JPH06244676A - Multi-stage connecting saw filter - Google Patents

Abstract

PURPOSE:To provide a surface acoustic wave(SAW) filter, for which frequency passing band characteristics and group delay characteristics are improved, so as to provide practical attenuation characteristics. CONSTITUTION:A SAW filter element 21 of lateral mode coupling is formed on the surface of a piezoelectric substrate 1 by parallelly providing two SAW resonators and a SAW filter element 22 of longitudinal mode coupling is formed parallelly to this SAW filter element 21. The other terminal of a crossing electrode 21a and one terminal of a crossing electrode 22a are electrically connected, an input/output electrode 24 and one terminal of the crossing electrode 21a as well as an input/output electrode 25 and the crossing electrode 22b are respectively electrically connected. Those electrodes are guided to the outside.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter to which surface acoustic waves propagating on a piezoelectric substrate are applied.

[0002]

2. Description of the Related Art A conventional technique will be described with reference to FIGS. FIG. 2 is a plan view of a conventional transverse mode coupled multi-stage connection surface acoustic wave filter. The piezoelectric substrate 1 is a thin plate of LiN
It is made of bO ₃ and is processed into a rectangular shape. Interdigitated electrodes 41a, 42a and reflector electrodes 41b, 41c, 42b, 42c are formed on the surface of the piezoelectric substrate 1, and two pairs of resonators are provided in which two surface acoustic wave resonators are provided in parallel. The surface acoustic wave filter elements 41 and 42 are configured. The interdigital electrodes 41a and 42a are electrically connected by a connecting electrode 43, and a multistage resonator type surface acoustic wave filter is configured as a whole. Electromagnetic shield electrodes 51, 5 are provided between the two surface acoustic wave filters 41, 42.
2 is formed up to the vicinity of the connecting electrode 43, and is provided for the purpose of mutually preventing the influence of electromagnetic waves (generally called direct waves) directly propagating between both filters or other elastic waves such as bulk waves. There is.

FIG. 3 is a plan view of a conventional multi-stage surface acoustic wave filter of longitudinal mode coupling. The piezoelectric substrate 1 is made of thin plate LiNbO ₃ and is processed into a rectangular shape. Interdigitated electrodes 61a and 61b are arranged in series on the surface of the piezoelectric substrate 1, and reflector electrodes 61c and 61d are formed on both sides of these interdigitated electrodes, which are surface acoustic wave filters of longitudinal mode coupling. The element 61 is configured, and the interdigital electrodes 62a and 62b are arranged in parallel with the element 61, and the reflector electrodes 62c and 62d are formed on both sides of the interdigital electrodes, respectively, and the longitudinal mode coupling surface acoustic wave filter element 61 is formed. And the two surface acoustic wave filter elements are electrically connected by the connecting electrode 63. Further, electromagnetic shield electrodes 51, 52 are formed between the two surface acoustic wave filters 41, 42 up to the vicinity of the connecting electrode 43 as in the conventional example, and the multi-stage connection resonator type surface acoustic wave filter is formed as a whole. It is configured.

Although not shown, these surface acoustic wave filters are housed in a package made of ceramic or the like, and finally the metallic lid and the metal layer of the package are hermetically sealed by seam welding or the like.

[0005]

The frequency pass band characteristic of the transverse mode coupled surface acoustic wave filter is generally difficult to obtain a wide pass band width as shown in FIG. Shows a bimodal characteristic as shown by A, and deteriorates the group delay characteristic. Further, as shown in FIG. 6, the longitudinal mode coupling generally has a single-peak characteristic as compared with that of the transverse mode coupling and can have a wide pass band width. However, outside the pass band, the influence of the side ropes is large. A lot of spurious that is thought to be caused was generated and deteriorated the damping characteristic.

The present invention has been made to solve the above problems, and in a multi-stage connection type surface acoustic wave filter, the frequency pass band characteristic and the group delay characteristic are excellent, and a practical attenuation characteristic can be obtained. The purpose is to obtain a surface acoustic wave filter that can be used.

[0007]

In order to solve the above-mentioned problems, a multi-stage connection type surface acoustic wave filter according to the present invention comprises:
A surface acoustic wave filter having at least two interdigital electrodes for exciting surface acoustic waves arranged in series on the surface of a piezoelectric substrate, and having reflector electrodes for reflecting the surface acoustic waves excited on both sides of these interdigital electrodes. A surface acoustic wave in which two elements are provided and two surface acoustic wave resonators having a crossing finger electrode for exciting the surface acoustic wave and reflectors for reflecting the surface acoustic wave excited on both sides of the crossing finger electrode are arranged in parallel. At least two filter elements are arranged in parallel, and the interdigital electrodes of the surface acoustic wave filter elements are electrically connected. When matching of impedance and the like is required due to the connection of the surface acoustic wave filter elements, it is preferable to provide an impedance matching circuit including a coil and a capacitor on the connecting electrode that connects the cross finger electrodes of both filter elements.

[0008]

Since at least the transverse mode coupled surface acoustic wave filter element and the longitudinal mode coupled surface acoustic wave filter element are formed on the surface of the piezoelectric substrate, excellent transverse mode coupling attenuation characteristics can be obtained by passing through the longitudinal mode coupling. It suppresses spurious that occurs outside the band, and the pass-band characteristic, which is the unimodal characteristic of the longitudinal mode coupling, reduces the occurrence of the bimodal characteristic of the transverse mode coupling, which complements the disadvantages of both. FIG. 4 is a graph showing the frequency pass band characteristic of the product of the present invention. FIG. 4 is a graph showing the frequency pass band characteristic of the surface acoustic wave filter of the transverse mode coupling shown in FIG. 5 (the same center frequency as the graph shown in FIG. 4). Frequency band characteristics of the longitudinal mode coupled surface acoustic wave filter shown in FIG.
It can be seen that the disadvantages of the same center frequency as in the graph shown in (4) complement each other.

