Surface acoustic wave filter of multistage connection type

Abstract: The surface acoustic wave filter of multi-stage connection type includes : a plurality of surface acoustic wave filters (15, 16) formed on a piezoelectric substrate (100) and a connection section (8) for electrically connecting the surface acoustic wave filters in a cascade connection. Each of the surface acoustic wave filters comprises interdigital transducers (1, 12) with two signal terminals for electric input and output (respectively 1, 11), the interdigital transducer including an excitation section (6) for exciting a surface acoustic wave and two terminal sections (5, 7) disposed on both sides of the excitation section, the two terminal sections serving as input or output terminal for electric signal and ground terminal. One of the signal terminal sections is connected to the connection section, and the electrode area of that signal terminal section is adjusted so that a parasitic capacitance formed between the signal terminal section and a grounding point of the surface acoustic wave filter is less than or equal to 0.01 pF.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter of multi-stage connection type. More particularly, the present invention relates to a surface acoustic wave filter in which a plurality of surface acoustic wave filters are connected in a multi-stage connection (cascade connection). 2. Description of the Related Arts Surface acoustic wave filters are gaining popularity as a filter for a transmitting/receiving circuit of a small communication apparatus such as a portable phone. A surface acoustic wave filter, especially resonator type surface acoustic wave filter is formed of an electrode pattern formed on a piezoelectric substrate, including an interdigital transducer (hereafter referred to as IDT) and reflectors disposed on both sides of the IDT. A surface acoustic wave is excited inside of this electrode pattern. Also, a surface acoustic wave filter of multi-stage connection type is proposed in which a plurality of surface acoustic wave filters are serially connected. Fig. 7 shows a construction of a conventional surface acoustic wave filter of multi-stage connection type, as known from e.g. JP06276046. This filter includes a plurality of surface acoustic wave filters (109, 110) of transversal coupling type connected serially via a metal electrode film (cascade connection section 102). Generally, IDT 107 includes an excitation section formed of a plurality of electrode fingers as well as a terminal-side bus bar 103 and a connection-side bus bar 104 disposed on upper and lower sides of the IDT. As an example of IDT in the surface acoustic wave filter, there has been developed a filter such that a line width of an electrode finger and an interval between electrode fingers are each 3.2 µm; an electrode period λ of IDT is 12.8 µm; a width of a terminal-side bus bar 103 and a connection-side bus bar 104 along an up-and-down direction of the drawing is about 15λ. Fig. 8 is a circuit block diagram of the surface acoustic wave filter shown in Fig. 12. A filter chip 113 of Fig. 8 corresponds to the surface acoustic wave filter formed on a piezoelectric substrate 100 of Fig. 7. Generally, matching circuits 111, 112 are attached to an input side and an output side of the filter chip 113 for providing an impedance matching with an external circuit. Also, in such a surface acoustic wave filter including connection via the cascade connection section 102, an impedance mismatching often occurs also at the cascade connection section 102, causing an extremely great loss at that section. Therefore, an external circuit (an inductor L in the drawing) as shown in Figs. 7 and 8 is added to provide an impedance matching at the cascade connection section 102. For this external circuit, various kinds of circuits are proposed (Japanese Unexamined Patent Publication Nos. Hei 5(1993)-129884, Hei 6(1994)-276046, Hei 6(1994)-310978, Hei 7(1995)-7369, Hei 7(1995)-22891, Hei 7(1995)-22892). However, the proposed matching circuits are accompanied by a drawback that the cost of the entire filter rises due to an increased area of the filter including this external circuit and the increased number of circuit elements, since the matching circuit is attached to an outside of the filter chip. Here, the impedance mismatching in the cascade connection section 102 is caused by a capacitance component included in parallel with an input impedance and an output impedance of each surface acoustic wave filter. The capacitance component is, for example, a "parasitic capacitance" existing in each surface acoustic wave filter. Fig. 9 is a view for explaining the parasitic capacitance at the IDT portion of the surface acoustic wave filter. The IDT includes an electrode finger section disposed at its center portion and bus bars 115 disposed on both sides of the