingers of reflector: 100
 Parallel resonators P1 and P3
 Length of an aperture: 40 .mu.m
 Number of pairs of electrode fingers: 90 pairs
 Number of electrode fingers of reflector: 100
 Parallel resonators P2
 Length of an aperture: 40 .mu.m
 Number of pairs of electrode fingers: 180 pairs
 Number of electrode fingers of reflector: 100
 It is noted that VSWR is an index which represents the reflection quantity
 of the surface acoustic wave filter 1 on the input or the output side. As
 VSWR in a pass band becomes smaller, the characteristic improves.
 Particularly, when the filter is used as the band filter of a portable
 telephone, it is desirable that VSWR has a value of up to about 2 in the
 pass band.
 In FIG. 2, it is seen that the maximum points E and F of VSWR exist in the
 band on the higher and lower frequency sides. This is a generally observed
 phenomenon, as understood in the description of the relationship of the
 difference between the resonant frequency of the series resonators and
 that of the parallel resonators to VSWR. This occurs due to the plural
 resonant phenomena in the band.
 Then, by comparison, it was investigated how the maximum point E on the
 lower frequency side and the maximum point F on the higher frequency side
 are different in this preferred embodiment and the conventional example.
 First, in the conventional example, the parallel resonators P1 through P3
 were changed at the same time while making sure that the resonant
 frequencies of the parallel resonators P1 through P3 were the same. The
 change of VSWR, caused in this case, is shown in FIG. 3.
 In FIG. 3, the frequency ratio of the parallel resonators is plotted on the
 abscissa, and VSWR is plotted on the ordinate. The frequency ratio is
 defined as f.sub.1 /f.sub.0 in which f.sub.0 represents the resonant
 frequency of the reference parallel resonator obtained by a conventional
 design technique and supposed to be optimum, and f.sub.1 represents the
 resonant frequency of the parallel resonators.
 Further, in FIG. 3, the solid line represents the VSWR characteristic on
 the higher frequency side which corresponds to the above-described maximum
 point F on the higher frequency side, and the broken line represents the
 VSWR characteristic on the lower frequency side which corresponds to the
 maximum point E on the lower frequency side.
 As seen in FIG. 3, with the frequency ratio decreased, that is, with the
 resonant frequency of the parallel resonators decreased, the maximum point
 of VSWR on the lower frequency side becomes smaller, namely, the
 characteristic is improved. However, with respect to the maximum point F
 of VSWR on the higher frequency side, as the resonant frequency is
 decreased, the characteristic is degraded to a greater degree.
 That is, it is seen that at a frequency ratio where VSWR on the lower
 frequency side is improved only by about 0.1-0.2, VSWR on the higher
 frequency side exceeds the tolerance, namely, exceeds the value of
 approximately 2.
 FIG. 4 illustrates the results obtained when the frequency of the second
 parallel resonator P2 only of the surface acoustic wave filter 1 of the
 above preferred embodiment was decreased. The frequency ratio of the
 parallel resonators is plotted on the abscissa. In this case, the
 frequency ratio is defined as (the resonant frequency of the second
 parallel resonator P2)/(the resonant frequency of the first and third
 parallel resonator P1 and P3), where the resonant frequencies of the first
 and third parallel resonators P1 and P3 are fixed and have a value similar
 to the above-described standard frequency of the conventional example as
 shown in FIG. 3.
 As seen in FIG. 4, as the resonant frequency of the second parallel
 resonator is reduced, the maximum point of VSWR on the lower frequency
 side becomes sufficiently small (see the broken line). On the other hand,
 as the resonant frequency of the second parallel resonator P2 is reduced,
 the VSWR characteristic on the higher frequency side is decreased, as
 shown by the solid line. However, the change of the VSWR characteristic on
 the higher frequency side is very small as compared with the conventional
 example shown in FIG. 3.
 It is seen that when the ratio of the resonant frequency of the second
 parallel resonator P2 relative to that of the first and third parallel
 resonators P1 and P3 is varied up to the value of about 0.990, VSWR on the
 higher frequency side is increased to a value of about 2.0, and the
 maximum point of VSWR on the lower frequency side is improved by about
 0.5.
 Accordingly, it is seen that by setting the resonant frequency of the
 second parallel resonator in the range defined by the above formula (1),
 the VSWR characteristic on the higher frequency side is reduced to a value
 of up to about 2, the VSWR characteristic on the lower frequency side is
 significantly improved, and thereby, the insertion loss is greatly reduced
 in a wide frequency range.
 Hereinafter, a more concrete example of the surface acoustic wave filter 1
 of the first preferred embodiment will be described.
 As seen in the plan view of FIG. 5, as the surface acoustic wave filter 1,
 the series resonators S1 and S2 and the parallel resonators P1-P3 each
 including a one-port SAW resonator were provided on a LiTaO.sub.3
 substrate 11. The electrode finger pitches of IDT of the parallel
 resonators P1-P3 were set as listed in Table 1 set forth below. That is,
 the electrode finger pitch of the second parallel resonator P2 was set at
 about 2.110 .mu.m, which is higher than the electrode finger pitch of
 about 2.100 .mu.m of the first and third parallel resonators P1 and P3.
 In this case, the resonant frequencies of the parallel resonators P1-P3 are
 listed below in Table 1.
