CN112787623B

CN112787623B - Surface acoustic wave filter

Info

Publication number: CN112787623B
Application number: CN202011583771.XA
Authority: CN
Inventors: 弗兰克·李; 廖庆嵩; 霍振选; 杨清清
Original assignee: Tiantong Ruihong Technology Co ltd
Current assignee: Tiantong Ruihong Technology Co ltd
Priority date: 2020-12-28
Filing date: 2020-12-28
Publication date: 2022-02-22
Anticipated expiration: 2040-12-28
Also published as: CN112787623A

Abstract

The invention discloses a surface acoustic wave filter, comprising: the piezoelectric resonator comprises a piezoelectric substrate, a plurality of resonators and a plurality of coupling capacitors, wherein the resonators and the coupling capacitors are integrated on the piezoelectric substrate; a plurality of coupling capacitors are connected in series between the input port and the output port; the resonator is connected in parallel between the connection node of the coupling capacitor and ground potential. The coupling capacitor and the resonator are connected in series, parallel and the like to form the surface acoustic wave filter, so that transmission of specific frequency signals is completed, and the function of the filter is realized. The problem of high loss caused by the fact that a plurality of resonators are connected in series in a conventional filter is solved by connecting a plurality of coupling capacitors in series in the filter, the coupling capacitors are easy to manufacture, the process difficulty is reduced, the coupling capacitors are smaller than the conventional resonators formed by interdigital electrodes and reflecting grids, and miniaturization of devices is realized by utilizing the coupling capacitors.

Description

Surface acoustic wave filter

Technical Field

The embodiment of the invention relates to the technical field of band-pass filters, in particular to a surface acoustic wave filter.

Background

The surface acoustic wave filter is widely applied to mobile communication terminals as a frequency selection device. The surface acoustic wave filter comprises resonators, the resonators are interdigital electrode structures manufactured on a piezoelectric substrate, and the resonators are connected in series or in parallel to form the surface acoustic wave filter to complete the function of the filter.

However, the existing filter uses a multi-order series-parallel structure for increasing out-of-band rejection, the loss of the series resonators is accumulated, which is not beneficial to meeting the low loss requirement, and the series resonators usually have higher resonant frequency than the parallel resonators, so for the high-frequency filter product, the electrode line width of the series resonators is extremely small, the process difficulty is greater, and the resonators need static capacitance values for meeting the design requirement, need larger chip area, and are not beneficial to miniaturization of devices.

Disclosure of Invention

The invention provides a surface acoustic wave filter to reduce loss, realize miniaturization of devices and reduce process difficulty.

An embodiment of the present invention provides a surface acoustic wave filter, including:

the piezoelectric resonator comprises a piezoelectric substrate, a plurality of resonators and a plurality of coupling capacitors, wherein the resonators and the coupling capacitors are integrated on the piezoelectric substrate;

a plurality of the coupling capacitors are connected in series between the input port and the output port;

the resonator is connected in parallel between the connection node of the coupling capacitor and ground potential.

Optionally, the resonator includes a first interdigital electrode and a reflective grating; the coupling capacitor comprises a second interdigital electrode, and a set angle is formed between the second interdigital electrode and the arrangement direction of the first interdigital electrode.

Optionally, the first interdigital electrode, the second interdigital electrode, and the reflective gate are made of at least one of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold, or an alloy containing at least one of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold as a main component.

Optionally, the surface acoustic wave filter further includes an inductor, the inductor and at least one of the resonators form a branch connected between a connection node of the coupling capacitor and a ground potential, and the inductor is connected between the resonator of the branch and the ground potential.

Optionally, the number of resonators on each branch is the same.

Optionally, the number of resonators on each branch is not exactly the same.

Optionally, each of the branches includes one or more resonators.

Optionally, the surface acoustic wave filter further includes an inductor, a first end of each of the resonators is connected to a connection node of the coupling capacitor, a second end of each of the resonators is connected to a first end of the same inductor, and a second end of the inductor is connected to the ground potential.

Optionally, the inductor is integrated in the piezoelectric substrate or the package substrate.

Optionally, the piezoelectric substrate material is piezoelectric single crystal or piezoelectric ceramic.

