JP4599758B2 - Surface acoustic wave resonator and ladder type surface acoustic wave filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave resonator (hereinafter referred to as a SAW resonator) used in a narrow band ladder type surface acoustic wave filter.
[0002]
[Prior art]
In recent years, surface acoustic wave filters (hereinafter referred to as SAW filters) have been widely used in the communication field, and are widely used particularly in mobile phones and the like because they have excellent characteristics such as high performance, small size, and mass productivity. One type of SAW filter is a ladder type surface acoustic wave filter (hereinafter referred to as a ladder type SAW filter), which has characteristics such as steep attenuation characteristics and low loss, and is therefore widely used for RF filters for mobile phones. Yes.
For example, in the European 1.8 GHz cellular phone system (DCS), a broadband RF filter having a bandwidth of 75 MHz (specific bandwidth 4.3%) is used for both transmission and reception. On the other hand, a GPS device that receives radio waves from a plurality of navigation satellites and obtains position information on the ground with high accuracy has a narrow band with a center frequency of 1.5 GHz and a bandwidth of 2 MHz (specific bandwidth is 0.12%). SAW filters are required.
[0003]
FIG. 5A is a diagram showing a basic section configuration of a ladder-type SAW filter, which is composed of SAW resonators Zp of parallel arms and SAW resonators Zs of series arms, and the reactance curves of the respective arms are the same. It is set as shown in FIG. That is, when the anti-resonance frequency of the parallel arm SAW resonator Zp and the resonance frequency of the series arm SAW resonator Zs are set to substantially coincide with each other, as shown in F of FIG. A band-pass filter is formed, an attenuation pole is formed between the resonance frequency of the parallel arm SAW resonator Zp and the anti-resonance frequency of the series arm SAW resonator Zs, and a filter having a low loss and a steep attenuation gradient is obtained. Further, when a filter having a steep attenuation slope or a filter having a large guaranteed attenuation is required, a ladder type basic section filter is connected in cascade as shown in FIG.
[0004]
As apparent from FIG. 5B, the bandwidth of the ladder-type SAW filter depends on the difference df = fa−fs between the resonance frequency fs of the SAW resonator and the antiresonance frequency fa. The resonance frequency difference df is expressed by the following equation by the capacitance ratio γ of the SAW resonator (ratio γ = C0 / C1 of the capacitance C0 to the motional capacitance C1).
df = fs / (2γ)
Therefore, the bandwidth of the ladder type SAW filter is determined by the capacitance ratio γ of the SAW resonator. That is, in order to obtain a narrow band ladder-type SAW filter, it is necessary to increase the capacitance ratio γ of the SAW resonator.
[0005]
FIG. 6A is a diagram showing a configuration of an electrode pattern of a SAW resonator used in a ladder-type SAW filter. The IDT electrode 21 is formed on the main surface of a piezoelectric substrate (not shown) along the propagation direction of the surface wave. And grating reflectors (hereinafter referred to as reflectors) 22a and 22b are arranged on both sides thereof. The IDT electrode 21 is composed of a pair of comb electrodes having a plurality of electrode fingers interleaved with each other, and forms a one-terminal pair SAW resonator having a lead electrode extending from each comb electrode as a terminal.
FIG. 6 (b) uses an electrode pattern as shown in FIG. 6 (a). The piezoelectric substrate has 42 ° YX LiTaO ₃ , the IDT electrode 21 has 85 pairs, and the reflectors 22a and 22b have numbers. Resonance characteristics of 75 SAW resonators with IDT electrode 21 having an electrode period λ of 2 μm, a crossing width of 40 μm, and an aluminum electrode film thickness of 1800 mm are shown in a Smith chart over a frequency range of 1.5 GHz to 2.5 GHz. It is. From the figure, it can be seen that the resonance and antiresonance frequencies are 1948.356 MHz and 2014.827 MHz, respectively, and the capacitance ratio γ is 14.4.
FIG. 3C shows the spurious of the SAW resonator with a return loss from a frequency of 1.75 GHz to 2.25 GHz. As is apparent from the figure, spurious of about 2.51 dB occurs in the design of the conventional regular SAW resonator. The Smith chart and the return loss are both obtained by simulation with 50Ω terminated in parallel with the SAW resonator.
