JP2002064358A - Compound surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sharpen the high frequency end of the passband curve of a compound surface acoustic wave(SAW) filter serially connecting first-third longitudinal coupling double mode SAW filters and an SAW resonator. SOLUTION: The compound SAW filter is configured by forming the first- third longitudinal coupling double mode SAW filters, the SAW resonator and an IDT electrode for capacitor on a piezoelectric substrate, serially connecting the first-third longitudinal coupling double mode SAW filters and the SAW resonator and parallel connecting the IDT electrode for capacitor between the stages.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a composite surface acoustic wave filter, and more particularly to a composite surface acoustic wave filter having improved passband characteristics and attenuation gradient.

[0002]

2. Description of the Related Art Recently, a surface acoustic wave filter (hereinafter referred to as SA) has been developed.
W filters) are widely used in the field of communications and have excellent characteristics such as high performance, small size, and mass productivity, and are therefore often used particularly in mobile phones and the like. The RF section of a cellular mobile phone is required to have a filter having a low loss, a wide band, and a steep attenuation gradient in order to separate adjacent transmission and reception waves. For example, the frequency of the cellular telephone system (PDC) in Japan is allocated as shown in FIG. That is, the receiving frequency band of the terminal is 810 to 843.
MHz (hereinafter referred to as RX1 band) and 870 MHz to 8
An 85 MHz band (hereinafter, referred to as an RX2 band) is allocated, and the RX2 band is a transmission frequency band 893MH of a terminal.
Since it is close to z to 898 MHz (TX1), RX
The attenuation gradient on the high frequency side of the receiving RF filter used in the two bands is required to be 10 dB or more (indicated by α in the figure) in the 893 to 898 MHz band.

FIG. 9 shows a conventional longitudinally-coupled dual-mode surface acoustic wave filter using a first- and third-order longitudinal modes (hereinafter referred to as a dual mode S).
FIG. 4 is a plan view showing the configuration of an AW filter (hereinafter referred to as an AW filter) on the main surface of the piezoelectric substrate 31 along the propagation direction of the surface wave.
The electrodes 32, 33, and 34 are arranged close to each other, and grating reflectors (hereinafter, referred to as reflectors) 35a and 35b are provided on both sides of the IDT electrodes. And IDT
The electrodes 32, 33, and 34 are each formed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and each of the outer IDT electrodes 33, 34 is connected to one of the comb-shaped electrodes and connected to the input terminal IN. At the same time, the other comb-shaped electrodes are respectively grounded. Furthermore, the central IDT electrode 3
One of the two comb electrodes is connected to the output terminal OUT, and the other comb electrode is grounded to form a dual mode SAW filter.

When a high frequency voltage is applied to the input terminal IN of the dual mode SAW filter shown in FIG.
Multiple modes are confined between 35b, of which ID
By properly setting the T electrode pattern, the first and third longitudinal modes are strongly excited, and function as a dual mode SAW filter using these two modes. As is well known, the bandwidth of a dual mode SAW filter depends on the frequency spacing of the two modes.

[0005] FIG. 10A shows a dual-mode SAW filter, in which a 36 ° YX LiTaO ₃ is used for a piezoelectric substrate 31 to constitute an RF filter for the RX2 band (870 MHz to 885 MHz).
31.5 pairs of central IDT electrodes 32 and outer IDT electrodes 3
FIG. 4 is a diagram showing filter characteristics of a dual mode SAW filter when 21.5 pairs of 3 and 34 are used, the intersection width is 30λ (λ is the wavelength of the surface acoustic wave to be excited), and the electrode thickness of the aluminum alloy is 170 nm. P is the passband standard (870 MHz to 885 at 2.5 dB)
MHz), Q is the attenuation band standard (10dB from 893MHz to 898MHz)
Above). FIGS. 10B and 10C are Smith charts when the input side and the output side of the filter are respectively terminated with 50Ω. As apparent from FIG. 10 (a), although the prototype satisfies the attenuation standard Q barely, there is a problem that the yield significantly deteriorates when mass-produced in consideration of manufacturing variations.