The invention provides a surface acoustic wave filter, which comprises a piezoelectric substrate, a plurality of resonators and a plurality of coupling capacitors, wherein the resonators and the coupling capacitors are integrated on the piezoelectric substrate; a plurality of coupling capacitors are connected in series between the input port and the output port; the resonator is connected between the connecting node of the coupling capacitor and the ground potential in parallel; the coupling capacitor and the resonator are connected in series, parallel and the like to form the surface acoustic wave filter to complete the function of the filter. The coupling capacitors are connected in series in the filter, so that the problem of high loss caused by the fact that a plurality of resonators are connected in series in a conventional filter is solved, the coupling capacitors are easy to prepare, the process difficulty is low, the coupling capacitors are smaller than the conventional resonators formed by interdigital electrodes and reflecting gates, and the miniaturization of devices is realized by more utilizing the coupling capacitors.

Drawings

Fig. 1 is a circuit diagram of a surface acoustic wave filter according to an embodiment of the present invention.

Fig. 2 is a circuit diagram of a resonator according to an embodiment of the present invention.

Fig. 3 is a schematic structural diagram of a coupling capacitor according to an embodiment of the present invention.

Fig. 4 is a circuit diagram of another saw filter according to an embodiment of the present invention.

Fig. 5 is a schematic structural diagram of an inductor according to an embodiment of the present invention.

Fig. 6 is a cross-sectional view of a surface acoustic wave filter according to an embodiment of the present invention.

Fig. 7 is a frequency response graph of a surface acoustic wave filter according to an embodiment of the present invention.

Fig. 8 is an enlarged view of the frequency response curve passband of a saw filter according to an embodiment of the present invention.

Fig. 9 is a circuit diagram of another saw filter according to an embodiment of the present invention.

Fig. 10 is a circuit diagram of another surface acoustic wave filter according to an embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some of the structures related to the present invention are shown in the drawings, not all of the structures.

Fig. 1 is a circuit diagram of a surface acoustic wave filter according to an embodiment of the present invention, and referring to fig. 1, optionally, the surface acoustic wave filter includes:

the piezoelectric resonator comprises a piezoelectric substrate, a plurality of resonators 100 and a plurality of coupling capacitors 200 which are integrated on the piezoelectric substrate;

a plurality of coupling capacitors 200 are connected IN series between the input port IN and the output port OUT;

the resonator 100 is connected in parallel between the connection node of the coupling capacitor 200 and the ground potential GND.

The resonator 100 and the coupling capacitor 200 may be integrated on a piezoelectric substrate by using a microelectronic process, or may be integrated on a piezoelectric substrate by using a photolithography process or other processes. On the piezoelectric substrate, it is easy to select a metal thin film pattern to manufacture the coupling capacitor 200, and the resonator 100 and the coupling capacitor 200 are integrated on the piezoelectric substrate, so that the integration level is improved.

The plurality of coupling capacitors 200 are connected IN series between the input port IN and the output port OUT, a capacitance value of each coupling capacitor 200 can be set as required, the capacitance values of the plurality of coupling capacitors 200 may be the same, may not be the same, or may be partially the same, and this embodiment is not particularly limited herein. A connection node of a coupling capacitor 200 exists between the input port IN and the coupling capacitor 200 connected to the input port IN, a connection node of a coupling capacitor 200 exists between two adjacent coupling capacitors 200 connected IN series, and a connection node of a coupling capacitor 200 exists between the output port OUT and the coupling capacitor 200 connected to the output port OUT. The resonators 100 may be single-ended surface acoustic wave resonators, at least one resonator 100 is connected in parallel between the connection node of the coupling capacitor 200 and the ground potential to form a branch, the resonant frequency of each resonator 100 may be set according to requirements, when there are multiple resonators 100 in one branch, the resonant frequencies of the multiple resonators 100 may be the same, may be different, or may not be completely the same, and the resonant frequencies of the resonators 100 in one branch and the resonators 100 in another branch may be the same, may be different, or may not be completely the same. Fig. 1 exemplarily shows that the surface acoustic wave filter includes a series assembly 201 formed by connecting six coupling capacitors 200 IN series IN sequence, one end of the series assembly 201 is connected to the input port IN, and the other end of the series assembly 201 is connected to the output port OUT, and the surface acoustic wave filter further includes five branches, and a branch exists at a connection node between every two adjacent coupling capacitors 200 connected IN series. Each branch may include a first resonator 1001 and a second resonator 1002, one end of the first resonator 1001 is connected to the connection node of the coupling capacitor 200, the other end of the first resonator 1001 is connected to one end of the second resonator 1002, and the other end of the second resonator 1002 is connected to the ground potential GND. The number of the coupling capacitors 200, the number of the branches, and the number of the resonators 100 on each branch can be set according to requirements, and the embodiment is not limited in detail herein.