When a ladder-type SAW filter is configured using a SAW resonator having a large spurious, for example, when a spurious as shown in FIG. 6C is present in a series arm SAW resonator, the SP shown in FIG. Spurious is generated on the high frequency side of the pass band, and the attenuation is deteriorated. Further, when the SAW resonator of the parallel arm has spurious, spurious is generated from the high frequency side in the pass band to the attenuation region, and the filter characteristics are deteriorated.
[0006]
Various attempts have been made to increase the capacitance ratio γ in order to realize a narrow-band ladder-type SAW filter. For example, the technique disclosed in JP-A-8-65089 and JP-A-9-167937 is a technique for increasing the capacitance ratio γ by adding a capacitor Cp in parallel to the SAW resonator as shown in FIG. . As a means for forming the capacitance Cp, there is a means for forming a capacitance by forming a pair of comb shapes on the piezoelectric substrate. FIGS. 8B and 8C show the Smith chart and return loss of the SAW resonator configured as shown in FIG. Although the values shown in FIG. 6A are used as the values of the constants, it is necessary to make the impedances of the SAW resonators the same in order to compare the spurious magnitudes generated in the SAW resonators. Yes, the number of pairs of regular IDT electrodes 21 shown in FIG. 8A is 73, the number of reflectors is 87, and the capacitance Cp is 0.22 pF. From the Smith chart, the resonance and antiresonance frequencies are found to be 1948.383 MHz and 2005.240 MHz, respectively, and it can be seen that the capacitance ratio γ is 16.9. It is larger than 14.4 obtained in FIG. 6, and the size of the spurious is about 2.39 dB from FIG.
[0007]
Japanese Patent Laid-Open No. 2000-49568 proposes an IDT electrode provided with a portion A arranged so that the phase of electrode fingers is shifted by 180 ° inside the IDT electrode, as shown in FIG. 9A. FIG. 6 shows the impedance of the SAW resonator with 75.5 pairs of normal IDT electrodes 23 of the SAW resonator, 5 pairs of pair K of inversion electrodes, 74 pieces of reflectors 25a and 25b, respectively. It was set to match that of (a). FIG. 9B is a Smith chart, and FIG. 9C is a return loss. When the IDT electrode 23 having such an electrode pattern is used, it can be seen from the Smith chart that the resonance and antiresonance frequencies are 1948.252 MHz and 2005.775 MHz, respectively, and the capacitance ratio γ is 16.7. Further, as apparent from the return loss, it was found that although the capacity ratio γ was as large as 16.7, many spurious components were generated and the size was as large as 2.47 dB.
[0008]
FIG. 10A shows a modified example of the IDT electrode pattern disclosed in Japanese Patent Laid-Open No. 2000-49568, in which excitation thinning weighting is applied to a region where the excitation phase is reversed. That is, if the upper and lower comb-shaped electrodes in the drawing are “+” and “−”, respectively, the electrode finger configuration of the A part is “++ − +”. In order to match the impedance of the SAW resonator with that of FIG. 6A, the number of normal IDT electrode pairs N is 68 pairs, the number of IDT electrode pairs K shown in part A is 17, and the number of reflectors M is 58. Set. FIG. 10B shows a Smith chart, and FIG. 10C shows a return loss. From the Smith chart, it can be seen that the resonance and anti-resonance frequencies are 1947.834 MHz and 2005.091 MHz, respectively, and the capacitance ratio γ is 16.8. Although the capacity ratio γ was improved to 16.8, it was found that the size of the spurious was rather increased to 3.69 dB.
[0009]
FIG. 11A is an example of an IDT electrode pattern disclosed in Japanese Patent Laid-Open No. 11-163664, and the electrode fingers are thinned out at regular intervals. For example, one thinning is performed for seven electrode fingers as shown in part A of FIG. FIG. 11B and FIG. 11C show the Smith chart and the return loss obtained by simulation for the resonance characteristics of the SAW resonator of the IDT electrode. In order to match the impedance with the SAW resonator of FIG. 6A, 14 sets of thinning electrodes K, one set of electrode fingers in part A in the figure, one set of normal type electrodes N, and 61 reflectors M, respectively did. From the Smith chart, it can be seen that the resonance and anti-resonance frequencies are 1968.914 MHz and 2025.927 MHz, respectively, and the capacitance ratio γ is 17.0.