Therefore, as shown in FIG.
A SAW resonator R is connected in series to the AW filter F, and the resonance frequency of the resonator R is arranged in the pass band of the dual mode SAW filter F, and the anti-resonance frequency is arranged on the high frequency side near the pass band. A composite surface acoustic wave filter (hereinafter, referred to as a composite SAW filter) has been proposed in which an attenuation pole is formed on the high frequency side near the passband by using the filter to improve the attenuation. That is, as shown in FIG. 11, on a substrate 31 on which a dual mode SAW filter F is formed, an IDT electrode 36 composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and reflectors on both sides thereof The SAW resonators R formed by arranging 37a and 37b are juxtaposed to form a dual mode SAW.
Piezoelectric substrate 3 of IDT electrodes 33 and 34 outside filter F
A lead electrode extends from each of the comb-shaped electrodes closer to the center, and is connected to the comb-shaped electrode closer to the center of the piezoelectric substrate 31 of the SAW resonator R, and the other comb-shaped electrode is connected to the input terminal IN to form a composite SAW. Configure filters.

FIG. 12A shows the composite SAW shown in FIG.
Using the electrode pattern of the filter, RX2 band (870MHz ~
885 MHz) to form an RF filter.
Double-mode SAW filter F using 36 ゜ YX LiTaO ₃
31.5 pairs of the central IDT electrode 32 and the outer IDT electrode 3
3 and 34 are 21.5 pairs, the intersection width is 30λ (λ is the wavelength of the surface acoustic wave to be excited), the electrode thickness of the aluminum alloy is 170 nm, and the logarithm of the IDT electrode 36 of the SAW resonator R is 100.5.
Pair, cross width 20λ, electrode thickness of aluminum alloy 170n
It is a figure which shows the filter characteristic of the compound SAW filter comprised as m, (b) and (c) are the Smith charts when the input side and the output side are respectively terminated by 50 (ohm). As is clear from FIG. 12A, the attenuation on the high frequency side near the pass band is greatly increased, and the attenuation standard Q can be sufficiently satisfied.

[0008]

However, using a composite SAW filter as shown in FIG.
As shown in FIG. 12 (a), when the band-pass RF filter is configured, the cutoff characteristic at the high-frequency end in the pass band is significantly deteriorated, and the prototype slightly satisfies the standard P. In consideration of the above, there is a problem that the product often becomes defective. This is clear from the Smith charts of FIGS. 12 (b) and 12 (c). Thus, even when a composite SAW filter of a type in which a SAW resonator is connected in series to a dual mode SAW filter is used, mass production of a composite SAW filter having a relatively narrow bandwidth of about RX2 band (bandwidth 15 MHz) is required. There is a problem that the yield is significantly deteriorated. SUMMARY OF THE INVENTION The present invention has been made to solve the above problem, and has as its object to provide a surface acoustic wave filter having a flat passband and an improved attenuation in a high frequency side near a passband.

[0009]