One coupling capacitor 200 and one branch can be regarded as one basic unit constituting the surface acoustic wave filter. As shown in fig. 1, when two resonators 100 are included in the branch, that is, one coupling capacitor 200 and two resonators 100 constitute one basic unit, and a plurality of basic units constitute a surface wave filter. The input port IN is connected with an input signal source, the input signal source is an alternating electric signal, the alternating electric signal passes through the coupling capacitor 200 and the resonator 100, and then is converted into an acoustic signal of surface acoustic wave through inverse piezoelectric effect (the inverse piezoelectric effect means that a crystal material receives current excitation and vibrates to generate ultrasonic waves), and is converted into an electric signal through positive piezoelectric effect (the positive piezoelectric effect means that the crystal material receives pressure of the ultrasonic waves and converts the pressure into the electric signal) to be output to the output port OUT, and the frequency of the resonator 100 determines the transmission and attenuation capacity of the filter to signals with different frequencies, so that the signals with specific frequencies are transmitted to the output port OUT from the input port IN, and signals with other frequencies are filtered OUT, and therefore the filtering function is achieved. In the process of signal transmission, the series coupling capacitor 200 and the resonator 100 on the branch circuit together perform the filtering function, and the series coupling capacitor 200 has small signal loss and is easy to manufacture.

The invention provides a surface acoustic wave filter, which comprises a piezoelectric substrate, a plurality of resonators and a plurality of coupling capacitors, wherein the resonators and the coupling capacitors are integrated on the piezoelectric substrate; a plurality of coupling capacitors are connected in series between the input port and the output port; the resonator is connected in parallel between the connection node of the coupling capacitor and the ground potential, and the coupling capacitor and the resonator are connected in series, parallel and the like to form the surface acoustic wave filter to complete the function of the filter. The problem of high loss caused by the fact that a plurality of resonators are connected in series in a conventional filter is solved by connecting a plurality of coupling capacitors in series in the filter, the coupling capacitors are easy to manufacture, the process difficulty is reduced, the coupling capacitors are smaller than the conventional resonators formed by interdigital electrodes and reflecting grids, and miniaturization of devices is realized by utilizing the coupling capacitors.

Fig. 2 is a circuit diagram of a resonator according to an embodiment of the present invention, and fig. 3 is a schematic structural diagram of a coupling capacitor corresponding to the resonator of fig. 2, and referring to fig. 2 and fig. 3, optionally, the resonator 100 includes a first interdigital electrode 110 and a reflective grid 120; the coupling capacitor 200 includes a second interdigital electrode 210, and the second interdigital electrode 210 is arranged at a predetermined angle with respect to the first interdigital electrode 110.

Fig. 2 schematically shows an exemplary structure of a single-ended surface acoustic wave resonator, an ac signal input from an input port IN converts an electrical signal into an acoustic signal of a surface acoustic wave through a first electrode strip 1100, and the acoustic signal is transmitted on a piezoelectric substrate along a direction perpendicular to the electrode strips, i.e., along the x direction shown IN fig. 2, and after a certain delay, the surface acoustic wave is transmitted to a second electrode strip 1101, and the acoustic signal of the surface acoustic wave is converted into an electrical signal by the second electrode strip and transmitted to an output port OUT for output. The reflection grating 120 reflects the surface acoustic waves which are diffused outwards in the transmission process, so that the loss of the surface acoustic waves is reduced. The ability of the saw filter to transmit and attenuate electrical signals of different frequencies is influenced by the internal parameters of the resonator 100, including the width of the electrode strips of the interdigital electrode, the distance between adjacent electrode strips, and the thickness of the electrode film.