In this case as well, the capacitance ratio γ is improved to 17.0, but the spurious is as large as 2.55 dB.
[0010]
Japanese Patent Laid-Open No. 8-23256 proposes a configuration of an IDT electrode that increases the thinning electrode index as it goes to both sides from the center of the IDT electrode when thinning the IDT electrode. The electrode pattern of the SAW resonator shown in FIG. 12A is subjected to thinning weighting that approximates the excitation intensity distribution to a Kaiser function with α = 2.1 (Acoustic wave device technology handbook (Yoshio Shibayama, Ohm) p211). Is. The resonance characteristics of the SAW resonator having this configuration with 108 pairs of IDT electrodes and 52 reflectors are obtained by simulation, and the Smith chart of FIG. 11B and FIG. It is a figure of the return loss shown in FIG. From the Smith chart, it can be seen that the resonance and antiresonance frequencies are 1948.309 MHz and 2005.711 MHz, respectively, and the capacitance ratio γ is 16.8. Although the spurious was improved to 1.71 dB, it was found that the spurious was still a large value exceeding 1.5 dB.
[0011]
[Problems to be solved by the invention]
However, when the SAW resonator is configured by using various conventional SAW resonator design methods as described above, a large spurious is generated. Therefore, when such a SAW resonator is used to form a narrow-band ladder-type SAW filter, there is a problem that the filter characteristics deteriorate due to spurious.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a SAW resonator that greatly improves the capacitance ratio and suppresses the magnitude of spurious generated.
[0012]
[Means for Solving the Problems]
Inventions of the surface acoustic wave device according to the present invention in order to achieve the above object, a grating reflectors disposed on both sides of the thinned weighted IDT electrode along the propagation direction of a surface wave on a main surface of a piezoelectric substrate In the surface acoustic wave resonator configured as described above, the IDT electrode does not include an excitation region in which the phase of the surface acoustic wave excitation is reversed, and the IDT electrode thinning weight is determined by the propagation direction of the surface wave. The surface acoustic wave resonator is characterized in that the left and right are different from each other.
In one embodiment , in the surface acoustic wave resonator in which the regular IDT electrode and the grating reflectors are arranged on both sides of the piezoelectric substrate along the propagation direction of the surface wave on the main surface of the piezoelectric substrate, at least a portion of the electrode fingers of the IDT electrode, characterized in that it is replaced by an odd present one by the electrode fingers columns connected to the same potential alternately.
In one embodiment, in a surface acoustic wave resonator formed by arranging a regular IDT electrode and a grating reflector on both sides of a main surface of a piezoelectric substrate along the propagation direction of a surface wave, one bus bar is provided. When the electrode finger connected to the electrode bar is represented by “+” and the electrode finger connected to the other bus bar is represented by “−”, at least a part of the electrode fingers of the normal type IDT electrode are thinned out by a plurality of thinned electrode finger rows “++++” --- it, characterized in that it was replaced with ".
In one embodiment, in a surface acoustic wave resonator formed by arranging a regular IDT electrode and a grating reflector on both sides of a main surface of a piezoelectric substrate along the propagation direction of a surface wave, one bus bar is provided. When the electrode finger connected to the electrode bus is represented by “+” and the electrode finger connected to the other bus bar is represented by “−”, at least a part of the electrode fingers of the normal type IDT electrode are thinned out by a plurality of electrode finger rows “++++++” ----- and "characterized in that substituted out with a plurality of thinned electrode fingers string" +++ --- ".
In one embodiment, the surface acoustic wave resonator includes a single IDT electrode thinned and weighted along the propagation direction of the surface wave on the main surface of the piezoelectric substrate, and a grating reflector on both sides thereof. Thus, the IDT electrode does not include an excitation region in which the phase of the surface acoustic wave excitation is reversed, and the thinning weight is asymmetric with respect to the propagation direction of the surface wave with respect to the center of the IDT electrode. When the electrode finger connected to one bus bar of the IDT electrode is represented as “+” and the electrode finger connected to the other bus bar is represented as “−”, at least a part of the electrode fingers of the IDT electrode are electrode fingers. It is a thinning electrode of the column “++++-”.
