JPH09116381A - Vertical double mode surface acoustic wave resonator filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the pass band width down to 0.1 to 0.25% in terms of the fractional band width and also to extremely reduce the package size by grounding the connection part of surface acoustic wave resonators formed on an LBO (lithium tetraborate) substrate via the electrostatic capacity. SOLUTION: A 1st stage surface acoustic wave resonator includes an input IDT 1 having an input terminal 7, a connection IDT 3 and the reflectors 4 which are provided at both sides of the IDT 1 and 3. A 2nd stage surface acoustic wave resonator includes an output IDT 2 having an output terminal 8, a connection IDT 3 and the reflectors 4 which are provided at both side of the IDT 2 and 3. These two resonators are placed on an LBO substrate, and the IDT 3 of both resonators are connected to each other via a connection bus bar 5. Then the bar 5 is grounded by the electrostatic capacity 6 consisting of a comb-like electrode. The fractional band width is reduced as the value of the capacity 6 is increased. The capacity 6 of about 0.8pF or more is required to secure the fractional band width of 0.25% or more.

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 utilizing a vertical double mode coupling, which has a narrow pass band and a small package size.

[0002]

2. Description of the Related Art A vertical double-mode surface acoustic wave resonator filter formed on a piezoelectric substrate, for example, a LiNbO ₃ substrate propagating in a 64 ° Y rotation cut X direction is known (for example,
Morita, Watanabe, Ogawa, Nakazawa, IEICE Transactions A, vo
l. J76-A No. 2 pp. 227-235 (1993)). A general configuration of this vertical double mode surface acoustic wave resonator filter includes a plurality of interdigital transducers (hereinafter abbreviated as IDTs), and a pair of grating reflectors (hereinafter abbreviated as reflectors) is provided outside them. ) Is arranged.
In the surface acoustic wave resonator filter having such a configuration, the pair of reflectors can be regarded as a cavity, and the excited surface acoustic wave causes multiple reflection between the reflectors and between the IDTs to generate a standing wave. FIG. 5 shows two IDTs 10a, 10 on the piezoelectric substrate 9.
It is the figure which showed typically the state in which the standing wave generate | occur | produced in the surface acoustic wave resonator filter which is provided with b and the reflector 4 was arrange | positioned at the both sides. In this case, the standing wave of the 0th-order symmetric mode and the standing wave of the 1st-order antisymmetric mode are excited, and F ₀ ,
Give the resonant frequency of F ₁ . In the case of a surface acoustic wave resonator using three IDTs, a zero-order symmetric mode standing wave and a second-order symmetric mode standing wave are excited, respectively
The resonance frequencies of ₀ and F ₂ are given (FIG. 6). Note that the first-order antisymmetrical standing wave is not excited because the charges cancel each other out.

Further, there is a vertical double-mode surface acoustic wave resonator filter in which the surface acoustic wave resonator filters having the above-mentioned structure are cascade-connected in multiple stages. Figure 7 (a) shows the input IDT1
(Output IDT 2), connection IDT 3, and surface acoustic wave resonators provided with reflectors 4 on both sides of the connection IDT for connection using a connection bus bar 5.
It is a vertical double-mode surface acoustic wave resonator filter having a 2IDT structure in which three of them are cascade-connected. Also, FIG. 7 (b)
Is an input IDT1 (output IDT2) and connection IDTs on both sides
3 and the connection IDTs 3 facing each other with the connection bus bar 5 between the surface acoustic wave resonators having the reflectors 4 on both sides thereof.
It is a vertical dual-mode surface acoustic wave resonator filter having a 3-IDT structure connected in cascade.

The vertical double mode surface acoustic wave resonator filter having the structure shown in FIGS. 7 (a) and 7 (b) has a series arm impedance Za and a parallel arm impedance Zb as shown in FIG.
It can be expressed as an equivalent circuit by a symmetric lattice circuit replaced by. The pass band can be formed by bringing the resonance point of Za and the antiresonance point of Zb, and the antiresonance point of Za and the resonance point of Zb close to each other, that is, by performing frequency matching.

[0005]

By the way, a system having a pass bandwidth of 0.1 to 0.25% in a specific bandwidth has been developed and put into practical use for some digital type mobile communication devices. Therefore, a surface acoustic wave filter having a specific bandwidth of about 0.1 to 0.25% is required. Furthermore, there is a strong demand for downsizing in mobile communication devices, and surface acoustic wave filters are no exception.

Lithium tetraborate (Li ₂ B ₄ O ₇ ) substrate (hereinafter LB
In the case of a vertical double-mode surface acoustic wave resonator filter fabricated on an (O substrate), the generally obtained specific bandwidth is 0.
It is about 3 to 0.5%, and a small and lightweight vertical double mode surface acoustic wave resonator filter has been put into practical use. As a means for narrowing the pass band width of the vertical double mode surface acoustic wave resonator filter, increasing the number of IDT pairs of the input IDT (output IDT) and the connection IDT can be mentioned. But,
When the IDT logarithm is increased, the amplitude ripple and the delay time ripple in the pass band increase, and there is a problem that the flatness of the amplitude characteristic and the group delay time characteristic cannot be satisfied.

