JP3459494B2 - Surface acoustic wave filter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter suitable as a high-frequency device. 2. Description of the Related Art With the spread of portable telephones and automobile telephones in recent years, the need for high-frequency filters of small size and high performance has been increasing. Conventionally, dielectric filters and surface acoustic wave filters have been known as high-frequency filters, but the latter is more suitable for miniaturization and high performance. Further, surface acoustic wave filters are classified into a multi-electrode type, a multi-mode type, and a composite type (ladder connection type) depending on the electrode structure. A band pass filter can be realized at a frequency of 800 MHz or more. In view of the fact that a matching circuit is not required due to loss, a composite type surface acoustic wave filter has attracted attention. [0004] This composite type surface acoustic wave filter is shown in FIG.
As shown in FIG. 1, a comb-shaped electrode 2 is formed on a piezoelectric substrate (LiNbO ₃ or the like) 1.
a, and one filter function unit 3 is provided with two one-port resonators 2 each including a reflector a and the grating reflectors 2b, 2b, and one of the one-port resonators 2 is electrically connected to the signal line 4. They are connected in series (this is referred to as a series arm resonator 2S), and the other one-port resonator 2 is electrically connected in parallel to a signal line (this is referred to as a parallel arm resonator 2P).
FIG. 9 shows the basic configuration of one filter function unit 3. [0005] The composite surface acoustic wave filter implements a bandpass filter by utilizing the difference in impedance between the series arm resonator 2S and the parallel arm resonator 2P. Hereinafter, the principle will be briefly described with reference to FIGS. The impedance of the series arm resonator 2S is jX _S , the impedance of the parallel arm resonator 2P is jX _P , the anti-resonance frequency of the parallel arm resonator 2P is f _ap , its resonance frequency is f _rP , and the anti-resonance of the series arm resonator 2S is _Assuming that the frequency is f _aS and the resonance frequency is f _rS , as shown in FIG.
By substantially matching the _ap with the resonance frequency _frS , as shown in FIG. 4B, a filter characteristic having the anti-resonance frequency _faS and the resonance frequency _frP as poles around the substantially matching point is obtained. Can be The number of resonators 2 depends on the insertion loss of the filter,
It is set based on the out-of-band suppression amount and the like. The filter function unit 3 is provided in three stages using three series arm resonators 2S and three parallel arm resonators 2P.
When LiTaO ₃ of ° YX (Y-cut X-axis propagation) is used, 8
In the 00 MHz band, a low-loss filter having a minimum insertion loss of 2 dB or less and an out-of-band suppression of 25 dB or more has been obtained. In addition, with this structure, the impedance of the series arm resonator 2S is 0 in the pass band, and the impedance of the parallel arm resonator 2P is sufficiently larger than 50Ω.
There is an advantage that impedance matching can be achieved and a matching circuit is unnecessary. By the way, the composite type surface acoustic wave filter has a disadvantage that the degree of freedom for changing the bandwidth is small due to the filter principle. That is, it is important to the resonance frequency f _rS of the series arm resonator 2S and the anti-resonance frequency f _ap of the parallel arm resonator 2P is substantially matched, not substantially match a resonant frequency f _rS and the anti-resonance frequency f _ap In order to obtain a filter having a large bandwidth in FIG.