The coupling capacitor includes the second interdigital electrode 210, and the second interdigital electrode 210 also has the same characteristics of propagating acoustic signals as the first interdigital electrode 110, so that the direction of the electrode strip of the second interdigital electrode 210 and the direction of the electrode strip of the first interdigital electrode 110 need to form a set angle, optionally, the set angle is 90 °, so that the second interdigital electrode 210 does not have a resonance function in a certain frequency range, and only plays a role of a capacitor. The angle of 90 ° set in this embodiment is only an exemplary preferred embodiment, and in other embodiments, the set angle may be other values.

In other embodiments, the coupling capacitor may also be a stacked structure of metal layer-dielectric layer-metal layer.

Optionally, the materials of the first interdigital electrode 110, the second interdigital electrode 210, and the reflective grating 120 each include at least one of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold, or an alloy containing at least one of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold as a main component.

The material of the first interdigital electrode 110, the second interdigital electrode 210, and the reflective grating 120 may be one metal material of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold, may also be another single metal material such as molybdenum, cobalt, and the like, and may also be an alloy made of any one of the above metal materials, which is not limited in this embodiment.

Fig. 4 is a circuit diagram of another saw filter according to an embodiment of the present invention, fig. 5 is a schematic structural diagram of an inductor according to an embodiment of the present invention, and fig. 6 is a cross-sectional view of the saw filter according to an embodiment of the present invention, which can be cut by the filter shown in fig. 4. Referring to fig. 4, optionally, the saw filter further includes an inductor 300, the inductor 300 and at least one resonator 100 form a branch connected between the connection node of the coupling capacitor 200 and the ground potential GND, and the inductor 300 is connected between the resonator 100 of the branch and the ground potential.

Illustratively, the surface acoustic wave filter shown IN fig. 4 is formed by sequentially connecting six coupling capacitors 200 IN series to form a series assembly 201, one end of the series assembly 201 is connected to the input port IN, and the other end of the series assembly 201 is connected to the output port OUT, and the surface acoustic wave filter further includes five branches, and one branch exists at a connection point between every two adjacent 2 coupling capacitors 200. Each branch includes a first resonator 1001, a second resonator 1002, and an inductor 300, one end of the first resonator 1001 is connected to the connection node of the coupling capacitor 200, the other end of the first resonator 1001 is connected to one end of the second resonator 1002, the other end of the second resonator 1002 is connected to one end of the inductor 300, and the other end of the inductor 300 is connected to the ground potential GND. The inductor 300 may be a spiral inductor, as shown in fig. 5, or may be an IPD inductor or other type of inductor, and the number of the coupling capacitors 200, the number of the branches, the number of the resonators 100 on each branch, and the number of the inductors 300 may be set according to the requirement, which is not limited in this embodiment.

Referring to fig. 6, a metal thin film layer 02 covers a piezoelectric substrate 01, a resonator and a coupling capacitor are etched through a photolithography process, the metal thin film layer 02 is connected with an inductor 300 sequentially through a first bonding pad 03, a gold ball 04, a second bonding pad 05, a via hole 06 and the inductor 300, in this embodiment, the inductor 300 is integrated in a package substrate 07, the inductor 300 is connected with an external circuit through the via hole 06 and a third bonding pad 08, and an epoxy film 09 covers one surface of the piezoelectric substrate 01, which is far away from the metal thin film layer 02, and forms a sealing structure with the package substrate 07 to isolate moisture. The piezoelectric substrate 01 of the filter is connected with a circuit through the gold ball 04, the gold ball 04 can generate a small parasitic inductance under high frequency, the size of the parasitic inductance can be influenced by the shape and the position of the gold ball 04, and the performance of the filter can be influenced by the parasitic inductance. When the inductor 300 is added between the resonator and ground potential, the value of the inductor is much larger than the value of the parasitic inductance, thereby eliminating the effect of the parasitic inductance on the filter.

