JPH06232687A - Surface acoustic wave resonator filter - Google Patents

Abstract

PURPOSE:To attain broad band processing with a simple configuration by providing a 2-dimension mode coupling structure in which longitudinal mode resonance in a longitudinal direction being a propagation direction of a surface acoustic wave and lateral mode resonance in the lateral direction perpendicular to the longitudinal direction are coupled. CONSTITUTION:Two longitudinal mode coupling resonators A, B are arranged side by side onto a piezoelectric substrate 20 so as to couple the lateral mode acoustically. An input IDT electrode 1A and a dummy electrode 2A are arranged in the longitudinal direction and reflectors 3A, 4A are provided to both sides and the IDT electrode 1A, the dummy electrode 2A, and the reflectors 3A, 4A form a 1st longitudinal mode resonator A. Moreover, an output IDT electrode 1B and a dummy electrode 2B are arranged in the longitudinal direction and reflectors 3B, 4B are provided to both sides and the IDT electrode 1B, the dummy electrode 2B, and the reflectors 3B, 4B form a 2nd longitudinal mode resonator B. Then the two pole longitudinal mode resonators A, B are coupled acoustically or electrically to synthesize the characteristics of the two resonators A, B on a frequency axis.

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 resonator filter, and more particularly to a surface acoustic wave resonator type bandpass filter having a wide band characteristic utilizing longitudinal mode resonance of a surface acoustic wave.

[0002]

2. Description of the Related Art An equivalent circuit of a conventional two-pole mode SAW resonator is as shown in FIG. 7A, which has two series resonance circuits 11 and 12 having resonance frequencies f11 and f12.
It is known that the phases of the two resonance frequencies f11 and f12 are opposite to each other. This reverse phase relationship is equivalently shown by the transformer 13.

When impedance matching is performed on the two-pole mode SAW resonator under a certain condition, a filter characteristic which becomes a pass band between the resonance frequencies f11 and f12 is shown in FIG.
(B) is obtained, and the center frequency of its pass band is f1. A monolithic crystal filter is well known as a filter that uses this principle. With a filter configuration, the insertion loss at the resonance frequencies f11 and f12 is 3 to 3 from the minimum insertion loss in the pass band (generally 0 dB).
It substantially coincides with the 3 dB point that is attenuated by dB.

As a conventional method for realizing the two-pole mode resonator, there are a longitudinal mode resonance type which produces two resonance modes in the propagation direction of a surface acoustic wave and a resonance mode which is transverse to the propagation direction. There are two types of transverse mode resonance type.

Regarding the two-pole mode resonator,
1984 IEEE 38th Annual Fre
quality Control Symposium
At "NARROW BANDPASS FILT
ER USING DOUBLE-MODE SAW
RESONATORS ON QUARTZ "(MT
anaka, et al. ) Paper and “Narrow Bandpass Double M” at the 15th EM Symposium of the Institute of Electrical Communication in 1986.
ode SAW Filter "(M. Tanaka,
et al. ) Is disclosed in detail in the paper.

FIGS. 8A and 8B show a concrete structure of this two-pole mode resonator. FIG. 8A shows two resonators a in the lateral direction perpendicular to the propagation direction of the surface acoustic wave.
It is a transverse mode coupling type filter in which and b are arranged close to each other. The resonator a has an IDT (interdigital) electrode 1a in the center, and reflectors 3a,
4a. Similarly, the resonator b has the IDT electrode 1b at the center.
And reflectors 3b and 4b on both sides of the IDT electrode 1b.
Have. The IDT electrode 1a is the input electrode, the IDT
The electrode 1b serves as an output electrode.

In this transverse mode coupling type filter, two modes (even symmetry mode indicated by f1 and odd symmetry mode indicated by f2) generated between the input side IDT electrode 1a and the output side IDT electrode 1b are coupled to each other. The band pass filter characteristic as shown in FIG. 7 (B) is obtained.

FIG. 8B shows a longitudinal mode coupling type filter in which two IDT electrodes 1 and 2 are arranged close to each other in the longitudinal direction which is the propagation direction of a surface acoustic wave. Reflectors 3 and 4 are arranged on both sides of the IDT electrodes 1 and 2, respectively.

