JPH0454011A - Longitudinal dual mode surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To form a small sized filter with a low loss by setting a resonance frequency of a surface acoustic wave resonator for suppression formed between sections of energy confinement type resonators connected in cascade in multi- stage to be a frequency requiring suppression. CONSTITUTION:A suppression use surface acoustic wave (SAW) resonator 30 is provided in parallel electrically with a connection part between an output interdigital electrode 23 of a 1st section and an input interdigital electrode 26 of a 2nd section. An interdigital electrode 31 provided to the SAW resonator 30 is interposed between a connection part and ground between sections and reflectors 32, 33 are arranged to both sides. An electrode finger pitch of the interdigital electrode 31 and inter-grating pitch of the reflectors 32, 33 are set so that a resonance frequency fr is coincident with a frequency fH requiring suppression. The impedance is minimized at the frequency fr and a sufficient suppression is obtained at the frequency fH.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a band-pass type, low-loss surface acoustic wave filter that utilizes vertical dual mode coupling.

[Prior Art] FIG. 5 shows a conventional vertical dual-mode SAW filter 1 constructed of an energy-trapped resonator that utilizes surface acoustic waves (SAW). This SAW filter 1 has two sets of comb-shaped electrodes 3 and 4 for excitation of surface waves on the surface of a piezoelectric substrate 2, and reflectors 5 and 6 are arranged on both sides thereof.

In such a configuration, as shown in FIG. 6, the energy of the surface wave excited between the reflectors 5 and 6 is confined, and zero-order and first-order longitudinal resonance modes appear. The frequency difference between these two modes changes depending on the absolute number of comb-shaped electrodes 3 and 4, and by matching the input and output impedance, a vertical double A mode SAW filter 1 is realized.

Furthermore, even higher-order resonance modes actually exist, for example, as shown in FIG. 7, in addition to two sets of comb-shaped electrodes 3 and 4 between reflectors 5 and 6 In some cases, another set of comb-shaped electrodes 7 is provided. In this case, it can be easily inferred that zero-order longitudinal resonance modes and second-order longitudinal resonance modes appear.

Note that in the configurations shown in FIGS. 5 to 7, the amount of suppression outside the passband is generally
and 7) are often insufficient due to their frequency characteristics, and usually two or more sections of the same are cascaded in multiple stages, with one section being a SAW filter with the above-mentioned configuration, that is, an energy-confined resonator. The configuration shown in FIG. 8 is adopted.

In the vertical dual mode SAW filter 1a shown in FIG. Two sets of comb-shaped electrodes 8 and 9 and reflectors 10 and 11 are provided. These two sections of energy confinement type resonators are connected in series by connecting the comb-shaped electrodes 4 and 8.

[Problems to be Solved by the Invention] However, when a configuration in which two or more sections of energy trapping resonators are cascaded in multiple stages, such as the vertical dual mode SAW filter 1a shown in FIG. Since the insertion loss within the band increases and the total area of the electrodes required also increases, the number of stages that can be connected in cascade is often limited, and as a result, it is not possible to secure a sufficient amount of suppression outside the passband. There is a problem.

FIG. 9 is a frequency characteristic diagram of a conventional vertical dual mode SAW filter. As shown in FIG. 9, the center frequency f in the passband. For example, when the frequency of an intermediate frequency stage is set at the frequency difference position fH determined from fH, it is desirable that the amount of suppression at fH is large. The amount of suppression was insufficient because it was not sufficiently attenuated.

Therefore, an object of the present invention is to provide a vertical dual mode surface acoustic wave filter that can obtain a sufficient amount of suppression outside the passband.

[Means for Solving the Problems] The present invention provides a piezoelectric substrate with at least two sets of comb-shaped electrodes arranged close to each other, and reflectors arranged on both sides of the comb-shaped electrodes. In order to solve the above-mentioned technical problem, the present invention is directed to a vertical dual-mode surface acoustic wave filter in which energy-trapped resonators are configured and these energy-trapped resonators are connected in cascade in multiple stages, and to solve the above-mentioned technical problems, A position on the piezoelectric substrate that has little influence on the propagation of surface waves in the filter, and is electrically parallel to the comb-shaped electrode at the connection portion between each section of the energy-trapped resonator. , is characterized by the connection of a suppression surface acoustic wave resonator.