[0009]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a plan view of a surface acoustic wave filter according to the present invention. The piezoelectric substrate 1 is made of thin plate LiNbO ₃ and is processed into a rectangular shape. On the surface of this piezoelectric substrate 1,
A transverse mode coupled surface acoustic wave filter element 21 having two surface acoustic wave resonators provided in parallel is formed. The surface acoustic wave filter element 21 is excited by the interdigital electrode 21a and the interdigital electrode. Reflector electrodes 21b and 21c are provided on both sides in the surface acoustic wave propagation direction. Also,
A longitudinal mode coupled surface acoustic wave filter element 22 is formed in parallel with the surface acoustic wave filter element 21, and the surface acoustic wave filter element 22 includes the interdigital electrode 22a,
22b are arranged in series, and reflector electrodes 22c and 22d are provided on both sides of these interdigital electrodes in the surface acoustic wave propagation direction. The other end of the crossing finger electrode 21a and one end of the crossing finger electrode 22a are electrically connected to each other by a connecting electrode 23, and the input / output electrode 24 and one end of the crossing finger electrode 21a and the input / output electrode 26 are connected.
And the cross finger electrode 22b are electrically connected to each other, and the electrodes are led to the outside. Further, electromagnetic shield electrodes 31 and 32 are formed between the surface acoustic wave filter elements, and respectively from the end of the reflector electrode 21b of the surface acoustic wave filter element 21 to the vicinity of the connecting electrode and the reflector electrode 2.
Similarly, it extends from the end 2c to the vicinity of the connecting electrode. These two surface acoustic wave filter elements constitute a multi-stage connection type surface acoustic wave filter 2 as a whole. The center frequencies of these surface acoustic wave filter elements may be designed to be the same frequency, but it may be necessary to shift the frequencies in accordance with the specifications of the required frequency characteristics of the pass band. Although not shown, the surface acoustic wave filter 2 is housed in a package made of ceramics such as alumina and having an extraction electrode to the outside, and after a necessary electrical connection is made, the metallic lid is attached to the package. The metal frame is joined to the metal frame by seam welding or the like and hermetically sealed.

When matching of impedances and the like is required in connection with each surface acoustic wave filter element, for example, as shown in FIG.
As shown in, it is preferable to provide an impedance matching circuit including a coil, a capacitor and the like on the central extraction electrode 33 extending from the connecting electrode that connects the interdigital electrodes of both filter elements. Further, in the above-mentioned embodiment, one combination of both the longitudinal mode coupling type and the transverse mode coupling type surface acoustic wave filter element is shown, but the present invention is not limited to this, and one of the two may be selected depending on the specifications such as required electrical characteristics. One and the other may be combined in one or more combinations.

[0011]

Since at least the surface acoustic wave filter element of the transverse mode coupling and the surface acoustic wave filter element of the longitudinal mode coupling are formed on the surface of the piezoelectric substrate, the excellent attenuation characteristic of the transverse mode coupling has the longitudinal mode coupling. Suppresses spurious waves that occur outside the passband of L, and the passband characteristic, which is the unimodal characteristic of the longitudinal mode coupling, reduces the occurrence of the bimodal characteristic of the transverse mode coupling. Multi-stage elastic surface with good electrical characteristics, in which spurious due to coupling between stages, which is likely to occur when connecting in multiple stages, is hard to occur, and overall good balance of frequency pass band characteristics, group delay characteristics, attenuation characteristics, etc. A wave filter can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment.

FIG. 2 is a plan view showing a conventional example.

FIG. 3 is a plan view showing a conventional example.

FIG. 4 is a graph showing frequency pass band characteristics according to the present invention.

5 is a graph showing frequency pass band characteristics of the conventional product shown in FIG.

6 is a graph showing frequency pass band characteristics of the conventional product shown in FIG.

[Explanation of symbols]

1 Piezoelectric Substrate 21, 41, 42 Transverse Mode Coupling Surface Acoustic Wave Filter Element 22, 61, 62 Longitudinal Mode Coupling Surface Acoustic Wave Filter Element 21a, 22a, 22b, 41a, 42a, 61a, 6
1b, 62a, 62b Interdigital finger electrodes 21b, 21c, 22c, 22d, 41b, 41c, 4
2b, 42c, 61c, 61d, 62c, 62d Reflector electrode 31, 32, 51, 52, 71, 72, 73, 74 Electromagnetic shield electrode

Claims (1)

[Claims] 1. A piezoelectric substrate is provided with at least two interdigital electrodes for exciting surface acoustic waves arranged in series on a surface of a piezoelectric substrate, and reflector electrodes for reflecting the excited surface acoustic waves are provided on both sides of these interdigital electrodes. In addition to providing the longitudinal mode coupling type surface acoustic wave filter element described above, a surface acoustic wave resonator having an interdigital electrode for exciting the surface acoustic wave and a reflector for reflecting the surface acoustic wave excited on both sides of the interdigital electrode is provided. At least two transverse mode coupling type surface acoustic wave filter elements arranged in parallel are selectively arranged in parallel, and the cross finger electrodes of each surface acoustic wave filter element are electrically connected. Surface acoustic wave filter.