Also in this longitudinal mode coupling type filter, the bandpass filter characteristic shown in FIG. 7B is obtained by the two mode couplings generated between the input side IDT electrode 1 and the output side IDT electrode 2.

[0010]

In the conventional surface acoustic wave filter of this type, it is necessary to widen the interval between the resonance frequencies in the two modes to be used in order to widen the band of the filter characteristics. Has restrictions on design parameters, which causes various problems such as increase in use impedance and deterioration of attenuation outside the band. Therefore, there is a drawback that there is a limit to widening the band.

An object of the present invention is to provide a surface acoustic wave resonator filter capable of widening the band with a simple structure.

[0012]

A surface acoustic wave resonator filter according to the present invention is a surface acoustic wave resonator filter including an interdigital electrode on a piezoelectric substrate, the surface acoustic wave resonator being arranged in a propagation direction of the surface acoustic wave. It is characterized by having a two-dimensional mode coupling type structure in which a longitudinal mode resonance in a certain longitudinal direction and a transverse mode resonance in a lateral direction perpendicular thereto are coupled.

In another surface acoustic wave resonator filter according to the present invention, the two-dimensional mode-coupling structure is formed on the piezoelectric substrate so that the first and second longitudinal mode-coupling resonators are transversely mode-coupled to each other. It is characterized in that the structures are arranged adjacent to each other.

Still another surface acoustic wave resonator filter according to the present invention is a surface acoustic wave resonator filter including interdigital electrodes on a voltage substrate, the longitudinal direction being a surface acoustic wave propagation direction. Characterized in that it has first and second longitudinal mode coupled resonators utilizing the longitudinal mode resonance, and these first and second longitudinal mode coupled resonators are connected in parallel between input and output. And

[0015]

The present invention will be described in detail below.

First, a two-pole mode SAW resonator A having resonance frequencies f11 and f12 and two resonance frequencies f21 and f22.
Consider the case where there is a pole mode SAW resonator B. When each resonator is impedance-matched and used as a filter, the filter characteristics are as shown in FIGS. 1 (A) and 1 (B), assuming that the center frequencies of the passbands are f1 and f2, respectively.

To combine the characteristics A and B of these two filters into one filter characteristic shown in FIG.
At the point where the two filter characteristics A and B intersect, the output signal power of each filter is 1/2 of the output signal power of each pass band.
And the two signals must be in phase.
The equivalent circuit of the resonators A and B satisfying this requirement is shown in FIG.
It becomes like (A). The equivalent circuit of the resonator A is a parallel connection configuration 14 of series resonance circuits having resonance frequencies f11 and f12, and that of the resonator B is a parallel connection configuration 15 of series resonance circuits having resonance frequencies f21 and f22. .

The resonance frequency f12 of the high frequency side of the resonator A is
And the output signal at the low-side resonance frequency f21 of the resonator B vectorally match (power and phase),
The relationship is shown by the transformer 16. The combined resonance characteristic at this time is as shown in FIG. 2 (B), and the impedance-matched filter characteristic is as wide as shown in FIG. 1 (A).

The structure of an embodiment of the present invention satisfying the above characteristics is shown in FIG. With reference to FIG. 3A, in this embodiment, two sets of the longitudinal mode coupling type resonators shown in FIG. 8B are arranged laterally adjacent to each other, and further, between these two resonators. It is acoustically coupled in transverse mode. That is, the SAW filter according to the present invention has a two-dimensional mode-coupling structure in which the longitudinal longitudinal mode resonance, which is the propagation direction of the surface acoustic wave, and the transverse transverse mode resonance perpendicular thereto are coupled.

On the piezoelectric substrate 20, two longitudinal mode coupling type resonators A and B are arranged close to each other so as to allow acoustic transverse mode coupling. The input IDT electrode 1A and the dummy electrode 2A are arranged in the vertical direction, and reflectors 3A and 4A are provided on both sides of the input IDT electrode 1A, the dummy electrode 2A, and the reflectors 3A and 4A, respectively. It constitutes the resonator A.

