JPH03222512A - Polarized type saw resonator filter - Google Patents

Abstract

PURPOSE:To obtain a steep cut-off characteristic without increasing the number of SAW resonators and to attain miniaturization by connecting surface acoustic wave(SAW) resonators at the input and output sides with a feedback capacitor. CONSTITUTION:SAW resonators 1-3 each comprising an interdigital transducer(IDT) 6 and SAW reflectors 7, 8 are formed in parallel on a piezoelectric substrate 5 and a capacitor 4 is formed thereon. Then an input terminal 9 connecting to the IDT 6 of the SAW resonator 1 and one terminal of the capacitor 4 are connected and the other terminal of the capacitor 4 is connected to an output terminal 10 connecting to the IDT 6 of the SAW resonator 3. Since the input side SAW resonator and the output side SAW resonator are interconnected via the capacitor in such a way, part of an output electric signal is fed back to the input SAW resonator. Thus, an attenuation pole frequency is formed to a higher frequency with respect to the center frequency of the band pass filter to obtain a steep cut-off characteristic and miniaturization is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to surface acoustic waves
The present invention relates to a polarized SAW resonator filter, which is a type of SAW resonator filter using c Wave+ (hereinafter referred to as SAW).

(Prior Art) In recent years, there has been a strong demand for smaller UHF/vHF band radios and smaller filters, and in particular, SAW resonator filters that are stable in narrow bands and have low insertion loss are attracting attention. This SAW resonator filter realizes a narrow band low loss filter by arranging two SAW resonators in parallel and utilizing acoustic coupling between them.

Figure 2 shows the SA used in the above SAW resonator filter.
It is an explanatory diagram of a W resonator, and fa) is an electrode configuration.

(bl, (cl indicates the displacement distribution (γ). Second
In fig.

The SAW excited by the interdigital transducer (hereinafter referred to as IDT) 77 generates a standing wave that is reflected by reflectors 12 and 13 provided close to both sides of the SAW, and the energy is transferred to the reflector 12. .13. At this time.

A cavity is constructed in the propagation direction (vertical direction) of the SAW, and the first order shown in (D, +iD, (il) in FIG.
Resonant modes with second-order and third-order displacement distributions (V') are excited.

In addition, in the direction perpendicular to the SAW propagation direction (lateral direction), in region A where the electrode fingers of IDT 17 intersect, the 5AWO propagation is lower than in region B on both sides of the 5AWO propagation due to reflection and perturbation by the electrode fingers. A wave path is constructed.

FIG. 3 is an explanatory diagram of a dual mode SAW resonator filter that applies the operation of such a SAW resonator, (,)
shows the electrode configuration, and (b) shows its displacement distribution. This dual-mode SAW resonator filter has two identical SAW resonators placed close to each other in parallel, and if the spacing between the two SAW resonators is sufficiently narrow, the acoustic Coupling occurs and two resonance modes as shown in FIG. 3(b) are excited. One has a displacement distribution that is symmetrical with respect to the centers of the two SAW resonators, and the other has a displacement distribution that is point symmetrical with respect to the center, with the former being a symmetric mode and the latter being antisymmetric. It's called a mode.

The area It, which is analyzed using the surface acoustic wave waveguide model consisting of five areas (area 1-V) shown in FIG. 3(a),
? / is a low speed region, and regions i, m, and v are high speed regions. Further, region (2) is an acoustic coupling region between two SAW resonators, and its width is defined as fG. In Figure 3, S
The AW propagation direction is the X direction, and the directions perpendicular to this are the two directions, and the piezoelectric substrate (
(not shown) is a uniform isotropic material, then SA
The propagation of W is a scalar potential W, (low velocity region)
, W.

(high speed region) is expressed by equations (1) and (2).

Here, Vs and V are the speeds of the SAW in the low speed region and high speed region, respectively, and ω is the resonance angular frequency.

Assuming that the boundary conditions are that the displacement and stress are continuous at each boundary and that no force is applied at the outermost edge, and that the width B of the outer high-speed region is sufficiently large, the above equation (
Equations (3) and (4) are obtained by solving 1) and (2).

Here, ■ is the propagation speed of the SAW in each mode. Equation (3) corresponds to the case where the displacements in regions ■ and
If the displacement in (2) is in the opposite direction, that is, it corresponds to antisymmetric Moge.

The equivalent circuit of such a dual mode SAW resonator filter can be represented in FIG. Here, f, L,
C and R are the resonant frequencies of the symmetric modes, respectively.

sss is the equivalent inductance, equivalent capacitance 2 equivalent resistance, fa, La, Ca, Ra are the resonant frequency 2 equivalent inductance of antisymmetric moat 9, equivalent capacitance 9 equivalent resistance, and Laki Ls, Rs KiR
It's Yu. Further, co is the parallel capacitance of each SAW resonator. According to circuit theory, this equivalent circuit is further divided into the fifth
Therefore, the dual-mode SAW resonator filter shown in FIG.
It can be seen that this is an AW resonator fill.

(Problem to be Solved by the Invention) However, with the non-polar SAW resonator filter having the above configuration, the attenuation characteristic for the center frequency of the pan Y-pass filter can only realize a characteristic that increases monotonically with respect to changes in frequency. In order to achieve steep damping characteristics, the number of momentum% SAW resonators is increased and the dual mode SAW
This requires a configuration in which resonator filters are connected in multiple stages, which has the disadvantage of increasing the size.

