Surface acoustic wave filter

Abstract

A saw filter includes a first saw resonator having a pair of terminals and a predetermined resonance frequency (f_rp), the first saw resonator being provided in a parallel arm of the saw filter. A second saw resonator has a pair of terminals and a predetermined resonance frequency (f_rs) approximately equal to a predetermined antiresonance frequency of the first saw resonator (f_ap). The second saw resonator is provided in a series arm of the saw filter. An inductance element is connected in series to the first saw resonator.

Description

Application Ser. No. 09/314,943, filed May 4, 1999 and copending application Ser. No. 09/925,942, filed Aug. 10, 2001, are each reissues of U.S. Pat. No. 5,631,612 (application Ser. No. 08/369,492, filed Jan. 6, 1995).

This application is a continuation of application No. 07/965,774, filed Oct. 23, 1992, now U.S. Pat. No. 5,559,481, patented Sep. 24, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to surface acoustic wave (SAW) filters, and more particularly to a ladder-type SAW filter suitable for an RF (Radio Frequency) filter provided in pocket and mobile telephones, such as automobile phone sets and portable phones.

2. Description of the Prior Art

In Japan, an automobile phone or portable phone system has a specification in which a transmission frequency band is ±8.5 MHz about a center frequency of 933.5 MHz. The ratio of the above transmission band to the center frequency is approximately 2%.

Recently, SAW filters have been employed in automobile phone or portable phone systems. It is required that the SAW filters have characteristics which satisfy the above specification. More specifically, it is required that the pass band width is so broad that 1) the ratio of the pass band to the center frequency is equal to or greater than 2%, 2) the insertion loss is small and equal to 5 dB-2 dB, and 3) the suppression factor is high and equal to 20 db-30 dB.

In order to satisfy the above requirements, SAW filters are substituted for conventional transversal filters. Generally, SAW elements are so connected that a ladder-type filter serving as a resonator is formed.

FIG. 1 is an equivalent circuit of a SAW filter disclosed in Japanese Laid-Open Patent Publication No. 52-19044. A SAW filter 1 shown in FIG. 1 comprises a SAW resonator 3 in a series arm 2, and a SAW resonator 5 in a parallel arm 4. The equivalent parallel capacitance C_OB of the resonator 5 in the parallel arm 4 is larger than the equivalent parallel capacitance C_OA of the resonator 3 in the series arm 2.

The SAW filter 1 shown in FIG. 1 has a characteristic shown in FIG. 2. A curve 6 shows an attenuation quantity v. frequency characteristic of the SAW filter 1. As indicated by arrows 7 shown in FIG. 2, the suppression factor increases as the equivalent parallel capacitance C_OB increases. However, as the equivalent parallel capacitance C_OB increases, the band width decreases, as indicated by arrows 8, and the insertion loss increases, as indicated by an arrow 9. Hence, the characteristic deteriorates, as indicated by a broken line 10. When trying to obtain a suppression factor equal to or larger than 20 dB, the band width is decreased so that the ratio of the pass band to the center frequency is equal to or smaller than 1%, and does not satisfy the aforementioned specification of the 800 MHz-band radio systems.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a SAW filter in which the above disadvantages are eliminated.

A more specific object of the present invention is to provide a SAW filter having a large band width, a large suppression factor, and a small insertion loss.

The above objects of the present invention are achieved by a SAW filter comprising: a first SAW resonator (21, R1A, R1B) having a pair of terminals and a predetermined resonance frequency (f_rp), the first SAW resonator being provided in a parallel arm (24) of the SAW filter; a second SAW resonator (23) having a pair of terminals and a predetermined resonance frequency (f_rs) approximately equal to the predetermined antiresonance frequency of the first SAW resonator (f_ap), the second SAW resonator being provided in a series arm (24) of the SAW filter; and an inductance element (25, L1) connected in series to the first SAW resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIG. 1 is an equivalent circuit diagram of a conventional SAW filter;

FIG. 2 is a graph of a characteristic of the conventional SAW filter shown in FIG. 1;

FIG. 3 is a circuit diagram of a SAW filter according to the present invention;

FIG. 4 is a block diagram of the basic structure of a filter circuit using a resonator;