electrode finger section. The electrode finger section includes a plurality of electrode fingers meshing with each other in a lattice-like configuration. The IDT is formed on a piezoelectric substrate 100 and is electrically insulated from a bottom 114 of the filter chip package. Now, it is believed that the parasitic capacitance exists in portions 116 and 117 enclosed by ellipses as shown in Fig. 9. In other words, one parasitic capacitance (117) is a capacitance of a plate capacitor formed between the bus bar 115 of IDT and the bottom 114 of the package; and the other capacitance (116) is a capacitance existing in the vicinity of a tip of each electrode finger in the electrode finger section of IDT and a connecting portion of an electrode finger adjacent the tip with the bus bar 115. The impedance mismatching at the cascade connection section 102 will be reduced if this parasitic capacitance decreases. SUMMARY OF THE INVENTION The purpose of the present invention is to decrease a parasitic capacitance for reducing an impedance mismatching at a cascade connection section without adding an external circuit to the cascade connection section in a surface acoustic wave filter of multi-stage connection type in which a plurality of surface acoustic wave filters are connected in cascade. The present invention provides a surface acoustic wave filter according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description of preferred embodiments of the invention, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a view showing a construction of a surface acoustic wave filter of multi-stage connection type according to the present invention; Fig. 2 is a graph representing a relationship between a width of a connection-side bus bar and a capacitance contained in an input impedance according to the present invention; Fig. 3 is a graph representing a relationship between the width of the connection-side bus bar and an insertion loss according to the present invention; Fig. 4 is a graph representing frequency relative to insertion loss characteristics when the width of the connection-side bus bar is 1λ according to the present invention; Fig. 5 is a graph representing frequency relative to insertion loss characteristics when the width of the connection-side bus bar is 15 λ according to the present invention; Fig. 6 is a view showing a construction of a multi-mode filter of longitudinal coupling type (when an area of a connection-side bus bar is reduced) according to the present invention; Fig. 7 is a view showing a construction of a conventional surface acoustic wave filter in multi-stage connection; Fig. 8 is a circuit block diagram of the conventional surface acoustic wave filter; and Fig. 9 is a view for explaining a parasitic capacitance at an IDT portion of a surface acoustic wave filter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface acoustic wave filter of multi-stage connection type according to the present invention comprises a plurality of surface acoustic wave filters formed on a piezoelectric substrate and a connection section for electrically connecting the surface acoustic wave filters in a cascade connection. An electrode area which forms a parasitic capacitance in the surface acoustic wave filters and in the connection section is adjusted so that the parasitic capacitance is below a predetermined level. The piezoelectric substrate to be used in the present invention may be formed of any material as long as a surface acoustic wave is generated thereon. The material may be a conventional one such as LiTaO 3 , LiNbO 3 , or Li 2 B 4 O 7 . However, the present invention is especially effective when quartz is used as the substrate material because its relative dielectric constant is small and a variation in adjustment of an input impedance is small. The surface acoustic wave filter produces a particularly excellent effect when a multimode filter of transversal coupling type, which is often used in multi-stage connection, is formed on a quartz substrate. However, the same effect may be obtained by using a surface acoustic wave filter of longitudinal coupling type or a filter of transversal type. The IDT and the connection section constituting the surface acoustic wave filter are formed of an electrically conductive metal film such as aluminum Al, gold Au, copper Cu, or titanium Ti. Further, in order to reduce the parasitic capacitance in the present invention, it is preferable that the electrode area of said one of the signal terminal sections is less than or equal to 1/15 of an electrode area of said other signal terminal section. One of the two terminal signal sections constituting the IDT is electrically connected to the connection section. Here, a "parasitic capacitance" is formed between said one of the two signal terminal sections and a grounding point of the surface acoustic wave filter. The grounding