The resonator 100, the coupling capacitor 200 and the inductor 300 jointly act to convert an electric signal input from the input port IN into an acoustic signal, and then the acoustic signal is converted into an electric signal and output to the output port OUT, and the resonator 100 only transmits a signal with a specific frequency, so that the filtering function of the filter is realized, the bandwidth of the surface acoustic wave filter can be widened after the inductor 300 is added, meanwhile, the zero point of the filter is added, and the influence of the parasitic inductance of the gold ball 04 on the filter during packaging is eliminated.

Fig. 7 is a frequency response curve graph of a surface acoustic wave filter according to an embodiment of the present invention, fig. 8 is an enlarged view of a frequency response curve passband of the surface acoustic wave filter according to the embodiment of the present invention, fig. 7 and fig. 8 are graphs obtained under a circuit structure of the surface acoustic wave filter shown in fig. 4, a center frequency of the filter is 2442MHz, 1db is 3.4% of a relative bandwidth, and a minimum loss is less than 0.8 db. Referring to fig. 7, the abscissa of fig. 7 represents frequency, the ordinate represents Attenuation, i.e., Attenuation suppression of an unwanted frequency signal after an input signal passes through a filter, the ordinate of fig. 8 represents Insertion Loss representing Loss of the input signal to a passband frequency range signal after passing through the filter, fig. 8 is a partially enlarged view of NN in the vicinity of the passband of fig. 7, and the ordinate generally represents Insertion Loss in the partially enlarged view.

Referring to fig. 4, 7 and 8, the abscissa 2.401GHz of m1 is the start frequency of the pass band of the filter, and the abscissa 2.483GHz of m2 is the end frequency of the pass band, i.e., (m1, m2) constitutes the pass band of the filter. The attenuation at the point m1 is-1.685 dB, the attenuation at the point m2 is-1.715 dB, and the minimum attenuation of the passband is-0.7, namely, the loss of the filter can be reduced to about-1.715 dB to-0.7 dB by adopting the structure that the coupling capacitors 200 are connected in series as shown in FIG. 4. In fig. 7, point a is a zero (zero refers to that when the system input amplitude is not zero and the input frequency makes the system output zero, the input frequency value is the zero), and after the inductor 300 is added, the zero a is added, so that the zero can improve the suppression capability without increasing the filter order, and better block the transmission of the signal with the out-of-band frequency.

With continued reference to fig. 4, optionally, the number of resonators 100 on each leg is the same.

For example, each branch in fig. 4 includes two resonators 100, and in other embodiments, the number of resonators 100 on each branch may be set according to requirements. In order to achieve the required performance of the filter, for example, to achieve the required bandwidth, the number of resonators 100 of each branch having the same number of resonators 100 may be adjusted, for example, two resonators 100 per branch may be added to three resonators 100 per branch, and the number of branches may be adjusted, for example, from the original five to seven or six, so as to achieve the required performance of the filter.

Fig. 9 is a circuit diagram of another saw filter according to an embodiment of the present invention, and referring to fig. 9, alternatively, the number of resonators 100 in each branch is not exactly the same.

The surface acoustic wave filter shown IN fig. 9 includes six coupling capacitors 200, the six coupling capacitors 200 are sequentially connected IN series to form a series assembly 201, one end of the series assembly 201 is connected to the input port IN, and the other end of the series assembly 201 is connected to the output port OUT, and the surface acoustic wave filter further includes five branches, and a branch is present at a connection point between each two adjacent coupling capacitors 200. Each branch comprises at least a resonator 100 and an inductor 300, one end of the inductor 300 is connected to the resonator 100, and the other end of the inductor 300 is connected to ground GND. The number of resonators 100 in five branches is not exactly the same, wherein three branches include two resonators 100, one branch includes one resonator 100, and another branch includes three resonators 100, in other embodiments, the number of resonators 100 in each branch may be different, the number of resonators 100 in each branch may be set as required, and when the number of branches is fixed, the number of resonators 100 in each branch may be changed to enable the saw filter to meet performance requirements.

With continued reference to fig. 4, optionally, one or more resonators 100 are included on each leg.

The number of resonators 100 in each branch may be one or more, for example, each branch shown in fig. 4 includes two resonators, and the number of resonators 100 in each branch may be set according to requirements, which is not specifically limited herein.