As shown in (b), the difference Δf ( _frS −
f _rP ) must be increased, and in this case, a ripple occurs in the band due to a difference between the anti-resonance frequency f _ap and the resonance frequency f _rS . Further, as shown in FIGS. 12A and 12B, when the difference Δf between the resonance frequencies is reduced, there is a problem that the pass band becomes convex. Therefore, in order to change the bandwidth while obtaining appropriate filter characteristics, the frequency difference between the resonance frequency and the anti-resonance frequency of each resonator 2 must be changed. As a method of changing the frequency difference, for example,
There is a method of changing the electromechanical coupling coefficient of the piezoelectric substrate 1. In this method, when the electromechanical coupling coefficient increases, the frequency difference between the resonance frequency and the antiresonance frequency of the resonator 2 increases, and when the electromechanical coupling coefficient decreases, the frequency difference between the resonance frequency and the antiresonance frequency of the resonator 2 increases. Uses the fact that it becomes smaller. Therefore, when a filter with a large bandwidth is required, a substrate with a large electromechanical coupling coefficient is used, and when a filter with a small bandwidth is required, a substrate with a small electromechanical coupling coefficient is used. . [0009] However, the conventional method of changing the frequency difference by changing the substrate to be used has the following problems. That is, as a piezoelectric substrate used for a high-frequency filter of 800 MHz or more, 36 ° Y-
XLiTaO ₃ (electromechanical coupling coefficient k ² = 0.047) and 6
4 ° YX LiNbO ₃ (electromechanical coupling coefficient k ² = 0.1
Since there are only two types of useful substrates, the frequency bandwidths required for each communication method (33 MHz, 25 MHz, 1
It is difficult to realize a filter that is sufficiently compatible with 7 MHz or the like. The present invention has been made in view of the above circumstances, and has as its object to provide a surface acoustic wave filter having a high degree of design freedom with respect to a bandwidth. A surface acoustic wave filter according to the present invention comprises two resonators each composed of a comb-shaped electrode and a grating reflector on a piezoelectric substrate, and one resonator is connected to a signal line. In a composite type surface acoustic wave filter having one or more filter function units that are electrically connected in series and the other resonator is electrically connected in parallel to a signal line, the series resonators are connected in series. The comb-shaped electrode of the resonator has one or more sets of electrode fingers that are continuously adjacent to each other. The comb electrode provided with the electrode fingers continuously adjacent to each other
This corresponds to a configuration in which a plurality of individual comb-shaped electrodes are provided with the adjacent electrode fingers as a boundary. In a resonator having a plurality of comb electrodes, the frequencies at the resonance point and the anti-resonance point change according to the number of the comb electrodes. That is, the resonance point and the anti-resonance point of the resonator can be arbitrarily changed. Therefore, a pass band width can be set arbitrarily with a surface acoustic wave filter in which a filter function unit is constituted by a resonator having such an arbitrary resonance point and an anti-resonance point. The present invention will be described below with reference to the drawings showing an embodiment thereof. FIG. 1 shows a composite type (ladder connection type) according to the present invention.
FIG. 3 is a schematic plan view showing the surface acoustic wave filter of FIG. In this surface acoustic wave filter, a one-port resonator 2 composed of comb-shaped electrodes 2a and grating reflectors 2b, 2b is formed on a piezoelectric substrate 1 composed of LiNbO _{3 of} 64 ° YX (Y-cut X-axis propagation). One one-port resonator 2 is electrically connected in series to the signal line 4 (this is referred to as a series arm resonator 2S), and the other The port resonator 2 is electrically connected in parallel to the signal line (this is called a parallel arm resonator 2P). The filter function units 3 are arranged in three stages. The surface acoustic wave filter shown in FIG. 1 has a center frequency f ₀ of the pass band of 902.5 MHz and a bandwidth of 25 MHz.
The electrode index of the comb-shaped electrode 2a and the electrode index of the comb-shaped electrode 2a of the parallel arm resonator 2P are each 120, and the length (opening length) of the electrode finger of the comb-shaped electrode 2a of the series arm resonator 2S.