FIGS. 5A, 5B and 5C are diagrams showing a one-terminal-pair SAW resonator;

FIGS. 6A and 6B are diagrams showing frequency characteristics of impedance and admittance of the one-terminal-pair SAW resonator;

FIGS. 7A to 7C are diagrams showing an immittance characteristic of a SAW resonator and a filter characteristic of the filter shown in FIG. 3 using that SAW resonator;

FIGS. 8A to 8C are diagrams showing the characteristics of the conventional SAW filter shown in FIG. 1;

FIGS. 9A and 9B are diagrams showing effects obtained when an inductance is connected in series to a resonator;

FIGS. 10A and 10B are diagrams showing effects obtained when n one-terminal-pair resonators are connected in series;

FIGS. 11A and 11B are diagrams showing an aperture length dependence on a parallel-arm resonator;

FIGS. 12A to 12C are diagrams showing an aperture length dependence on a series-arm resonator;

FIG. 13 is a circuit diagram of a SAW filter according to a first embodiment of the present invention;

FIG. 14 is a diagram showing a band characteristic of the filter shown in FIG. 13;

FIGS. 15A and 15B are diagrams showing effects obtained when an inductance is added to a parallel-arm resonator;

FIG. 16 is a plan view of the structure of the SAW filter shown in FIG. 13 with a lid removed therefrom;

FIG. 17 is a cross-sectional view taken along a line XVII--XVII shown in FIG. 16;

FIG. 18 is a diagram of a SAW filter according to a second embodiment of the present invention;

FIG. 19 is a diagram showing a band characteristic of the filter shown in FIG. 18;

FIGS. 20A and 20B are diagrams showing effects based on the ratio of the aperture length of the parallel-arm resonator to the aperture length of the series-arm resonator;

FIG. 21 is a diagram of a SAW filter according to a third embodiment of the present invention;

FIG. 22 is a diagram showing a band characteristic of the filter shown in FIG. 21;

FIG. 23 is a diagram of a SAW filter according to a fourth embodiment of the present invention;

FIG. 24 is a diagram showing a band characteristic of the filter shown in FIG. 23;

FIG. 25 is a circuit diagram of a SAW filter according to a fifth embodiment of the present invention;

FIG. 26 is a diagram showing a band characteristic of the filter shown in FIG. 25;

FIG. 27 is a circuit diagram of a SAW filter according to a sixth embodiment of the present invention;

FIG. 28 is a diagram showing a first one-terminal-pair SAW resonator shown in FIG. 27;

FIG. 29 is a diagram showing a band characteristic of the filter shown in FIG. 27;

FIG. 30 is a diagram showing the influence of the reflector setting position on the width of a ripple;

FIG. 31 is a plan view of the structure of the SAW filter shown in FIG. 27 with a lid removed therefrom;

FIG. 32 is a diagram showing a variation of the first one-terminal-pair SAW resonator shown in FIG. 27;

FIG. 33 is a diagram showing another variation of the first one-terminal-pair SAW resonator shown in FIG. 27;

FIG. 34 is a circuit diagram of a SAW filter according to a seventh embodiment of the present invention;

FIG. 35 is a diagram showing the relation between the film thickness of the electrode and the ripple occurrence position;

FIG. 36 is a diagram showing a state in which a ripple arising from reflectors of a parallel-arm resonator has been dropped into a high-frequency attenuation pole;

FIGS. 37A, 37B and 37C are diagrams showing a film thickness' dependence on the pass band characteristic of a resonator-type filter;

FIGS. 38A and 38B are diagrams showing the results of an experiment concerning the film thickness' dependence on the insertion loss and the ripple occurrence position;

FIG. 39 is a diagram of a first one-terminal-pair SAW resonator according to an eighth embodiment of the present invention;

FIG. 40 is a diagram showing a band characteristic of the SAW filter shown in FIG. 39;

FIG. 41 is a diagram showing a variation of the first one-terminal-pair SAW resonator used in the eighth embodiment of the present invention;

FIG. 42 is a plan view of a structure which realizes inductors used in the filter shown in FIG. 13;

FIG. 43 is a diagram of another structure which realizes inductors used in the filter shown in FIG. 13;