On the other hand, as a surface acoustic wave resonator filter manufactured on a quartz substrate, a longitudinally coupled double mode filter, a laterally coupled double mode filter and the like have been put into practical use, but in order to obtain desired characteristics. Needs to increase the IDT logarithm. Therefore, there is a problem that the package size becomes large and it is difficult to cope with the recent demand for miniaturization of mobile communication devices.

Further, in the case of the LiTaO ₃ substrate and the LiNbO ₃ substrate, since the electromechanical coupling coefficient is large and the specific bandwidth is 3 to 5%, it cannot be applied to the surface acoustic wave filter for this purpose.

An object of the present invention is to achieve an area ratio smaller than that of a surface acoustic wave filter, which has a narrow pass band width of 0.1 to 0.25% in a specific bandwidth and a package size of a quartz substrate, which has been difficult to realize conventionally. An object of the present invention is to provide a vertical double-mode surface acoustic wave resonator filter that is significantly smaller than half or less.

[0010]

The vertical double mode surface acoustic wave resonator filter of the present invention is a surface acoustic wave resonator having a plurality of interdigital transducers and a grating reflector formed on a lithium tetraborate substrate. In a vertical double-mode surface acoustic wave resonator filter in which multiple elements are cascade-connected, a vertical double-mode surface acoustic wave characterized in that a connecting portion of the surface acoustic wave resonator is grounded via an electrostatic capacitance. It is a resonator filter. Further, the electrostatic capacitance is formed by a comb-shaped electrode formed on a lithium tetraborate substrate or a capacitance mounted in a package, which is a vertical double-mode surface acoustic wave resonator filter.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A plurality of IDs formed on an LBO substrate
Passbandwidth is obtained by grounding the connection part of T and the surface acoustic wave resonator filter equipped with a reflector on the outside of these in a vertical double-mode surface acoustic wave resonator filter, which is cascade-connected in multiple stages. Can be made narrower and the attenuation characteristic near the band can be improved.

By using the LBO substrate as the piezoelectric substrate, it is possible to make the size significantly smaller than that in the case of using the quartz substrate having the same electrical characteristics. Since the LBO substrate has a higher reflectance than the quartz substrate, energy can be trapped with a small number of reflectors, and the package size can be reduced. As the LBO substrate, 40 ° to 45 ° rotated X-cut Z-propagating lithium tetraborate having good temperature characteristics and a large electromechanical coupling coefficient can be used.

The value of the electrostatic capacitance depends on the characteristic impedance of the surface acoustic wave resonator filter and is set according to the electrode structure of the filter. The electrostatic capacitance is formed by forming a comb-shaped electrode for electrostatic capacitance at a position where it does not interfere with the surface acoustic wave excited by the surface acoustic wave resonator, or by providing the package with an electrostatic capacitance. Capacitance can be formed on the substrate while keeping the package size small.

[0014]

EXAMPLE A vertical double-mode surface acoustic wave resonator filter as shown in FIG. 1 (a) was produced on a 45 ° rotating X-cut Z-propagating lithium tetraborate substrate. A first-stage surface acoustic wave resonator having an input IDT 1 having an input terminal 7, a connection IDT 3 and a reflector 4 arranged on both sides thereof, and an output terminal 8.
Forming a second surface acoustic wave resonator having one output IDT2, one connection IDT3, and reflectors 4 on both sides thereof, and connecting these surface acoustic wave resonators to each other. The two were connected by a connecting bus bar 5. The connection bus bar 5 was grounded via the electrostatic capacitance 6 formed by the comb-shaped electrode.

The pattern of the comb-shaped electrodes was formed by vacuum-depositing an aluminum metal film, forming a resist pattern by a well-known photolithography technique, and then etching the aluminum metal film. In addition, the comb-shaped electrode is of a so-called normal type in which the overlapping lengths of the paired electrode fingers are substantially equal.

The number of comb-shaped electrode pairs of the input IDT (output IDT) is 4
2.5 pairs, 38.5 pairs of IDT electrodes for connection, and aperture length of 7.
1λ, the number of reflectors is 70 (note that λ is twice the reflector pitch). The terminating impedance was matched as 262Ω.

FIG. 2 shows a change in the relative bandwidth when the size of the capacitance 6 is changed. It can be seen that the specific bandwidth becomes narrower as the capacitance becomes larger. In order to reduce the specific bandwidth to 0.25% or less, it is sufficient to connect a capacitance of approximately 0.8 pF or more. Here, the value of the specific bandwidth is based on the standardized frequency obtained by standardizing the frequency at the center frequency.