According to a first aspect of the present invention, there is provided a composite surface acoustic wave filter according to the present invention, wherein one of the surface acoustic wave filters provided on a main surface of a piezoelectric substrate is provided. A composite surface acoustic wave filter characterized in that a terminal and one terminal of a surface acoustic wave resonator are connected in series, and a capacitance is connected in parallel at this connection point. According to a second aspect of the present invention, one terminal of the surface acoustic wave filter provided on the main surface of the piezoelectric substrate and one terminal of the surface acoustic wave resonator are connected in series, and a capacitance is connected to this connection point. A composite surface acoustic wave filter in which a capacitor is connected in parallel and a capacitor is connected in parallel to the other terminal of the surface acoustic wave filter. The invention according to claim 3 is the composite surface acoustic wave filter according to claim 1 or 2, wherein the capacitor is realized by an IDT electrode formed on a piezoelectric substrate. According to a fourth aspect of the present invention, the surface acoustic wave filter has three IDT electrodes and grating reflectors arranged on both sides thereof along the propagation direction of the surface acoustic wave.
2. A next-order / third-order longitudinally-coupled dual-mode filter, wherein the surface acoustic wave resonator includes an IDT electrode and grating reflectors on both sides thereof.
4. The composite surface acoustic wave filter according to any one of Items 1 to 3.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 is a plan view showing a configuration of a composite SAW filter according to the present invention, in which three IDs are provided on a main surface of a piezoelectric substrate 1 along a propagation direction of a surface wave.
A first-order to third-order longitudinally-coupled double-mode surface acoustic wave filter (double-mode SAW filter) 2 having a T electrode and grating reflectors on both sides thereof, and a grating reflector disposed on an IDT electrode and both sides thereof; Surface acoustic wave resonator (SAW resonator) 3 and an IDT electrode 4 for a capacitor substantially orthogonal to the propagation direction of the surface acoustic wave, and two IDT electrodes arranged outside the dual mode SAW filter 2. The lead electrodes extend from the comb-shaped electrodes near the center of the piezoelectric substrate 1 and are connected in series with the comb-shaped electrodes near the center of the piezoelectric substrate 1 of the SAW resonator 3, and the capacitor IDT electrodes 4 are connected to the lead electrodes. Connect in parallel. Furthermore, SA
The comb-shaped electrode of the W resonator 3 near the end of the piezoelectric substrate 1 is connected to the input terminal IN with a bonding wire or the like.
A comb-shaped electrode near the end of the piezoelectric substrate 1 of the IDT electrode arranged at the center of the dual-mode SAW filter and the output terminal OUT are connected by a bonding wire or the like, and the other comb-shaped electrode is grounded, respectively. Configure filters.

FIG. 2 (a) shows a dual mode SAW filter using 36 ゜ YX LiTaO ₃ for the piezoelectric substrate 1 to form an RX 2-band RF filter using the electrode pattern of the composite SAW filter of FIG. The center IDT electrode 2 has 31.5 pairs, the outer two IDT electrodes 21.5 pairs, the intersection width is 30λ (λ is the wavelength of the surface to be excited), and the electrode thickness of the aluminum alloy is
170 nm, and the number of IDT electrode pairs of the SAW resonator 3 is 100.5.
Pair, intersection width is 20λ, and log of IDT electrode 4 for capacitor is
FIG. 13 is a diagram showing passband characteristics of a composite SAW filter having a cross width of 73 μm (capacitance: 1.2 pF), which satisfies both a passband standard P and a high-band attenuation standard Q near the passband. You can see that it is. FIGS. 7B and 7C are Smith charts on the input side and output side when the composite SAW filter is terminated with 50Ω. The circle diagram traces the vicinity of 50Ω near the pass band of the filter. I understand.

A feature of the present invention is that a SAW resonator is connected in series to a dual mode SAW filter, and a capacitor is connected in parallel between the stages to achieve impedance matching with a terminating resistor of 50Ω, and to provide a pass band, especially This is to sharpen the cutoff characteristic on the high frequency side in the passband.