FIG. 44 is a circuit diagram of a SAW filter according to an eleventh embodiment of the present invention;

FIG. 45 is a perspective view of the SAW filter shown in FIG. 44;

FIGS. 46A and 46B are diagrams showing an immittance characteristic of a SAW resonator in which the resonance frequency is higher than the anti-resonance frequency;

FIGS. 47A, 47B and 47C are diagrams showing variations in the band characteristic of the ladder-type filter observed when the difference between the resonance frequency and the antiresonance frequency increases from zero;

FIGS. 48A and 48B are diagrams showing how to measure the characteristics of the SAW resonator;

FIG. 49 is a graph showing admittance and immittance characteristics of SAW resonators in the series arm and the parallel arm;

FIG. 50 is a diagram showing the frequency dependence on the product of bx;

FIG. 51 is diagram showing an equivalent circuit in which a part of the circuit shown in FIG. 44 is expressed by means of L and C;

FIG. 52 is a diagram showing the relation between |bx_max| and Δf/f_rs;

FIG. 53 is a diagram showing the relation between k² and τ;

FIG. 54 is a circuit diagram of a SAW filter according to a twelfth embodiment of the present invention;

FIG. 55 is a perspective view of the SAW filter shown in FIG. 54;

FIG. 56 is a diagram showing a filter characteristic of the SAW resonator shown in FIG. 53;

FIG. 57 is a diagram showing a characteristic obtained when an output-side admittance of the filter shown in FIG. 64 is reduced;

FIGS. 58A and 58B are circuit diagrams of unit sections;

FIGS. 59A, 59B and 59C are circuit diagrams showing multi-connections of unit sections;

FIG. 60 is a diagram showing a connection of two four-terminal circuits and an interface therebetween;

FIGS. 61A, 61B and 61C are circuit diagrams showing unit section connecting ways;

FIG. 62 is a diagram showing how n unit sections are cascaded;

FIGS. 63A, 63B and 63C are circuit diagrams showing how ladder-type circuits are configured using the unit sections;

FIG. 64 is a circuit diagram of a

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows an overview of a SAW filter 20 according to the present invention. The SAW filter 20 comprises a first SAW resonator 21 having a pair of terminals, a parallel arm 22, a second SAW resonator 23 having a pair of terminals, a series arm 24, and a inductor 25. The first resonator 21 connected to the parallel arm 22 has a predetermined resonance frequency f_rp. The second resonator 21 connected to the series arm 24 has a predetermined resonance frequency f_rs approximately equal to an antiresonance frequency f_ap of the first resonator 21. The inductor 25 is connected in series to the first resonator 21, and provided in the parallel arm 22.

The principle of the SAW filter 20 will now be described. Use of image parameters is convenient to verify whether or not a resonance circuit has a filter characteristic. The details of image parameters are described in the following document: Yanagisawa et al., "Theory and Design of Filters", Sanpo Shuppan, Electronics Sensho, pp.192-pp.203, 1974.

First of all, a basic ladder-type circuit having a filter characteristic will be described with reference to FIG. 4. Two black boxes 30 and 31 shown in FIG. 4 are respectively SAW resonators. For the sake of simplicity, it will now be assumed that the SAW resonators 30 and 31 are respectively reactance circuits having no resistance, and that the impedance Z of the resonator 30 provided in the series arm is equal to jx, and the admittance Y of the resonator 31 provided in the parallel arm is equal to jb.

According to the image parameter method, an image transfer quantity γ (a complex number) defined in the following equation has the important meaning: $\begin{array}{cc}\exp \⁡\left(\mathrm{\γ}\right)=\sqrt{V_1\·I_1/V_2\·I_2}& \left(1\right)\end{array}$

where V₁ and I₁ denote an input voltage and an input current, respectively, and V₂ and I₂ denote an output voltage and an output current, respectively. The equation (1) can be rewritten as follows: $\begin{array}{cc}\begin{array}{c}\tanh \⁡\left(\mathrm{\γ}\right)=\tanh \⁡\left(\mathrm{\α}+j\⁢\⁢\mathrm{\β}\right)\\ =\sqrt{\left(B\·C\right)/\left(A\·D\right)}\end{array}& \left(2\right)\end{array}$

where A, B, C and D denote parameters of an F matrix showing the whole circuit shown in FIG. 4. When the value expressed by the equation (2) is an imaginary number, the two-terminal-pair circuit shown in FIG. 4 has a pass band characteristic. With the above value being a real number, the circuit shown in FIG. 4 has an attenuation characteristic. The ABCD parameters can be rewritten using the above-mentioned x and b:

A=1

B=jx

C=jb

D=1-bx (3).