point of the surface acoustic wave filter is, for example, a metallic film on a package base in which the piezoelectric substrate is mounted. The grounding point is located on a side opposite to the surface of the piezoelectric substrate on which the IDT and the like are formed. In addition to this, the parasitic capacitance may possibly be formed between various sites of the surface acoustic wave filters and the ground. The present invention mainly aims at reducing the parasitic capacitance formed at one site (117) shown in Fig. 9. Specifically, a first parasitic capacitance 117 is formed between one signal terminal section of the IDT and the grounding point of the surface acoustic wave filter. In order to reduce the first parasitic capacitance 117, the area of said one signal terminal section is preferably as small as possible. Fig. 1 shows a construction of the surface acoustic wave filter of multi-stage connection type for reducing the first parasitic capacitance 117 according to the present invention. Referring to Fig. 1, two multimode filters of transversal coupling type 15, 16 are connected in two stages via a cascade connection section 8. It is to be noted that the construction is not specifically limited to the one shown in Fig. 1. The multimode filter 15 of transversal coupling type in the first stage is a surface acoustic wave filter of signal input side, to which an electric signal is inputted from an input terminal 4. A center portion of the filter 15 is an input-side IDT 1, and two reflectors 2, 3 are disposed on both sides of the IDT 1. The IDT 1 includes an excitation section and two signal terminal sections 5, 7. The excitation section is formed of a plurality of electrode fingers 6. Hereafter, the signal terminal section connected to the input terminal 4 is referred to as a terminal-side bus bar 5, and the signal terminal section connected to the cascade connection section 8 is referred to as a connection-side bus bar 7. The multimode filter 16 of transversal coupling type and the output-side IDT 12 have the same construction as shown above. However, the output terminal 11 is a portion for outputting the detected surface acoustic wave as an electric signal. Electrode patterns for the IDTs 1, 12, the reflectors 2, 3, 13, 14 and the cascade connection section 8 in Fig. 1 are all formed of a metal film such as aluminum on the same piezoelectric substrate. According to a first embodiment of the present invention shown in Fig. 1, an electrode area of the connection-side bus bar 7 is sufficiently small. Referring to Fig. 1, the terminal-side bus bars 5, 10 and the connection-side bus bars 7, 9 constituting the IDTs 1, 12 each have a rectangular shape of equal width which is elongated in a right-and-left direction of the document sheet. Therefore, the widths 21, 23 of the connection-side bus bars 7, 9 are allowed to be smaller than the widths 22, 24 of the terminal-side bus bars 5, 10 in order to reduce the area. This allows the parasitic capacitance formed between the connection-side bus bars 7, 9 and the ground to be reduced. A principle for the reduction of parasitic capacitance is explained below. Generally, the multimode filter of transversal coupling type shown in Fig. 1 is formed on a surface of a piezoelectric substrate 100. The piezoelectric substrate 100 is mounted on the metal film formed on the base of a filter package. A rear surface of the piezoelectric substrate 100 on which the electrode patterns of the IDT, reflectors, and the cascade connection section are not formed is in contact with the housing which is usually connected to the ground. Since the piezoelectric substrate itself is made of an insulator such as quartz, the first parasitic capacitance 117 is formed via the piezoelectric substrate 100 between the connection-side bus bars 7, 9 and the rear surface of the piezoelectric substrate 100 which is in contact with the housing. The first parasitic capacitance can be approximated by a capacitance of a plate capacitor. Generally, the capacitance of the plate capacitor is represented by the following formula: C = ε 0 ·ε r ·S/t wherein C is a capacitance of the plate capacitor (the first parasitic capacitance); ε 0 is a vacuum dielectric constant; ε r is a relative dielectric constant of the piezoelectric substrate; S is the electrode area of the connection-side bus bars 7, 9; and t is a thickness of the piezoelectric substrate. From this formula, it is understood that the first parasitic capacitance C is in proportion to the electrode area S of the connection-side bus bars 7, 9. Therefore, the first parasitic capacitance decreases by reduction of the electrode area S of the connection-side bus bars 7, 9. Fig. 2 is a graph showing a relationship between the widths 21, 23 of the connection-side bus bars 7, 9 and the capacitance value contained in an input impedance of the multimode filters 