FIG. 3 shows a second embodiment according to the present invention. The difference from FIG.
A lead electrode extends from the comb-shaped electrode near the end of the piezoelectric substrate 1 and connects the lead to the output terminal OUT.
A capacitor I formed on the piezoelectric substrate 1 is connected to the lead electrode.
That is, the DT electrodes 5 are connected in parallel. FIG. 4 (a) shows the use of the electrode pattern shown in FIG. 3 to form an RF filter for the RX2 band, using 36 ° YX LiTaO ₃ for the piezoelectric substrate 1 and the IDT electrode at the center of the dual mode SAW filter 2. To
31.5 pairs, 21.5 pairs of outer two IDT electrodes, cross width 30
λ (λ is the wavelength of the surface acoustic wave to be excited), the electrode thickness of the aluminum alloy is 170 nm, the number of IDT electrodes of the SAW resonator 3 is 100.5, the cross width is 20λ, and the number of IDT electrodes 4 for capacitors is 36. , Crossing width 73μm (1.2p in capacity)
F), a composite SA having 12 pairs of IDT electrodes 5 for capacitors and a cross width of 73 μm (0.4 pF in capacitance)
FIG. 3 is a diagram illustrating passband characteristics of a W filter, and it can be seen that the cutoff characteristics in the high band within the passband are improved from the filter characteristics illustrated in FIG. 2. This is clear by comparing the input side and output side Smith charts shown in FIGS. 4B and 4C with those of FIG.

FIG. 5 shows a third embodiment according to the present invention. The difference from the second embodiment shown in FIG. 3 is that a SAW resonator 6 is further connected in series on the input side. A feature of the third invention is that by connecting two SAW resonators 3 and 6 in series, the attenuation characteristic of the filter is improved using the anti-resonance frequency of the SAW resonator.
The anti-resonance frequencies may be the same, or they may be arranged slightly apart. However, it is desirable that the resonance frequency of each SAW resonator be located at the center of the band. FIG. 6 (a)
Uses 36 電極 YX LiTaO ₃ for the piezoelectric substrate 1 to constitute an RF filter for the RX2 band using the electrode pattern of FIG.
The center IDT electrode of the dual mode SAW filter 2 is 31.5
Pair, 21.5 pairs of outer two IDT electrodes, cross width 30λ
(Λ is the wavelength of the surface to be excited), the electrode thickness of the aluminum alloy is 170 nm, the number of IDT electrode pairs of the SAW resonator 3 is 100.5, the cross width is 20λ, the number of IDT electrode pairs of the SAW resonator 6 is 100.5, Intersection width 10λ, IDT for capacitors
The number of electrodes 4 is 36 pairs, and the cross width is 73 μm (1.2 p
F), a composite SA having 12 pairs of IDT electrodes 5 for capacitors and a cross width of 73 μm (0.4 pF in capacitance)
FIG. 4 is a diagram illustrating passband characteristics of a W filter, wherein a curve A is a passband characteristic and a curve B is an attenuation characteristic. It can be seen that the ripple in the pass band is about 0.5 dB, the cutoff characteristic at the high frequency end of the pass band is steep, and the attenuation band sufficiently suppresses spurious. FIG. 6B is a Smith chart on the output side, and it can be seen that the circle diagram traces around 50Ω near the pass band of the filter.

FIG. 7 shows an example of a one-input / two-output reception RF filter used in a PDC system, in which one package 11 has two filters, an RX1 band RF filter 12 and an RX2 band RF filter 13. This is an example. In a relatively wide filter (bandwidth of 33 MHz) such as an RX1 band RF filter, even if a SAW resonator is connected in series to a dual-mode SAW filter, the cutoff characteristic at the upper end of the passband hardly deteriorates. There is no need to provide capacitance between stages and at the output end. On the other hand, it has been experimentally found that the present invention is effective when the band is relatively narrow (bandwidth 15 MHz) such as the RX2 band.

In the above, the dual mode SAW filter and the SA
The cut-off characteristic at the end of the pass band is improved by connecting a capacitor composed of an IDT electrode in parallel between the stage and the W resonator. However, the cut-off characteristic at the end of the pass band is not necessarily required, and the capacitor is suitable for impedance matching. If so, another electrode pattern may be used, or an element such as a chip-type capacitor may be used. Also, an example in which 36 ° YX LiTaO ₃ is used for the piezoelectric substrate has been described, but another cutting angle, for example, 36 ° to 42 ° may be used. Needless to say, the present invention can be applied to other piezoelectric substrates such as lithium niobate, lithium tetraborate, and langasite as the piezoelectric substrate. In the above embodiment, the outer ID of the primary-tertiary dual mode SAW filter
Although the resonator is connected to the input / output end extending from the T electrode, the same effect can be obtained even if the resonator is connected to the output side although the ripple is slightly increased.