Hence, the following equation (4) can be obtained from the equation (2) using the above ABCD parameters: $\begin{array}{cc}\tanh \⁡\left(\mathrm{\γ}\right)=\sqrt{\mathrm{bx}/\left(\mathrm{bx}-1\right)}.& \left(4\right)\end{array}$

When 0<bx<1, that is, when b and x have the same sign and are small values, the entire circuit shown in FIG. 4 has a pass band characteristic. When bx<0 or bx>1, that is, when the b and x have different signs or the product of bx is a large value, the circuit shown in FIG. 4 has an attenuation characteristic.

In order to qualitatively understand the frequency characteristics of b and x, the impedance and admittance of the SAW resonators will not be considered.

As shown in FIG. 5A, a SAW resonator having pair of terminals comprises an interdigital electrode 40 (see "Nikkel Electronics", November 29, pp.76-pp.98, 1976). A reference number 41 indicates a pair of electrodes, 42 indicates an aperture length (crossing width), and 43 indicates an interdigital electrode period. When the resistance of the interdigital electrode 40 is neglected, the SAW resonator shown in FIG. 5A has an equivalent circuit 45 shown in FIG. 8B, in which C₀ denotes the electrostatic capacitance of the interdigital electrode 40, C₁ and L₁ denote equivalent constants. Hereinafter, the equivalent circuit 45 is depicted by symbol 46 shown in FIG. 5C.

FIGS. 6A and 6B qualitatively show an impedance vs. frequency characteristic (A) of the equivalent circuit shown in FIG. 5B, and an admittance vs. frequency characteristic (B) thereof. The characteristics shown in FIGS. 6A and 6B are double resonance characteristics in which two resonance frequencies f_r and f_a exist. It will be noted that a resonator having a crystal has a double resonance characteristic. When the resonators respectively having a double resonance characteristic are arranged in the series and parallel arms, respectively, and an antiresonance frequency f_ap of the parallel arm is made approximately equal to a resonance frequency f_rs of the series arm, a circuit can be configured which has a band-pass-type filter characteristic having the center frequencies f_ap and f_rs. This is because, as shown in an immittance vs. frequency characteristic shown in FIG. 7A, the relation 0<bx<1 is satisfied in a frequency range around the center frequency f_ap≈f_rs and that frequency range is a pass band, while the relation bx>1 is satisfied in a frequency range slightly away from the center frequency and the relation bx<0 is satisfied in a frequency range far away from the center frequency, the latter two frequency ranges serving as attenuation bands. Hence, the SAW filter shown in FIG. 4 has a qualitative filter characteristic 47 shown in FIG. 7B.

A description will now be given of the factors that determine the band width in the resonator-type SAW filters. As is seen from FIGS. 7A and 7B, the band width is mainly dependent on the difference between the resonance frequency f_r and the antiresonance frequency f_a of each of the two resonators. The band width increases as the above difference increases, while the band width decreases as the difference decreases. The resonance frequency f_r and the antiresonance frequency f_a can be determined using the following equations, using the equivalent circuit constants shown in FIG. 5B: $\begin{array}{cc}{f}_r=1/\left[2\&CenterDot;\mathrm{\&pi;}\&it;\sqrt{\left(C_1\times L_1\right)}\right]& \left(5\right)\\ f_a=f_r\&CenterDot;\sqrt{\left(1+1/\mathrm{\&tau;}\right)}& \left(6\right)\\ \mathrm{\&tau;}=C_0/C_1& \left(7\right)\end{array}$

where τ denotes the capacitance ratio. The ratio of the pass band to the center frequency (Δf/f_o) is mainly dependent on the difference between f_r and f_a, and is therefore expressed in the following expression, using the equations (6) and (7):

Δf/f₀=2(f_a-f_r)/(f_a+f_r)=2/(4τ+1) (8).