15, 16 of transversal coupling type according to the first embodiment of the present invention. An ST-cut quartz with a thickness (t) of about 350 µm is used for the piezoelectric substrate 100. A multimode filter of transversal coupling type shown in Fig. 1 is formed on the piezoelectric substrate 100. In the IDTs 1, 12 and the reflectors 2, 3, 13, 14 of the multimode filter of transversal coupling type, the electrode period is λ = 12.5 µm; the electrode film thickness is 0.36 µm; the number of electrode finger pairs in the IDTs 1, 12 is 250; the overlapping width of the electrode fingers 6 is 7λ; and the width (22, 24) of the terminal-side bus bars 5, 10 is approximately 15λ. Fig. 2 shows a capacitance value when the width 21, 23 of the connection-side bus bars 7, 9 is 15λ and a capacitance value when the width is 1λ. Fig. 2 shows that the capacitance value is about 2.29 pF when the width of the connection-side bus bars 7, 9 is the same (i.e. 15λ) as that of the terminal-side bus bars 5, 10, whereas the capacitance value is about 2.23 pF when the width of the connection-side bus bars is smaller (i.e. 1λ) than that of the terminal-side bus bars. Theoretically, according to the formula (1), the capacitance decreases by about 0.06 pF when the width of the connection-side bus bars 7, 9 is reduced from 15λ to 1λ. This theoretical decrease in the capacitance is approximately the same as the observed capacitance shown above. Further, when the width of the connection-side bus bars 7, 9 is 1λ, it is calculated from the formula (1) that, although the first capacitance 117 still exists, it is as small as 0.004 pF. Therefore, as shown in Fig. 2, it is appreciated that the observed capacitance value contained in the input impedance of the filter is in the order of several picofarads, and the first parasitic capacitance fraction (0.004 pF) contained therein is smaller by more than two orders of magnitude. Thus, the fact that the first parasitic capacitance 117 is smaller (below 0.01 pF) by more than two orders of magnitude as compared with the capacitance fraction of the input impedance usually present in the filter implies that the first parasitic capacitance has been reduced to an almost negligible value. Fig. 3 is a graph showing a relationship between the width 21, 23 of the connection-side bus bars 7, 9 and the loss in the cascade connection section 8 according to the first embodiment of the present invention. Fig. 3 shows that the loss decreases by about 0.6 dB when the width 21, 23 of the connection-side bus bars 7, 9 is 1λ, as compared with the case in which the width is 15λ. Thus, it is appreciated that the loss, which is a cause of impedance mismatching, tends to decrease if the width of the connection-side bus bars 7, 9 is reduced. In other words, reduction in the electrode area of the connection-side bus bars 7, 9 decreases the loss and reduces the impedance mismatching at the cascade connection section 8. Here, it is to be noted that the construction shown in Fig. 1 is merely an example, and it is not specifically limited as long as the electrode area of the connection-side bus bars 7, 9 is sufficiently small. Namely, the width 21, 23 of the connection-side bus bars need not be reduced, and the connection-side bus bars need not have a rectangular shape. Examples The present invention is now explained by way of embodiments showing reduction of parasitic capacitance. These embodiments are given only for exemplary purposes and are not limitative of the present invention. Embodiment 1 This shows an embodiment of a surface acoustic wave filter including multimode filters of transversal coupling type connected in two-stage connection in which the width of the connection-side bus bar is sufficiently small and is smaller than the width of the terminal-side bus bar. The view showing the construction of this embodiment is similar to the one shown in Fig. 1. The piezoelectric substrate is an ST-cut quartz; the electrode period of the IDT is λ = 12.5 µm; the thickness of the electrode film is 0.36 µm; the overlapping width of the electrode fingers is 7λ; the number of electrode pairs is 250; the width of the terminal-side bus bar is 15λ; and the width of the connection-side bus bar is 1λ. The connection-side bus bar and the terminal-side bus bar each have a rectangular shape and their length along the up-and-down direction of the document sheet is the same, so that the electrode area of the connection-side bus bar is 1/15 of the electrode area of the terminal-side bus bar. Fig. 4 shows filter characteristics (insertion loss relative to frequency) when the width of the connection-side bus bar is 1λ according to the present invention. Fig. 5 shows filter characteristics (insertion loss relative to frequency) when the width of the connection-side bus bar is 15λ, as a comparison with Fig. 6. Comparing Figs. 4 and 5, it is understood