The output IDT electrode 1B and the dummy electrode 2B are vertically arranged, and reflectors 3B and 4B are provided on both sides of the output IDT electrode 1B and the dummy electrode 2B. Longitudinal mode resonator B
Are configured.

The dummy electrodes 2A and 2B correspond to one IDT electrode of the longitudinal mode resonator shown in FIG. 8B and have a function acoustically equivalent to that of the IDT electrode. The dummy electrodes 2A and 2B will be described in more detail. In the structure of FIG. 3A, the transverse mode coupling is the same as the transverse mode coupling between two adjacent IDT waveguides, like the conventional transverse mode resonator of FIG. 8A.

On the other hand, the longitudinal mode coupling is shown in FIG.
Similar to the longitudinal mode resonator, the resonance mode due to the multiple reflection between the reflectors 3 and 4, the resonance mode due to the multiple reflection between the IDT electrodes, and the resonance mode inside the IDT electrode are used.

Dummy electrode 2 in the structure of FIG.
A and 2B have a structure equivalent to the IDT electrode except for the structure in which the IDT electrode is electrically short-circuited. Therefore, when the two resonators A and B are laterally adjacent to each other, the dummy electrodes 2A and 2B are present, so that the two resonators A and B are adjacent to each other.
Is equivalent to a uniform grating in-waveguide and therefore the desired transverse mode coupling is obtained.

In terms of vertical coupling, the dummy electrodes 2A,
2B needs to be acoustically equivalent to the IDT electrode and is a longitudinal mode resonator due to the presence of the dummy electrode,
FIG. 8B shows the multiple reflection resonance between the reflectors, the multiple reflection resonance between the IDT electrodes, and the resonance of the IDT electrodes themselves.
It will be generated and used in the same manner as the conventional longitudinal mode resonator of.

The electrodes are arranged so that two longitudinal mode resonances occur in the vertical direction, but in the horizontal direction, there are two modes, an even symmetric mode (f1) and an odd symmetric mode (f2). Since SAW propagation occurs, two transverse mode characteristics are obtained in even and odd symmetric modes, respectively.

This is shown by an equivalent circuit in FIG. 2 (A).
As a result, the resonance circuit 14 exhibits an even symmetric mode, and the resonance circuit 15 exhibits an odd symmetric mode. Each resonance circuit 1
Reference numerals 4 and 15 respectively have two series resonance circuits showing two longitudinal modes.

With this structure, the high-frequency side longitudinal mode resonance f12 of the even symmetric mode and the low-frequency side longitudinal mode resonance f21 of the odd-symmetric mode.
It is necessary to match and as shown in FIG. For that purpose, it is possible to appropriately select the number N of pairs of the IDT electrodes, the vertical interval between the IDT electrodes, and the vertical interval between the IDT electrode and the reflector.

By doing so, the combined resonance characteristic shown in FIG. 2B can be obtained. FIG. 4 (A),
The actual measurement result is shown in FIG. 4B, and FIG.
It is an enlarged view of the vicinity of 50 MHz. Figure 3
When the filter of (A) is used with impedance matching, flat characteristics in the pass band can be obtained as shown in FIGS. 5 (A) and 5 (B). FIG. 5B is an enlarged view of the vicinity of 250 MHz.

This SAW filter uses an ST-cut quartz substrate, and conventionally has a relative band (3 dB band / center frequency).
Was about 0.1%, but in this experimental example,
A wide band of 5% is possible. Further, FIG. 2 (A)
As can be seen from the equivalent circuit of, since the even symmetric mode and the odd symmetric mode are phase-inverted (by the transformer 16),
Outside the band, the signals in each mode cancel each other, and the attenuation rate improves.

FIG. 3B is a diagram showing the configuration of another embodiment of the present invention. In this example, two longitudinal mode coupling resonators A and B are connected in parallel with each other between the input and output, and the transverse mode can be considered to be electrically coupled.

The resonator A is a pair of IDT electrodes 1A and 2A.
And reflectors 3A and 4A arranged on both sides thereof. The resonator B is composed of a pair of IDT electrodes 1B and 2B and reflectors 3B and 4B arranged on both sides thereof. Then, the IDT electrodes 1A and 2B are electrically connected, and I
The DT electrodes 2A and 1B are electrically connected, and as a result, two resonators A and B are provided between the input IN and the output OUT.
Are connected in parallel with each other.