It can be seen from the equation (8) that the capacitance ratio τ is the main factor which determines the ratio of the pass band to the center frequency. However, as set forth in Japanese Laid-Open Patent Publication No. 52-19044, the capacitance ratio is much dependent on the type of substrate material used for the interdigital electrode. For example, an ST-cut crystal having a small electromechanical coupling coefficient has a capacitance ratio τ equal to or greater than 1300, while a 36°C Y-cut X-propagation LiTaO₃ substrate having a large electromechanical coupling coefficient has a capacitance ratio τ of approximately 15. The ratio of the pass band to the center frequency is 0.04% for ST-cut crystal, and 3.3% for the 36°C Y-cut X-propagation LiTaO₃ substrate. Hence, the band width is much dependent on the substrate material.

The band width decreases as the equivalent parallel capacitance C_OB increases in order to improve the side lobe suppression factor according to Japanese Laid-Open Patent Publication No. 52-19044.

The above phenomenon will now be described with reference to FIGS. 8A, 8B and 8C. As is seen from the previous description of the principle of the present invention, as the admittance value increases while f_r and f_a of the parallel resonator (see, FIG. 8C) are kept constant, the product of bx has a negative sign and increases, as shown in FIG. 8A. However, the bx product increases around the center frequency, and hence the range of bx>1 increases. Hence, the pass band in which the relation 0<bx<1 stands is narrowed, and a sufficient pass band cannot be obtained. This phenomenon is indicated by arrows in FIG. 8B.

The following two conditions must be satisfied in order to eliminate the above disadvantages. The first condition is to increase the difference between the resonance frequency f_r and the antiresonance frequency f_a in at least one of the resonators provided in the series and parallel arms (see FIG. 8C). The second condition is to increase either the impedance or admittance of the above-mentioned one of the resonators. As the impedance or admittance increases, the side lobe attenuation quantity increases. When the above two conditions awe satisfied, the side lobe attenuation quantity can be improved while the pass band is improved or prevented from being narrowed.

Regarding the first condition, it is effective to provide an inductor L connected in series to a SAW resonator having a pair of terminals in order to increase the difference between f_r and f_a. FIGS. 9A and 9B respectively show an impedance vs. frequency characteristic of a SAW filter in which an inductor having an inductance of 8 nH is connected to a resonator, and an admittance vs. frequency characteristic thereof. The parameters of the equivalent circuits of the SAW resonators used for obtaining the characteristics are illustrated in FIGS. 9A and 9B.

FIG. 9A shows an impedance characteristic curve 50 obtained before the inductor L is connected to the resonator, and an impedance characteristic curve 51 obtained after the inductor is connected thereto. FIG. 9B shows an admittance characteristic curve 52 obtained before the inductor L is connected to the resonator, and an admittance characteristic curve 53 obtained after the inductor L is connected thereto.

It can be seen from FIG. 9A that the inductance L increases the distance between the resonance frequency f_r and the antiresonance frequency f_a. In the graph of FIG. 9A, the distance is increased by approximately 30 MHz. This is because, as shown in FIG. 9A, the inductance L functions to shift the impedance characteristic curve of the original resonator upwards to the plus side by ωL, and hence the resonance frequency f_r changes to f_r'. In this case, the antiresonance frequency f_a has little variation. The admittance, which is the reciprocal of the impedance, changes, as shown in FIG. 9B. In this case, the resonance frequency f_r changes to f_r'.

Regarding the aforementioned second condition, the admittance value increases due to the inductance L, as shown in FIG. 9B. However, as shown in FIG. 9A, the impedance value decreases in frequencies outside of the pass band. Hence, if the inductance L is added to the resonator provided in the series arm, it is necessary to provide an additional means for increasing the impedance value. The above additional means is, for example, an arrangement in which a plurality of identical SAW resonators are connected in series to each other (cascaded).