Fig. 10 is a circuit diagram of another saw filter according to an embodiment of the present invention, and referring to fig. 10, optionally, the saw filter further includes an inductor 300, a first end a1 of each resonator 100 is connected to the connection node of the coupling capacitor 200, a second end a2 of each resonator 100 is connected to a first end B1 of the same inductor 300, and a second end B2 of the inductor 300 is connected to a ground potential GND.

The surface acoustic wave filter of the present embodiment includes six coupling capacitors 200, an inductor 300, and five arms, the six coupling capacitors 200 being connected IN series IN this order between the input port IN and the output port OUT, each arm including a resonator 100, the first end a1 of the resonator 100 of each arm being connected to the connection node, the second end of the resonator 100 being connected to the first end B1 of the inductor 300, and the second end B2 of the inductor 300 being connected to the ground potential GND. All the arms of the saw filter of the present embodiment are connected to the ground potential GND through the same inductor 300, and in other embodiments, some of the resonators 100 of the arms may be directly connected to the ground potential GND, and some of the resonators 100 of the arms may be connected to the ground potential GND through the same inductor 300. In this embodiment, the resonators 100 on each branch are all connected in parallel, in other embodiments, the resonators on the branches may be connected in parallel, and when each branch includes a plurality of resonators 100, the connection manner of the resonators is not specifically limited herein.

The number of coupling capacitors 200, the number of branches, the number of resonators 100 on each branch, the connection manner of resonators 100, the number of inductors 300, and the connection manner between branches can be set according to the requirement, and this embodiment is not limited in detail herein.

The resonator 100, the coupling capacitor 200 and the inductor 300 jointly act to convert an electric signal input from the input port IN into an acoustic signal, and then the acoustic signal is converted into an electric signal and output to the output port OUT, and the resonator 100 only transmits a signal with a specific frequency, so that the filtering function of the filter is realized, the bandwidth of the surface acoustic wave filter can be widened after the inductor 300 is added, meanwhile, the zero point of the filter is added, and the influence of the parasitic inductance of the gold ball on the filter during packaging is eliminated.

When the size of the inductor is small, the inductor can be integrated in a piezoelectric substrate or a packaging substrate, so that the miniaturization of the filter is facilitated, and the integration degree is improved.

The piezoelectric single crystal may be lithium titanate, lithium tantalate, or other piezoelectric single crystal materials, and this embodiment is not limited in detail herein. The piezoelectric material has the property of piezoelectric effect, so that the surface acoustic wave filter can convert an electric signal into an acoustic signal and then into an electric signal.

It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A surface acoustic wave filter, comprising:

the resonator comprises a first interdigital electrode and a reflecting grating; the coupling capacitor comprises a second interdigital electrode which is arranged at 90 degrees with the first interdigital electrode, so that the coupling capacitor has no resonance function and only plays a role of a capacitor in a specific frequency range;

the resonator is connected between a connection node of the coupling capacitor and the ground potential in parallel;

and the inductor and at least one resonator form a branch circuit which is connected between the connecting node of the coupling capacitor and the ground potential, and the inductor is connected between the resonator of the branch circuit and the ground potential.

2. The surface acoustic wave filter as set forth in claim 1, wherein the materials of said first interdigital electrode, said second interdigital electrode, and said reflection grating each include at least one of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold, or an alloy having at least one of aluminum, copper, titanium, tungsten, platinum, nickel, chromium, and gold as a main component.

3. A surface acoustic wave filter as set forth in claim 1, wherein said resonators are equal in number on each of said branches.

4. A surface acoustic wave filter as set forth in claim 1, wherein the number of said resonators is not identical in each of said branches.

5. A surface acoustic wave filter as set forth in claim 1, wherein each of said branches includes one or more of said resonators.

6. A surface acoustic wave filter as set forth in claim 1, further comprising an inductor, a first end of each of said resonators being connected to a connection node of said coupling capacitor, a second end of each of said resonators being connected to a first end of the same inductor, and a second end of said inductor being connected to said ground potential.

7. A surface acoustic wave filter as set forth in claim 1 or claim 6, characterized in that said inductor is integrated in said piezoelectric substrate or package substrate.

8. A surface acoustic wave filter as set forth in claim 1, wherein said piezoelectric substrate material is a piezoelectric single crystal or a piezoelectric ceramic.