Is set to 70 μm, and the length (opening length) of the electrode finger of the comb electrode 2a of the parallel arm resonator 2P is set to 140 μm. The comb-shaped electrode 2a of the series arm resonator 2S is formed with a pair of continuously adjacent electrode fingers. FIG. 2 is an enlarged schematic plan view showing the comb-shaped electrode 2a of one series arm resonator 2S in the surface acoustic wave filter of FIG. In this figure, continuously adjacent electrode fingers are indicated by circles. This series arm resonator 2S
The electrode index of the comb-shaped electrode 2a is 120 as described above. The electrode fingers are arranged so that there are 60 electrode fingers on both sides of the adjacent electrode fingers. FIG. 3 is a schematic plan view showing the comb-shaped electrode 2a of the series arm resonator 2S in an enlarged manner, similarly to FIG. 2, showing a modification of the comb-shaped electrode 2a. In the comb-shaped electrode 2a in FIG. 3, two pairs of electrode fingers that are continuously adjacent are formed. The electrode index of the comb-shaped electrode 2a in FIG. 3 is 120 as described above, and 40 electrode fingers are present on both sides of the continuous adjacent electrode finger as described above. is there. FIG. 4 is an enlarged schematic plan view of the comb-shaped electrode 2a of the series arm resonator 2S, similarly to FIG. 2, showing a modified example of the comb-shaped electrode 2a. In the comb-shaped electrode 2a in FIG. 4, three pairs of electrode fingers that are continuously adjacent to each other are formed. The electrode index of the comb-shaped electrode 2a in FIG. 4 is 120 as described above, and 30 electrode fingers are present on both sides of the continuous adjacent electrode fingers. is there. FIG. 5 is a schematic plan view showing the comb-shaped electrode 2a of the series arm resonator 2S in an enlarged manner, similarly to FIG. 2, showing a modification of the comb-shaped electrode 2a. In the comb-shaped electrode 2a in FIG. 4, two sets of continuously adjacent electrode fingers are formed, and one set of the continuously adjacent electrode fingers is
The other pair of continuously adjacent electrode fingers formed on one electrode side of the comb-shaped electrode 2a is formed on the other electrode side of the comb-shaped electrode 2a. Then, the formation positions of the two sets of successively adjacent electrode fingers do not overlap each other. As shown in FIGS. 2 to 5 described above, the comb-shaped electrode 2a having successively adjacent electrode fingers corresponds to a structure having a plurality of individual comb-shaped electrodes with the adjacent electrode finger as a boundary. . That is, in the case of FIG. 2, the comb-shaped electrode is divided into two,
3 and 5, the comb-shaped electrode is divided into three, and in FIG. 4, the comb-shaped electrode is divided into four. In a resonator including a plurality of comb-shaped electrodes (divided comb-shaped electrodes), the frequencies of the resonance point and the anti-resonance point change according to the number of comb-shaped electrodes (the number of divisions). FIG. 6 is a graph showing the frequency-attenuation characteristic of the resonator 2. The comb-shaped electrode having the structure shown in FIG. The three configurations of a comb-shaped electrode having a configuration of 3 (indicated by a thick line in FIG. 3) and a conventional comb-shaped electrode (indicated by a dotted line in FIG. 3) are shown. I have. As is apparent from FIG. 6, the distance between the highest attenuation point (anti-resonance point) and the resonance point in the resonator having the comb electrode divided into two is determined by that of the resonator having the normal electrode. It is 7 MHz narrower than the distance between the anti-resonance points and the resonance points. Further, the distance between the anti-resonance point of the resonator having the divided comb-shaped electrode and the resonance point is smaller by 11 MHz than the distance between the anti-resonance point and the resonance point of the resonator having the normal electrode. That is, in the resonator 2 having the divided comb-shaped electrodes 2a, the resonance point and the anti-resonance point of the resonator 2 can be arbitrarily changed according to the number of divisions. Therefore, with the surface acoustic wave filter of the present invention in which a filter function unit is constituted by such a resonator having an arbitrary resonance point and an anti-resonance point, the pass bandwidth can be arbitrarily set. FIG. 7 is a graph showing frequency-attenuation characteristics of the surface acoustic wave filter shown in FIG. For comparison, the frequency-attenuation characteristic of the conventional surface acoustic wave filter shown in FIG. 8 is also shown. As is apparent from FIG. 7, with the surface acoustic wave filter of the present invention, the frequency characteristics can be set so that the pass band width is 25 MHz, and even if the band is narrowed in this way, the conventional example can be used. As shown in FIG. 12, when the difference Δf between the resonance frequencies is reduced, the pass band does not become convex. Also, the frequency bandwidth required for each communication method (33 MHz, 2