[0017]

According to the present invention, the cutoff characteristics at the high-pass end of the composite SAW filter and the attenuation on the high-frequency side near the passband are provided in the first and second aspects of the present invention. And an excellent effect that it can be improved.
According to the third aspect of the present invention, since the required capacitance is formed by the IDT electrode provided on the same piezoelectric substrate, there is an effect that the mask can be manufactured in substantially the same process at the same time as the size reduction.
According to a fourth aspect of the present invention, the surface acoustic wave filter is realized by a primary-tertiary longitudinally-coupled dual-mode SAW filter, and the resonator is an ID.
This is realized by a T electrode and grating reflectors arranged on both sides of the T electrode.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration of a composite SAW filter according to the present invention.

FIG. 2 (a) is a passband characteristic of the composite SAW filter according to the present invention, and FIGS. 2 (b) and (c) are Smith charts on the input side and the output side, respectively.

FIG. 3 is a plan view showing a configuration of a composite SAW filter according to a second invention.

FIG. 4A is a passband characteristic of the composite SAW filter according to the second invention, and FIGS. 4B and 4C are Smith charts on the input side and the output side, respectively.

FIG. 5 is a plan view showing a configuration of a composite SAW filter according to a third invention.

FIG. 6A is a filter characteristic of a composite SAW filter according to the third invention, and FIG. 6B is a Smith chart.

FIG. 7 is an example of an RF filter for a PDC mounted with an RF filter for an RX1 band (810 MHz to 843 MHz) and an RF filter for an RX2 band (870 MHz to 885 MHz).

FIG. 8 is a diagram showing frequency allocation of the PDC mobile phone system.

FIG. 9 is a diagram showing a configuration of a conventional first-order to third-order longitudinally coupled dual-mode SAW filter.

10A is a diagram showing the filter characteristics of a conventional primary-tertiary-order longitudinally coupled dual-mode SAW filter, FIG.
(C) is a Smith chart on the input side and the output side, respectively.

FIG. 11 shows a conventional composite SAW filter in which a SAW resonator is connected in series to a primary-tertiary-order longitudinally coupled dual-mode SAW filter.

12A is a diagram showing the filter characteristics of the composite SAW filter of FIG. 11, and FIGS. 12B and 12C are Smith charts on the input side and the output side, respectively.

[Explanation of symbols]

 1. Piezoelectric substrate 2. First-order / third-order longitudinally-coupled dual-mode SAW filter 3, 6, SAW resonator 4, 5, IDT electrode for capacitor A. Pass-band characteristic of filter B. Filter Attenuation characteristics

Claims (4)

[Claims] 1. One terminal of a surface acoustic wave filter provided on a main surface of a piezoelectric substrate and one terminal of a surface acoustic wave resonator are connected in series, and a capacitor is connected in parallel at this connection point. A composite surface acoustic wave filter characterized by the above-mentioned. 2. One terminal of a surface acoustic wave filter provided on the main surface of a piezoelectric substrate and one terminal of a surface acoustic wave resonator are connected in series, and a capacitor is connected in parallel to this connection point. And a capacitor connected in parallel to the other terminal of the surface acoustic wave filter. 3. An IDT wherein the capacitor is formed on a piezoelectric substrate.
3. The method according to claim 1, wherein the electrode is realized by an electrode.
A composite surface acoustic wave filter according to claim 1. 4. A first-order third-order longitudinally-coupled dual-mode filter in which the surface acoustic wave filter has three IDT electrodes and grating reflectors on both sides thereof along the propagation direction of the surface acoustic wave. 4. The composite surface acoustic wave filter according to claim 1, wherein said surface acoustic wave resonator comprises an IDT electrode and grating reflectors disposed on both sides thereof.