Even with this configuration, the equivalent circuit is the same as that shown in FIG. Here, it is necessary for the signals of the two resonators to have an opposite phase relationship, and therefore, in FIG. There is. That is, the IDT electrodes 1A and 2 of the resonator A are
In A, the upper interdigitated electrode is grounded and I of the resonator B is
The lower interdigitated electrodes of the DT electrodes 1B and 2B are grounded.

As another method, the ID of the resonator A is
The IDT of the resonator B with respect to the distance between the T electrodes 1A and 2A
By shifting the distance between the electrodes 2A and 2B by n / 2 wavelengths (n is an odd number), the signals of the two resonators A and B can also have an antiphase relationship.

FIGS. 6 (A) and 6 (B) show actual characteristic results of the filter having the structure of FIG. 3 (B).
(A) is an enlarged view around 250 MHz.

[0036]

As described above, according to the present invention, the two-pole mode SAW resonators are acoustically or electrically coupled and the characteristics of the two resonators are combined on the frequency axis. There is an effect that the filter characteristics can be broadened.

[Brief description of drawings]

FIG. 1A and FIG. 1B are diagrams showing characteristic examples of a filter according to the present invention.

FIG. 2A is an equivalent circuit diagram of a filter according to the present invention,
(B) is a filter characteristic diagram thereof.

3A is a configuration diagram of an embodiment of the present invention, and FIG. 3B is a configuration diagram of another embodiment of the present invention.

4 (A) and 4 (B) are diagrams showing the actual measurement results of the filter characteristics of one embodiment of the present invention, and FIG. 4 (B) is a partially enlarged view of (A).

5A and 5B show actual measurement results of characteristics when the filter of one embodiment of the present invention is used with impedance matching, and FIG. 5B is a partially enlarged view of FIG. .

6A and 6B show actual measurement results of characteristics when a filter according to another embodiment of the present invention is used with impedance matching, and FIG. 6A is a partially enlarged view of FIG. is there.

FIG. 7A is an equivalent circuit diagram of a conventional SAW resonator,
(B) is a characteristic diagram thereof.

FIG. 8A is a diagram showing a configuration example of a conventional transverse mode coupling filter, and FIG. 8B is a diagram showing a configuration example of a conventional longitudinal mode coupling filter.

[Explanation of symbols]

 1A, 1B IDT electrode 2A, 2B IDT electrode (dummy electrode) 3A, 3B, 4A, 4B reflector 20 Piezoelectric substrate

Claims (5)

[Claims] 1. A surface acoustic wave resonator filter including interdigital electrodes on a piezoelectric substrate, wherein a longitudinal mode resonance in a longitudinal direction which is a propagation direction of a surface acoustic wave and a lateral transverse direction perpendicular thereto. A surface acoustic wave resonator filter having a two-dimensional mode-coupling structure in which mode resonance is coupled. 2. The two-dimensional mode-coupling structure is a structure in which first and second longitudinal mode-coupling resonators are arranged adjacent to each other on the piezoelectric substrate so as to perform transverse mode coupling with each other. The surface acoustic wave resonator filter according to claim 1. 3. The surface acoustic wave resonator filter according to claim 2, wherein the transverse mode couplings of the first and second longitudinal mode coupling resonators are acoustic couplings with each other. 4. Each of the first and second longitudinal mode coupled resonators is arranged adjacent to the interdigital electrode of its own resonator in the longitudinal direction and to the interdigital electrode of another resonator. 4. The surface acoustic wave resonator filter according to claim 3, further comprising dummy electrodes adjacently arranged so as to acoustically couple in the lateral direction. 5. A surface acoustic wave resonator filter including interdigital electrodes on a voltage substrate, the first and second surfaces utilizing longitudinal mode resonance in a longitudinal direction of a surface acoustic wave propagation direction. 1. A surface acoustic wave resonator filter, comprising: the longitudinal mode-coupled resonator, and the first and second longitudinal mode-coupled resonators connected in parallel between the input and the output.