5 MHz and 17 MHz) can also be solved. In the above-described embodiment, the comb-shaped electrodes divided by successive electrode fingers have the same number of electrode indices. However, it is not limited to the same number. Needless to say, even when the divided comb-shaped electrodes have different electrode indices, the same effect as in the above embodiment can be exerted. Although the comb-shaped electrode is divided in the series arm resonator 2S, the parallel arm resonator 2S
In P, the comb-shaped electrode may be divided by electrode fingers that are continuously adjacent. Furthermore, it is not limited to the 800 MHz band specification,
In any frequency band, the composite surface acoustic wave filter of the present invention can exert the above-described effects. As described above, according to the present invention, the frequency difference between the resonance frequency and the antiresonance frequency of the series arm resonator and / or the parallel arm resonator can be changed. The disadvantage of obtaining a filter without making the resonance frequency substantially equal to the anti-resonance frequency can be eliminated. In addition, there is an effect that the insufficiency when trying to obtain the frequency bandwidth required in each communication method by changing the material of the piezoelectric substrate can be solved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view showing a composite type surface acoustic wave filter of the present invention. FIG. 2 is an enlarged schematic plan view showing a series arm resonator of the surface acoustic wave filter of FIG. 1; FIG. 3 is a schematic plan view showing a modification (3 divisions) of the resonator of the present invention. FIG. 4 is a schematic plan view showing a modified example (divided into four) of the resonator of the present invention. FIG. 5 is a schematic plan view showing a modified example (3 divisions) of the resonator of the present invention. FIG. 6 is a graph showing frequency-attenuation characteristics of the resonator of the present invention (split type) and a conventional resonator (normal type). FIG. 7 is a graph showing frequency-attenuation characteristics of the surface acoustic wave filter of the present invention and a conventional surface acoustic wave filter. FIG. 8 is a schematic plan view showing a conventional composite type surface acoustic wave filter. FIG. 9 is a schematic plan view showing a filter function unit. 10A shows frequency-impedance characteristics of a series arm resonator and a parallel arm resonator, and FIG. 10B shows frequency-attenuation when both resonators of FIG. 10A are used. It is a graph which showed the quantity characteristic. 11A shows frequency-impedance characteristics of a series arm resonator and a parallel arm resonator, and FIG. 11B shows frequency-attenuation when both resonators of FIG. It is a graph which showed the quantity characteristic. 12A shows frequency-impedance characteristics of a series arm resonator and a parallel arm resonator, and FIG. 12B shows frequency-attenuation when both resonators of FIG. It is a graph which showed the quantity characteristic. [Description of Signs] 1 Piezoelectric substrate 2 Resonator 2S Series arm resonator 2P Parallel arm resonator 2a Comb electrode 2b Grating reflector 3 Filter function unit 4 Signal line

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Taizo Kobayashi               2-5-5 Keihanhondori, Moriguchi-shi, Osaka               Sanyo Electric Co., Ltd. (72) Inventor Kosuke Takeuchi               2-5-5 Keihanhondori, Moriguchi-shi, Osaka               Sanyo Electric Co., Ltd. (72) Inventor Kenichi Shibata               2-5-5 Keihanhondori, Moriguchi-shi, Osaka               Sanyo Electric Co., Ltd.                (56) References JP-A-7-79128 (JP, A)                 JP-A-55-107319 (JP, A)

Claims (1)

(57) Claims 1. A resonator comprising a comb-shaped electrode and a grating reflector is provided on a piezoelectric substrate, and one resonator is electrically connected to a signal line in series. A composite surface acoustic wave filter having at least one filter function unit formed by electrically connecting the other resonator to a signal line, wherein the comb-shaped electrode of the serially connected resonator is provided. Has one or more pairs of electrode fingers that are continuously adjacent to each other.