[Operation] According to the present invention, the resonant frequency of the suppression surface acoustic wave resonator configured between each section of the energy confinement type resonators connected in multi-stage cascade is set to the frequency at which suppression is required. As a result, a trap type filter is constructed at that frequency, resulting in sufficient suppression.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain a sufficient amount of suppression without relying on multi-stage cascade connections, and therefore it is possible to realize a small surface acoustic wave filter with low loss. .

Moreover, since the passband characteristics can be controlled by the suppression surface acoustic wave resonator, it is also extremely effective in improving the performance of the surface acoustic wave filter.

[Embodiment] FIG. 1 is a configuration diagram of a vertical dual mode SAW filter 20 according to a first embodiment of the present invention.

Referring to FIG. 1, the SAW filter 20 includes a piezoelectric substrate 2
1. On this piezoelectric substrate 21, a two-section energy trapping resonator is constructed.

The energy confinement type resonator of the first section includes two sets of comb-shaped electrodes 22 and 23 arranged in close proximity, and reflectors 24 and 25 arranged on both sides of these (comb-shaped electrodes 22 and 23).

The energy confinement resonator of the second section comprises two sets of comb-shaped electrodes 26 and 27 arranged in close proximity and reflectors 28 and 29 arranged on either side of these comb-shaped electrodes 26 and 27.

By connecting the comb-shaped electrode 23 in the first section and the comb-shaped electrode 26 in the second section, the energy trapping resonators in the first and second sections are connected in cascade.

The feature of this embodiment is that a suppression SAW resonator 30 is electrically connected in parallel to the connection between the output side comb-shaped electrode 23 in the first section and the input side comb-shaped electrode 26 in the second section. This is what we have in place. S.A.
The W resonator 30 includes a set of comb-shaped electrodes 31 and reflectors 32 and 33 placed on both sides thereof.

The SAW resonator 30 is placed on the piezoelectric substrate 21 at a position where it does not interfere with the surface waves excited by the comb-shaped electrodes 22, 23, 26, 27.

The SAW filter 20 shown in FIG. 1 utilizes longitudinal zero-order and longitudinal first-order resonance modes, similar to the one shown in FIG. 6.

FIG. 2 is a configuration diagram of a vertical dual mode SAW filter 20a according to a second embodiment of the present invention.

Referring to FIG. 2, the SAW filter 20a includes a piezoelectric substrate 21a, similar to the SAW filter 20 described above. On this piezoelectric substrate 21a, two cascade-connected energy trapping resonators are constructed.

In order to clarify the comparison with the SAW filter 20 shown in FIG. 1, the reference numbers used in FIG. 1 are attached to corresponding elements. Between the vessel 24 and the reflector 25, in addition to the comb-shaped electrodes 22 and 23, another set of comb-shaped electrodes 34 is arranged. In the energy confinement type resonator of the second section, in addition to the square-shaped electrodes 26 and 27, another set of comb-shaped electrodes 35 is arranged between the reflector 28 and the reflector 29.

In this way, since the first and second energy confinement type resonators each have a comb-shaped electrode divided into three parts, the vertical zero-order and vertical second-order resonators are similar to those shown in FIG. It uses the resonance mode of

Furthermore, in order to cascade-connect the energy trapping resonators of the first and second sections, the comb-shaped electrodes 23 and 34 in the first section and the comb-shaped electrodes 26 and 35 in the second section are connected.

Also in this second embodiment, as in the first embodiment, the output side comb-shaped electrodes 23 and 34 of the first section are
A SAW resonator 30 for suppression is provided electrically in parallel at an electrode section extending from the coupling section of the second section to the coupling section of the input side comb-shaped electrodes 26 and 35. The SAW resonator 30 includes a rectangular electrode 31 and reflectors 32 and 33 placed on both sides thereof.

In both the first and second embodiments described above,
Reflectors 24, 25, 28, 29 and associated comb electrodes 22
．． 23. 26. 27. 34. 35, as well as the comb-shaped electrode 22. 23°26.27,3
4.35 The spacing between the comb-shaped electrodes 22.
23. 26. 27. The Q of the resonator can be improved by setting the electrode finger pitch to an integral multiple of 34°35 and making the electrode finger pitch about 2% smaller than the interlattice pitch of the reflectors 24, 25, 28, and 29. Ru. It is well known that it is desirable to titrate such conditions in designing a vertical dual mode SAW filter.

Below, in particular, regarding the first embodiment shown in FIG.
The effect will be explained.