Furthermore, at least some of the electrode fingers of the IDT electrode may be thinned electrodes of an electrode finger row “++++++”.
Further, the present invention is a ladder-type surface acoustic wave filter characterized by using the surface acoustic wave resonator according to the present invention described above or the surface acoustic wave resonator according to an embodiment described above .
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.
FIG. 1A is an electrode pattern showing the configuration of a SAW resonator according to the present invention. The IDT electrode 1 is formed on the main surface of a piezoelectric substrate (not shown) along the propagation direction of the surface wave and on both sides thereof. The reflectors 2a and 2b are arranged. The IDT electrode 1 is composed of a pair of comb electrodes having a plurality of electrode fingers interleaved with each other, and forms a one-terminal pair SAW resonator having a lead electrode extending from each comb electrode as a terminal.
[0014]
A feature of the present invention resides in the construction method of the IDT electrode 1, and as shown in A 1, A 2, A 3,..., Ai,. It is not symmetrical with respect to the center of the IDT electrode 1 but is applied according to the Kaiser function α toward the right in the figure. For example, in FIGS. 1A and 1B, α = 1.9, the number N of IDT electrode pairs is 105.5 pairs, and the number of reflectors is M in order to match the impedance dance of the SAW resonator of FIG. It is a figure which shows a Smith chart when simulating by setting 54 each, and a return loss. From FIG. 5B, the resonance and antiresonance frequencies are 1948.379 MHz and 2005.386 MHz, respectively, and the capacity ratio γ is improved to 16.8. Further, it can be seen that spurious generated on the high frequency side from the anti-resonance frequency is improved to values lower than 1.46 dB and 1.5 dB from FIG.
[0015]
FIG. 2A shows a second embodiment according to the present invention, which is characterized by a configuration method of the IDT electrode 3. That is, as shown in FIG. 2 (a), when an electrode finger connected to one bus bar is represented as "+" and an electrode finger connected to the other bus bar is represented as "-", at least a part of the regular IDT electrode Is replaced with the IDT electrode A in the electrode finger array thinned out as “++++-” as shown on the left side of the figure. Smith chart of the number of sets of IDT electrodes A set to 20 sets, 65 pairs of normal type electrodes N, and 35 sets of reflectors M in order to match impedance with the SAW resonator of FIG. The loss is shown in FIGS. 2 (b) and 2 (c). From the Smith chart, it can be seen that the resonance and anti-resonance frequencies are 1948.293 MHz and 2005.099 MHz, respectively, and the capacitance ratio γ is 16.8, which is improved. Further, from FIG. 2 (c), it was found that the spurious on the high frequency side is 1.30 dB, which is improved from the conventional one.
[0016]
FIG. 3A is a third embodiment according to the present invention, and among the regular IDT electrodes, the left electrode in the figure is an IDT electrode A1 of the “++++ ----” type, and “ It is replaced with an excitation thinning-out IDT electrode combined with a “++++-” type IDT electrode A2. Resonance characteristics of SAW resonators with three sets of IDT electrodes A1-A1-A2-A1-A2 to match impedance, 67 pairs of regular IDT electrodes N, and 30 reflectors M each. FIG. 3B is a Smith chart, and FIG. 3C is a return loss. From the Smith chart, the resonance and antiresonance frequencies are 1948.179 MHz and 2004.782 MHz, respectively, and the capacity ratio γ is improved to 17.0. It can be seen that the spurious generated on the high frequency side of the anti-resonance frequency is greatly suppressed to 1.04 dB.
[0017]
The common points of Embodiments 1 to 3 according to the present invention shown in FIGS. 1 to 3 are that the thinning-out electrode is composed of an electrode in which three or more odd-numbered electrode fingers are continuously connected, and the IDT. The arrangement of the thinned electrodes in the electrode is asymmetric with respect to the center of the IDT electrode, and the region where the excitation is out of phase is not included. However, as shown in part A of FIG. 4, the outermost part of the IDT electrode is not formed in the outermost part of the IDT electrode, even if an even number of electrode fingers having the same potential continue. The number of electrode fingers with the same potential may be even or odd.