FIGS. 10A and 10B show an impedance characteristic curve 56 of a resonance arrangement in which n identical SAW resonators, each having a pair of terminals, are cascaded. As shown in FIGS. 10A and 10B, the impedance value of the resonance arrangement having the n cascaded resonators is n times that of the single resonator. The resonance frequency of the resonator with the inductor L connected thereto is f_r". That is, the difference between f_r" and f_a of the resonance arrangement with the inductor L connected thereto is slightly smaller than the difference between f_r' and f_a of a single resonator with the inductor L connected thereto. However, the difference between f_r" and f_a of the resonance arrangement with the inductor L connected thereto is larger than that without the inductor L. It is possible to further increase the difference between the resonance frequency and the antiresonance frequency by using a larger inductance L.

In order to increase the band width, it is also possible to select the antiresonance frequency f_ap of the parallel arm resonator and the resonance frequency f_rs of the series arm resonator so that f_rs>f_ap. In this case, the condition bx<0 occurs around the center frequency, and hence the aforementioned pass band condition is not satisfied. Hence, there is a possibility that an insertion loss and a ripple may increase. However, by controlling Δf=f_rs-f_ap, it is possible to substantially suppress the increase in the insertion loss and the ripple and to expand the increase in the pass band.

A description will now be given of embodiments of the present invention. The embodiments which will be described are based on a simulation. Hence, this simulation will be described first, as well as the results of comparisons between the experimental results and the simulation in order to show the validity of the simulation.

The equivalent circuit shown in FIG. 5B easily simulates the characteristic of the SAW resonator having a pair of terminals, while that equivalent circuit is not suitable for simulating, with high accuracy, variations in the number of figure pairs, the aperture length and the electrode thickness, and the effects of a reflector. With the above in mind, the inventors have proposed an improved simulation which uses a Smith's equivalent circuit model and expands a transfer matrix to analyze the SAW resonators (see O. Ikata et al., "1990 ULTRASONIC SYMPOSIUM Proceedings, vol. 1, pp.83-pp.86, 1990; the disclosure of which is hereby incorporated by reference).

FIG. 11A is a graph showing the results of the simulation (calculation) for an arrangement in which a SAW resonator having a pair of terminals is disposed in the parallel arm. FIG. 11B is a graph showing the results of the experiment on an arrangement in which a one-terminal-pair SAW resonator including an interdigital electrode made of Al-2% Cu and having a film thickness of 1600 Å is disposed in a parallel arm, and bonding wires (L=1.5 nH) having a length of 3 mm are connected to the interdigital electrode. It can be seen from FIGS. 11A and 11B that the calculation values match the experiment values with respect to variations in the resonance points (f_r1, f_r2, f_r3) as well as the attenuation quantities observed around the resonance points for different aperture lengths (a=60, 150, 300 μm).

FIG. 12A is a graph showing the results of the simulation for an arrangement in which a SAW resonator having a pair of terminals is disposed in the series arm (see, FIG. 12C). The bonding pads used in the experiment which will be described later were slightly large, and the simulation was carried out taking into account a stray capacitance 0.5 pF of the bonding pads. FIG. 12B is a graph showing the results of the experiment on an arrangement in which a SAW resonator having a pair of terminals is disposed in the series arm. It can be seen from FIGS. 12A, 12B and 12C that the antiresonance frequencies f_a1, f_a2 and f_a3 do not depend on the aperture length and that the simulation results match the experimental results regarding variations in the attenuation quantity around the resonance frequencies.

Hence, it will be apparent from the above that the results of a simulation of the filter with the combination of the resonators disposed in the parallel and series arms match the results of the experiment. The embodiments described below are based on the result of simulations.

FIG. 13 shows a SAW filter 60 according to a first embodiment of the present invention. In Japan, an automobile and portable telephone system has a specification in which the ±8.5 MHz range about a center frequency of 933.5 MHz is a transmission band for mobile telephones and the ±8.5 MHz range about a center frequency of 878.5 MHz separated from 933.5 MHz by -55 MHz is a seriesconventionalisr₁ ) does not have a high impedance. Hence, according to the present embodiment, the phase rotation line S is connected in series to the filter F2.

As shown in FIG. 79, the direction of phase rotation caused by the line S is opposite to the directions shown in FIGS. 75 and 77. However, as shown in FIG. 80, suitable matching of the filter F2 can be obtained. In this case, the length of the line S formed on the glass-epoxy substrate is approximately 25 mm, and the length of the line S formed on the ceramic substrate is approximately 26 mm.