 More particularly, the resonant frequency of the second parallel resonator
 P2 is about 1938 MHz, which is lower than the resonant frequency of about
 1948 MHz of the first and third parallel resonators P1 and P3. The
 above-described frequency ratio of the parallel resonators is about
 0.9952.
 For comparison, as a conventional example, a surface acoustic wave filter
 was produced in the same manner as the above-described preferred
 embodiment except that all of the electrode pitches of IDT of the parallel
 resonators P1-P3 were set at about 2.100 .mu.m.
 TABLE 1
 Electrode
 finger pitch Electrode Frequency Frequency
 of first finger pitch of first of
 preferred of conventional preferred conventional
 embodiment example embodiment example
 Resonator (.mu.m) (.mu.m) (MHz) (MHz)
 P1 2.100 2.100 1948 1948
 P2 2.110 2.100 1938 1948
 P3 2.100 2.100 1948 1948
 FIG. 6A illustrates the VSWR characteristics of the surface acoustic wave
 filters of the above preferred embodiment and the comparison example, and
 FIG. 6B illustrates the insertion loss-frequency characteristics thereof.
 In FIGS. 6A and 6B, the solid lines represent the characteristics of the
 present preferred embodiment, and the broken lines indicate of the
 conventional example.
 As seen in FIGS. 6A and 6B, the VSWR characteristics in the band of the
 surface acoustic wave filter 1 of the present preferred embodiment are
 improved as compared with characteristics of the surface acoustic wave
 filter of the conventional example. Particularly, VSWR is effectively
 reduced on the lower frequency side, and thereby, the loss on the lower
 frequency side is reduced by about 0.5 dB.
 Thus, by making the electrode pitch of IDT of the second parallel resonator
 P2 longer than that of the first and third parallel resonators, the
 impedance relevance on the lower frequency side is greatly improved, and
 the reflection characteristic is also significantly improved.
 FIG. 7 is a circuit diagram of a surface acoustic wave filter according to
 a second preferred embodiment of the present invention. The surface
 acoustic wave filter 21 of this preferred embodiment is configured in a
 manner similar to that of the surface acoustic wave filter 1 of the first
 preferred embodiment except that three series resonators SI through S3 are
 arranged in the series arm. That is, one series resonator is added as
 compared with the surface acoustic wave filter 1 of the first preferred
 embodiment.
 In the above-described surface acoustic wave 21, an LiTaO.sub.3 substrate
 was used. One-port SAW resonators defining the series resonators S1-S3 and
 the parallel resonators P1-P3 were disposed on the LiTaO.sub.3 substrate
 used as a piezoelectric substrate, in a manner similar to that of the
 first preferred embodiment.
 Table 2 below lists the electrode finger pitches of IDT of the first,
 second and third parallel resonators P1-P3. In addition, the resonant
 frequencies of the parallel resonators P1-P3 are also listed in Table 2.
 For comparison, as a conventional example, a surface acoustic wave filter
 was produced which was configured in a manner similar to that of the
 second preferred embodiment except that the electrode pitches of IDT of
 the parallel resonators P1-P3 were set at about 2.100 .mu.m as listed in
 the following Table 2. The resonant frequencies of the parallel resonators
 P1-P3 in the surface acoustic wave filter of the conventional example are
 also listed in the following Table 2.
 TABLE 2
 Electrode
 finger pitch Electrode Frequency Frequency
 of second finger pitch of second of
 preferred of conventional preferred conventional
 embodiment example embodiment example
 Resonator (.mu.m) (.mu.m) (MHz) (MHz)
 P1 2.100 2.100 948 948
 P2 2.110 2.100 938 948
 P3 2.100 2.100 948 948
 FIG. 8 illustrates the VSWR characteristics of the surface acoustic wave
 filter 21 of the second preferred embodiment and the surface acoustic wave
 filter of the conventional example which were prepared in the
 above-described manner, and FIG. 9 illustrates the insertion
 loss-frequency characteristics of the filters.
 In FIGS. 8 and 9, the solid lines represent the characteristic of the
 surface acoustic wave filter of the second preferred embodiment, and the
 broken lines represent the characteristics of the conventional example.
 As seen in FIGS. 8 and 9, in the second preferred embodiment, the VSWR
 characteristics on the lower frequency side can be effectively improved.
 Accordingly, as apparent from FIG. 9, the loss can be reduced in a wide
 frequency range, particularly on the lower frequency side, in the band.
 In the above-described first and second preferred embodiments, the resonant
 frequency of the second parallel resonator P2 is varied relative to that
 of the first and third parallel resonators P1 and P3 by changing the
 electrode pitch. However, in order to vary the resonant frequency, various
 methods may be used to change the electrode finger width, a thin film
 capable of varying the sound velocity is adhered to the interdigital
 electrodes constituting the IDT, and the like, in addition to the manner
 by changing the electrode pitch. Thus, the manner for varying the resonant
 frequency of the second parallel resonator P2 with respect to the first
 and third parallel resonators P1 and P3 has no particular limitation.
 Further, in the first and second preferred embodiments, the ladder type
 surface acoustic wave filters each containing three parallel resonators
 are described. The present invention is not restricted to such ladder type
 surface acoustic wave filters, and is applicable to a surface acoustic
 wave filter containing at least four parallel resonators.
 While preferred embodiments of the invention have been disclosed, various
 modes of carrying out the principles disclosed herein are contemplated as
 being within the scope of the following claims. Therefore, it is
 understood that the scope of the invention is not to be limited except as
 otherwise set forth in the claims.