[0018]
【The invention's effect】
Since the present invention is configured as described above, the invention according to claim 1 exhibits an excellent effect that a SAW resonator that suppresses spurious generated in the vicinity of the high frequency side of the resonance frequency is obtained. The invention according to claim 2 exhibits an excellent effect that a SAW resonator in which the size of spurious is suppressed can be obtained. The inventions according to claims 3 and 4 show the effect that a SAW resonator with suppressed spurious can be realized. The invention according to claim 5 has an excellent effect that a filter with improved attenuation characteristics in the vicinity of the pass band can be realized.
[Brief description of the drawings]
1A is a diagram showing an electrode pattern of a SAW resonator according to the present invention, FIG. 1B is a Smith chart, and FIG. 1C is a diagram showing a return loss.
2A is a diagram showing electrode patterns of a SAW resonator according to a second embodiment of the present invention, FIG. 2B is a Smith chart, and FIG. 2C is a diagram showing return loss.
3A is a diagram showing an electrode pattern of a SAW resonator according to a third embodiment of the present invention, FIG. 3B is a Smith chart, and FIG. 3C is a diagram showing a return loss.
FIG. 4 is a diagram showing an electrode pattern of a SAW resonator according to the present invention.
5A is a diagram showing a basic section of a ladder filter, FIG. 5B is a diagram showing reactance curves and filter characteristics of parallel arms and series arms, and FIG. 5C is a ladder type in which the basic sections are connected in cascade. It is a filter.
6A is a diagram showing an electrode pattern of a conventional SAW resonator, FIG. 6B is a Smith chart, and FIG. 6C is a diagram showing a return loss.
FIG. 7 is a diagram illustrating spurious generated in a ladder-type SAW filter.
8A and 8B are diagrams showing the configuration of a SAW resonator in which a capacitor is connected in parallel to an IDT electrode, FIG. 8B is a Smith chart, and FIG. 8C is a diagram showing return loss.
9A is a diagram showing an electrode pattern of a SAW resonator arranged so that the phase of an electrode finger is shifted by 180 ° inside an IDT electrode, FIG. 9B is a Smith diagram, and FIG. 9C is a diagram showing a return loss. It is.
10A is a diagram showing an electrode pattern of a SAW resonator in which excitation thinning weighting is applied to a region where the excitation phase is reversed, FIG. 10B is a Smith chart, and FIG. 10C is a return loss. FIG.
11A is a diagram showing an electrode pattern of a SAW resonator in which electrode fingers are thinned out at regular intervals, FIG. 11B is a Smith chart, and FIG. 11C is a diagram showing return loss.
12A is a diagram showing an electrode pattern of a SAW resonator in which Kaiser function thinning distribution is applied to electrode fingers of an IDT electrode, FIG. 12B is a Smith chart, and FIG. 12C is a diagram showing return loss. .
[Explanation of symbols]
IDT electrodes 2a, 2b, 4a, 4b, 6a, 6b .. Grating reflector A, A1, A2, A3, A4, A5,... Ai.

Claims (2)

A surface acoustic wave resonator in which one IDT electrode thinned and weighted along the propagation direction of a surface acoustic wave and a grating reflector on both sides thereof are arranged on the main surface of the piezoelectric substrate,
The IDT electrode does not include an excitation region in which the phase of the surface acoustic wave excitation is reversed,
The thinning weight is asymmetric with respect to the propagation direction of the surface acoustic wave with respect to the center of the IDT electrode,
When the electrode finger connected to one bus bar of the IDT electrode is represented as “+” and the electrode finger connected to the other bus bar is represented as “−”,
Ri withdrawal electrode der at least a portion of the electrode fingers electrode fingers columns of the IDT electrode "+++ ---"
At least a portion of the electrode fingers electrode fingers column "+++++ -----" surface acoustic wave resonator, wherein the thinning electrode der Rukoto of the IDT electrode.   A ladder type surface acoustic wave filter comprising the surface acoustic wave resonator according to claim 1.

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