that, in the pass band, the insertion loss in Fig. 4 is about 5.2 dB, whereas the insertion loss in Fig. 5 is about 5.8 dB, showing an improvement of about 0.6 dB in the insertion loss according to the construction of Embodiment 1. In other words, by allowing the electrode area of the connection-side bus bar to be about 1/15 of the electrode area of the terminal-side bus bar, the parasitic capacitance can be decreased to reduce the impedance mismatching. Although the above-described Embodiment 1 shows a construction of a multimode filter of transversal coupling type, it is also possible to reduce the parasitic capacitance and to reduce the impedance mismatching in the case of a multimode filter of longitudinal coupling type. Fig. 6 shows a construction of a multimode filter of longitudinal coupling type in which the electrode area of the connection-side bus bar is reduced (for example, to 1λ). Here, each surface acoustic wave filter is composed of two IDTs, one of the IDTs being interconnected by a metal electrode film at a connection portion. The reduction of electrode area can be achieved, for example, by allowing the connection-side bus bar to have a width of about 1λ. When each IDT is allowed to have a dimension similar to those shown in Embodiment 1, the multimode filter of longitudinal coupling type shown in Fig. 6 can also provide characteristics similar to those shown in Figs. 2 and 3, achieving reduced parasitic capacitance and reduced impedance mismatching. As shown above, the present invention provides reduced impedance mismatching, since the parasitic capacitance formed between the connection-side bus bar and the grounding point of the surface acoustic wave filter can be reduced as compared with the conventional filter construction, since the electrode area of the connection-side bus bar of IDT connected to the connection section is sufficiently small and is smaller than the electrode area of the terminal-side bus bar.

Claims: A surface acoustic wave filter of multi-stage connection type comprising: a plurality of surface acoustic wave filters (15,16) formed on a piezoelectric substrate (100) and a connection section (8) for electrically connecting the surface acoustic wave filters in a cascade connection, each of the surface acoustic wave filters comprising an interdigital transducer section (1, 12) comprising a signal terminal (4, 11) for electric input or output, an excitation section for exciting a surface acoustic wave, a terminal side bus bar (5, 10) and a connection side bus bar (7, 9) disposed on both sides of the excitation section, said bus bars (5, 10, 7, 9) serving as input or output terminal for electric signal and ground terminal, said connection side bus bar being connected to said connection section (8), wherein one of said connection side bus bars (7, 9) of one of said wave filters has an electrode area adjusted so that a parasitic capacitance formed between said connection side bus bar (7, 9) and the rear surface of the piezoelectric substrate (100) is less than or equal to 0.01 pF, or has an electrode area less than an electrode area of the terminal side bus bar (5, 10) of said wave filter. A surface acoustic wave filter of multistage connection type according to claim 1, wherein the connection side bus bar and the terminal side bus bar of the interdigital transducer of one of said filters are both rectangular and have a respective width W1 and W2 such that a ratio W1/W2 is comprised between 1 and 1/15. A surface acoustic wave filter of multi-stage connection type according to claim 1, wherein the electrode area of the connection side bus bar (7, 9) of the interdigital transducer of one of said filters is less than or equal to 1/15 of an electrode area of the terminal side bus bar (5,10) of said interdigital transducer. A surface acoustic wave filter of multi-stage connection type according to any one of the preceding claims, wherein the surface acoustic wave filter is a multimode filter of transversal coupling type. A surface acoustic wave filter of multi-stage connection type according to any of claims 1 to 3, wherein the surface acoustic wave filter is a multimode filter of longitudinal coupling type. A surface acoustic wave filter of multi-stage connection type according to any of claims 1 to 5, wherein the piezoelectric substrate is formed of quartz or LiTaO 3 or LiNbO 3 or Li 2 B 4 O 7 . A surface acoustic wave filter of multi-stage connection type according to any of claims 1 to 6, wherein said acoustic wave filters (15, 16) and said connection sections (8) are formed of an electrically conductive metal film such as aluminium (Al), or gold (Au), or copper (Cu), or titanium (Ti).

Inventors: IKATA OSAMU, SATOH YOSHIO, TSUTSUMI JUN, IKATA, OSAMU, SATOH, YOSHIO, TSUTSUMI, JUN
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1998
Main classification: H03H9
IPC codes: H03H9