In the first embodiment shown in FIG. 1, as described above, a suppression SAW resonator 3 is installed at the connection between each section.
0 is set. A comb-shaped electrode 31 included in the SAW resonator 30 is interposed between the connecting portion between each section and the ground, and reflectors 3 are provided on both sides of the comb-shaped electrode 31.
2 and 33 are placed. Here, the electrode finger pitch of the square-shaped electrode 31 and the interlattice pitch of the reflectors 32 and 33 are set so that the resonance frequency matches the frequency that requires suppression. The impedance-frequency characteristics obtained in this manner are shown in FIG. In FIG. 3, f is the resonant frequency, and at this frequency f1 the impedance is minimum.

When the SAW resonator 30 having such an effect is provided at the connection between sections as described above, the frequency f
, a trap type filter is constructed, and the frequency characteristics of the entire SAW filter 20 are as shown in FIG.

As can be seen from FIG. 4, if the resonant frequency f of the SAW resonator 30 is set in advance to match the frequency f□ where suppression is required, a sufficient amount of suppression can be achieved at the frequency f□. You will get it.

The operation has been explained above in relation to the first embodiment shown in FIG. 1, but as in the second embodiment shown in FIG. The same thing can be said even when electrodes are provided. Furthermore, the same holds true even when the number of divisions of the comb-shaped electrode increases.

Further, the same thing can be said in the case of a SAW filter including an energy-trapped resonator in which three or more sections are cascaded in multiple stages.

In each of the above-described embodiments, the suppression SAW resonator 30 serving as a trap filter is configured by the comb-shaped electrode 31 and the reflectors 32 and 33. There are no so-called IDT multi-pair resonators, and two such resonators are used as one section.
The suppression SAW resonator may be configured by resonators having a configuration in which sections or more are connected in cascade in multiple stages.

Also, if suppression is required at two or more frequencies,
SAW resonators having respective frequencies as resonant frequencies may be connected in parallel.

Furthermore, in the first and second embodiments, <C-shaped electrode 2
3 and 26 and the ground of the comb-shaped electrode 31,
They may be connected on the same piezoelectric substrate 21 (or 21a).

Furthermore, when a piezoelectric substrate such as LiTaO3 having a large electromechanical coupling coefficient is used, spurious components sometimes appear in the high pass band. By setting the formed capacitance to an appropriate value, the position of the spurious can be freely controlled, so that the passband characteristics can be easily adjusted.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a vertical dual mode SAW filter 20 according to a first embodiment of the present invention. FIG. 2 is a block diagram showing a vertical dual mode SAW filter 20a according to a second embodiment of the present invention. FIG. 3 shows the SAW resonator 3 used in the first embodiment.
It is a figure which shows the impedance-frequency characteristic of 0. FIG. 4 is a diagram showing the frequency characteristics of the vertical dual mode SAW filter 20 according to the first embodiment. FIG. 5 is a block diagram showing a conventional vertical dual mode SAW filter 1. As shown in FIG. FIG. 6 is a conceptual diagram showing two mode distributions of the SAW filter 1 shown in FIG. FIG. 7 is a conceptual diagram showing two mode distributions in a modified example of the SAW filter 1 shown in FIG. FIG. 8 is a configuration diagram showing a vertical dual mode SAW filter 1a as another conventional example. FIG. 9 is a diagram showing the frequency characteristics of the SAW filter 1a shown in FIG. 8. In the figure, 20, 20a are vertical dual mode SAW filters, 21.21a is a piezoelectric substrate, 22°23.26.2
7, 31.34.35 is a comb-shaped electrode, 24.25.28
．． 29, 32, and 33 are reflectors, and 30 is a SAW resonator for suppression. Written by j11shiLE, Niko2

Claims (1)

[Claims] At least two sets of comb-shaped electrodes are arranged in close proximity to each other on a piezoelectric substrate, and reflectors are arranged on both sides of the comb-shaped electrodes, thereby forming a multi-section energy-confined resonator. In a vertical dual-mode surface acoustic wave filter in which type resonators are connected in cascade in multiple stages, a position on the piezoelectric substrate that has little influence on the propagation of surface waves in the filter, and is located at a position of the energy-trapped resonator. A surface acoustic wave resonator for suppression is connected to the connection portion between each section so as to be electrically parallel to the comb-shaped electrode.
Vertical dual mode surface acoustic wave filter.