FIG. 3 shows the pass characteristics of the vertical double mode surface acoustic wave resonator filter and the frequency response of Za and Zb of the symmetric lattice circuit when the magnitude of the capacitance is 1.5 pF. FIG. 4 shows the pass characteristic and the frequency response of Za and Zb of the vertical double mode surface acoustic wave resonator filter which is not provided. The frequency on the horizontal axis is a standardized frequency standardized by the center frequency of the pass band.

The specific bandwidth was 0.19% when the electrostatic capacitance was 1.5 pF, and was 0.35% when the electrostatic capacitance was not provided, and a narrow band of about 50% could be realized. In addition, the attenuation at a frequency 1.002 times the center frequency is 7 dB when no capacitance is provided, whereas the attenuation of 20 dB is larger when a capacitance of 1.5 pF is provided. It was The attenuation characteristics near the band were also significantly improved by providing a capacitance.

The resonance point (minimum point) of Za is from the high frequency side.
Resonance points (minimum points) of A0, A1, Zb from the high frequency side B0, B1, Za
The anti-resonance point (maximum value) of is from A0 ', A1' from the high frequency side, and the anti-resonance point (maximum value) of Zb is from B0 ', B1' from the high frequency side. From the frequency responses of Za and Zb in FIGS. 3 and 4, it can be seen that by providing the capacitance, the resonance points A0 and B0 and A1 and B1 come close to each other, and as a result, the pass band width becomes narrow.

The above embodiment is a vertical double mode surface acoustic wave resonator filter in which two surface acoustic wave resonators having a 2IDT structure are connected in cascade. The input IDT (output) as shown in FIG. Vertical dual mode surface acoustic wave resonator filter in which two surface acoustic wave resonators of 3IDT structure using one connection IDT) and two connection IDTs are cascaded Also in the case of the wave resonator filter, the band narrowing can be similarly realized by grounding the connection portion via the electrostatic capacitance.

[0022]

EFFECT OF THE INVENTION A vertical double-mode surface acoustic wave resonator filter in which a plurality of IDTs formed on an LBO substrate as described above and a surface acoustic wave resonator provided with a reflector on the outside thereof are cascade-connected in multiple stages. By grounding the connecting part via capacitance, the passband width is narrower than before, and as a result, it is an ultra-compact mobile communication system suitable for digital systems with low selectivity, in-band amplitude and delay time deviation. It is possible to provide a vertical double mode surface acoustic wave resonator filter.

[Brief description of the drawings]

FIG. 1 (a) is a diagram showing a configuration of a vertical double mode surface acoustic wave resonator filter including two IDTs according to an embodiment of the present invention. (b) is a diagram showing a configuration of a vertical double mode surface acoustic wave resonator filter including three IDTs according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a magnitude of a capacitance connected to a connection portion and a specific bandwidth in a vertical double mode surface acoustic wave resonator filter including two IDTs according to an example.

FIG. 3 is a diagram showing a frequency response when a connecting portion of a vertical double-mode surface acoustic wave resonator filter including two IDTs of an example is grounded via a capacitance of 1.5 pF.

FIG. 4 is a diagram showing a frequency response when a connecting portion of a vertical double-mode surface acoustic wave resonator filter including two IDTs of an example is not subjected to a capacitance.

FIG. 5 is a schematic diagram showing a state of a standing wave excited on a substrate of a surface acoustic wave resonator having two IDTs and a reflector on the outside thereof.

FIG. 6 is a schematic diagram showing a state of a standing wave excited on a substrate of a surface acoustic wave resonator provided with a reflector outside the three IDTs.

FIG. 7A is a diagram showing an example of a configuration of a conventional vertical double-mode surface acoustic wave resonator filter including two IDTs. (b) is a diagram showing an example of a configuration of a conventional vertical double mode surface acoustic wave resonator filter including three IDTs.

FIG. 8 is a diagram showing an equivalent circuit when the vertical double-mode surface acoustic wave resonator filter is replaced with a symmetric lattice circuit.

[Explanation of symbols]

1 Input IDT 2 Output IDT 3 Connection IDT 4 Reflector 5 Connection Busbar 6 Capacitance 7 Input Terminal 8 Output Terminal 9 Piezoelectric Substrate 10a, 10b, 10c Interdigital Transducer (IDT) Za Series Arm Impedance Zb Parallel Arm Impedance

Claims (2)

[Claims] 1. A vertical double mode surface acoustic wave resonator filter in which a plurality of interdigital transducers formed on a lithium tetraborate substrate and a surface acoustic wave resonator provided with a grating reflector are connected in cascade in multiple stages. A vertical double-mode surface acoustic wave resonator filter, wherein a connecting portion of the surface acoustic wave resonator is grounded via a capacitance. 2. A vertical double-mode surface acoustic wave resonator filter, wherein the capacitance is formed by a comb-shaped electrode on the substrate or a capacitance mounted in a package.