A variation of the configuration shown in FIG. 78 can be made by providing the inductor L in the same manner as shown in FIG. 74. It is also possible to further provide the capacitor C in the same manner as shown in FIG. 76.

The band center frequencies f₁ and f₂ of the sixteenth through nineteenth embodiments of the present invention are not limited to 887 MHz and 932 MHz.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

Claims

1. A band-pass filter having a pair of band-pass filter input terminals and plural pairs of band-pass filter output terminals, comprising:

a pair of saw filters having respective pass bands and comprising a plurality of one-port saw resonators connected in a ladder structure, each having at least a first stage located at a side of the pair of band-pass filter input terminals and a series-arm resonator located at the first stage, a pair of input terminals and a pair of output terminals;

the pair of band-pass filter input terminals being commonly connected to the respective pairs of input terminals of the pair of saw filters;

the plurality of pairs of band-pass filter output terminals being connected to the respective pairs of output terminals of the pair of saw filters; and

an inductance element located between at least one of the saw filters located at the first stage and the pair of band-pass filter input terminals and directly connected between the respective pair of input terminals of the at least one of the saw filters and thereby in parallel to said at least one of the saw filters.

2. A saw filter comprising:

a plurality of first saw resonators, each having a pair of terminals and a predetermined resonance frequency (f_rp), said first saw resonators being connected in respective, parallel arms of the saw filter;

a plurality of second saw resonators, each having a pair of terminals and a predetermined resonance frequency (f_rs) approximately equal to an antiresonance frequency (f_ap) of each of the first saw resonators, said second saw resonators being provided in series arms of the saw filter; and

inductance elements respectively connected in series with the first saw resonators in the parallel arms and formed of wires.

4. A band-pass filter having a predetermined pass-band characteristic and comprising:

a plurality of saw resonators connected in a ladder formation, said plurality of resonators being connected in respective said serial arms and parallel arms; and

bonding inductance elements, said parallel arms of said ladder formation being connected to ground via respective said bonding inductance elements.

6. A band-pass filter having a pair of band-pass filter input terminals and plural pairs of band-pass filter output terminals, comprising:

a pair of saw filters having respective, different pass bands and each saw filter having a pair of saw filter input terminals and a pair of saw filter output terminals and comprising a plurality of one-port saw resonators connected in a latter structure between the input and output terminals and including at least a first stage having a series-arm saw resonator connected to one of the pair of input terminals;

a pair of saw filters having respective pass bands and comprising a plurality of one-port saw resonators connected in a ladder structure, each having at least a first stage located at a side of the pair of band-pass filter input terminals and a series-arm resonator located at the first stage, a pair of input terminals and a pair of output terminals;

the pair of band-pass filter input terminals being commonly connected to the respective pairs of input terminals of the pair of saw filters;

the plurality of pairs of band-pass filter output terminals being connected to the respective pairs of output terminals of the pair of saw filters.

7. A band-pass filter having a predetermined pass-band characteristic and comprising:

a plurality of saw resonators connected in a ladder configuration of respective serial arms and parallel arms, said plurality of saw resonators being connected in respective said serial arms and parallel arms; and

bonding inductance elements respectively connecting said parallel arms of said ladder configuration to ground.

9. An rf saw filter having a relative bandwidth equal to or greater than 2%, a suppression factor equal to or larger than 20 db, and an insertion loss less than or equal to 5 db comprising:

a plurality of first saw resonators, each having a pair of terminals and a predetermined resonance frequency, said first saw resonators being connected in respective, parallel arms of said saw filter;

a plurality of second saw resonators, each having a pair of terminals and a predetermined resonance frequency approximately equal to an antiresonance frequency of each of said first saw resonators, said second saw resonators being provided in series arms of said saw filter; and #20#

inductance elements respectively connected in series with said first saw resonators in the parallel arms.

16. A saw filter comprising:

a first saw resonator in a parallel arm of said saw filter and having a predetermined resonance frequency;

a second saw resonator in a series arm of said saw filter and having a predetermined resonance frequency approximately equal to an antiresonance frequency of said first saw resonator; and

an inductance element connected in series with said first saw resonator, said inductance element functioning to increase the admittance of the parallel arm and decrease the resonance frequency.