The present invention has been made to eliminate the above-mentioned drawbacks, and by connecting the SAW resonators on the input and output sides with a feedback capacitor, a steep cutoff characteristic can be achieved without increasing the number of SAW resonators. It is an object of the present invention to provide a small-sized polarized SAW resonator filter that can realize the following.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a plurality of Ss including interdigital electrodes and reflectors on the same piezoelectric substrate.
In a SAW resonator filter in which AW resonators are formed adjacently in parallel, a capacitor is formed on the piezoelectric substrate, and the input side SAW resonator and the output side SAW resonator are electrically connected by the capacitor. be.

(Function) In a SAW resonator filter that employs multiple SAW resonators adjacent to each other in parallel, each SAW resonator realizes a SAW standing wave between its IDT and reflectors on both sides, and each SAW resonator Since the imager generates acoustic coupling with the adjacent SAW co-imager, it operates as a non-polar SAW resonator filter. In the present invention, since the input side SAW resonator and the output side resonator are connected by a capacitor, a part of the electric signal on the output side is fed back to the input side SAW resonator, so that the bandpass filter is An attenuation pole frequency is realized on the higher side with respect to the center frequency, and a steep cutoff characteristic is obtained.

(Example) FIG. 1 is a schematic diagram of an embodiment of the present invention.

- Three SAW resonators Z, 2, and J each consisting of an IDT 6 and SAW reflectors 7 and 8 provided close to both sides of the IDT 6 are formed in parallel on the piezoelectric substrate 5, and the
A capacitor 4 made of IDT is formed below the AW resonator 3,
The input terminal 9 connected to the IDT 6 of the SAW resonator 1 is connected to one end of the capacitor 4, and the IDT of the SAW resonator 3 is
The other end of the capacitor 4 is connected to the output terminal 10 connected to the capacitor 6. Note that the SAW resonators 1 and 2
,3. The capacitor 4 and the like can be formed using a semiconductor process by depositing metal such as aluminum or gold on the piezoelectric substrate 5 by a vapor deposition method, a suction method, or the like.

The operating principle of this SAW resonator filter is similar to the dual mode SAW resonator filter shown in FIG.

That is, in FIG. 1, the electrical signal input to the input terminal 9 is applied to the IDT 6 of the SAW resonator 1 and the SA
W, a standing wave is realized between the IDT 6 and the reflectors 7゜8 on both sides of it, and the SAW energy is converted to the reflector 7゜8.
, trapped between 8. This SAW resonator 1 causes acoustic coupling with a second SAW resonator 2 arranged close to each other in parallel, and furthermore, this SAW resonator 2 causes an acoustic coupling with a third SAW resonator 3 arranged close to each other in parallel. Acoustic coupling occurs between the two. Therefore, the SAW energy reaches the SAW resonator 3, is converted into an electrical signal, and is output from the output terminal 10 to the outside. Further, a part of the electrical signal is fed back to the input side via the capacitor 4. This capacitance 4 is realized by forming IDT electrode fingers on the piezoelectric substrate 5 as described above, and its capacitance CT is expressed by equation (5).

CT=K(ε,+1) (5) The equivalent circuit of the polarized SAW resonator filter having the above configuration can be expressed in the same way as the equivalent circuit of the SAW resonator filter of FIG. 3 can be expressed by FIG. hand.

This can be represented by FIG. This equivalent circuit is similar to the polarized equivalent circuit of a conventional crystal monolithic filter, and an attenuation pole frequency is realized at a frequency higher than the center frequency of the bandpass filter. Therefore, steep cutoff characteristics can be obtained.

(Effects of the Invention) As described above in detail, according to the present invention, a capacitor is formed on the same piezoelectric substrate as the SAW resonator filter, and the input side SAW resonator and the output side SAW resonator are connected by the capacitor. Since they are electrically connected, an attenuation pole frequency is realized on the higher side of the center frequency of the fill.

A polarized SAW resonator filter that is small and has steep cutoff characteristics can be obtained.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, and FIG. 2 is a SAW
An explanatory diagram of the resonator, Figure 3 is a conventional dual mode IF SAW
An explanatory diagram of a resonator filter, Fig. 4 is an equivalent circuit diagram of the double mode SAW resonator filter shown in Fig. 3, and Fig. 5 is an equivalent conversion circuit diagram obtained by converting the equivalent circuit shown in Fig. 4. , FIG. 6 is an equivalent circuit diagram of the polarized SAW resonator filter of FIG. 1. 1.2.3...SAW resonator, 4...capacitance, 5...
・Piezoelectric substrate, 6...IDT, 7, 8...Reflector, 9
...Input terminal, lO...Output terminal. Figure 6 (b) SAW Chi (Saimata Aoi's Mochi εθFl Rero Diagram

Claims (1)

[Claims] In a SAW resonator filter in which a plurality of SAW resonators each including an interdigital electrode and a reflector are formed adjacently in parallel on the same piezoelectric substrate, a capacitance is formed on the piezoelectric substrate, and the input side SAW is Resonator and output side SA
A polarized SAW resonator filter, characterized in that it is electrically connected to a W resonator.