JP2005295202A - Surface acoustic wave filter and duplexer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain wider and flatter attenuation outside a pass band without increasing an insertion loss. <P>SOLUTION: In a surface acoustic wave filter in which one or more series arm resonators are connected to one or more parallel arm resonators in a ladder shape, the distance between one bass bar and the tip of an electrode finger connected to another bass bar is set at a value 1λ-5λ in the IDTs of the series arm resonators 21-23, where the cycle of an electrode finger pair is λ. When the filter has two or more series arm resonators, the distance is differentiated between the two or more resonators. The cross length of the electrode fingers is a value within a range between 5λ to 20λ. This surface acoustic wave filter configures a duplexer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave filter and a duplexer, and more particularly, to a surface acoustic wave filter (hereinafter referred to as a SAW filter) in which a series arm resonator and a parallel arm resonator are connected in a ladder shape, and a duplexer including the filter.

  In mobile communication devices such as cellular phones, SAW filters have been widely used as high-frequency band filters in recent years because they are small and suitable for high functionality. Such a SAW filter includes a comb-shaped electrode (Interdigital Transducer / hereinafter referred to as IDT) that excites surface acoustic waves, and a reflector that is disposed on both sides of the IDT and traps the surface acoustic waves excited by the IDT. A SAW resonator is formed on a piezoelectric substrate, and a filter having a predetermined pass band is configured using this resonator.

  As a resonator connection structure, a circuit configuration in which a resonator is connected in a ladder type is known. That is, this ladder-type SAW filter has a series arm connecting the input end and the output end, and a parallel arm connecting the series arm and the reference potential, and the one-pair SAW resonator as a series arm resonator in the series arm. However, a pair of SAW resonators are similarly connected to the parallel arms as parallel arm resonators. The number of connected resonators is appropriately selected according to the target characteristics of the filter.

  Further, there is the following patent document as a disclosure of such a SAW filter.

JP 11-340774 A Japanese Patent No. 3255128

  By the way, it is desired that the band pass filter suppresses the insertion loss in a predetermined pass band and obtains a large attenuation outside the pass band.

  However, the ladder-type SAW filter can obtain a large attenuation in the frequency region near the extreme of the pass band, but has a characteristic that the attenuation decreases rapidly as the frequency goes away from the pass band. Therefore, for example, when configuring a duplexer that demultiplexes transmission / reception signals, the bandwidth of the attenuation band is often the same as the bandwidth of the pass band. Therefore, in such attenuation characteristics, the pass band of the counterpart filter is used. In some cases, sufficient attenuation cannot be obtained.

  On the other hand, if such characteristics outside the pass band are to be improved, the insertion loss in the pass band increases, causing characteristic deterioration in the pass band.

  Various design devices such as the configuration of the resonator itself forming the filter, the number of connections, and the arrangement configuration have been devised, but it is not easy to obtain a wide and high attenuation outside the passband.

  In addition, the invention of the above-mentioned Patent Document 1 is intended to flatten the attenuation frequency characteristic in the pass band to widen the band, and cannot improve the characteristic in the attenuation band. Furthermore, the invention of the above-mentioned Patent Document 2 aims to increase the steepness of the filter characteristics in the pass band.

  Accordingly, an object of the present invention is to provide a SAW filter that can obtain a broader flat attenuation outside the passband than before without increasing the insertion loss.

  In order to solve such a problem, the first SAW (surface acoustic wave) filter of the present invention is a SAW filter in which one or more series arm resonators and one or more parallel arm resonators are connected in a ladder shape. In the IDT (interdigital transducer) of at least one series arm resonator, the distance between one bus bar and the tip of the electrode finger connected to the other bus bar is defined as the period of the electrode finger pair λ. In this case, the value is larger than 1λ and 5λ or less.

  The inventor conducted experiments and studies on a ladder-type SAW filter. The above-described configuration relating to the first SAW filter of the present invention and the configurations of the following second and third SAW filters are based on such experimental results (details will be described later with reference to the drawings).

  That is, in the ladder-type SAW filter, the distance (hereinafter referred to as the gap length) between one bus bar in the IDT of the series arm resonator and the tip of the electrode finger connected to the other bus bar is defined as the period of the electrode finger pair ( If it is larger than the pitch ([Lambda]) of the electrode fingers, a spike-like peak appears on the high frequency side of the anti-resonance point in the frequency-impedance characteristic, and the impedance of the filter is increased. This peak is considered to be due to the secondary resonance mode generated in the gap between the IDT bus bar and the electrode finger because the frequency position changes when the gap length is changed.

  In the SAW filter of the present invention, the peak due to the secondary resonance mode is used to increase the attenuation outside the passband of the filter and to flatten the attenuation region.

  Specifically, the relationship between the gap length and the frequency is such that when the gap length is made to coincide with the period λ of the electrode finger pair (1λ), the secondary resonance mode peak (resonance frequency) is SAW. It almost overlaps with the anti-resonance frequency of the main resonance mode of the resonator, and the effect of increasing the attenuation amount due to the increase in impedance cannot be obtained.

  On the other hand, when the gap length is made larger than 1λ, the peak of the secondary resonance mode becomes higher (shifts toward a higher frequency). At the same time, as the gap length increases, the amount of increase in impedance due to the secondary resonance mode peak decreases, and when it is 5λ or more, the peak of the secondary resonance mode does not appear clearly. Moreover, even if such a secondary resonance mode is generated, the resonance point and antiresonance point of the main resonance mode are hardly affected, and no deterioration in the characteristics of the passband is observed.

  Therefore, in the SAW filter of the present invention, the IDT gap length of the series arm resonator is set to a value exceeding 1λ and not more than 5λ. As a result, a secondary resonance mode (waveguide mode) is generated, and the frequency position of the secondary resonance mode is set to a frequency higher than the antiresonance point of the filter attenuation region, so that the portion corresponding to the attenuation region of the ladder filter As a result, the attenuation outside the passband is larger than that of the conventional SAW filter.

  The second SAW filter of the present invention has two or more series arm resonators in the first SAW filter, and the IDT of the first series arm resonator among the two or more series arm resonators. The distance is a first value greater than 1λ and less than or equal to 5λ, and the distance in the IDT of the second series arm resonator is a second value different from the first value greater than 1λ and less than or equal to 5λ. .

  As described above, when the gap length of the IDT of the series arm resonator is changed in the ladder-type SAW filter, the frequency position of the secondary resonance mode peak shifts. Therefore, if the gap length of each resonator is set to a different value for a plurality of series arm resonators, a plurality of secondary resonance mode peaks can be generated in the attenuation region.

  In the second filter of the present invention, the gap lengths of the plurality of series arm resonators are set to different values within the range of the above 1λ and 5λ or less, whereby a plurality of secondary resonance mode peaks having different frequencies are obtained. And a flat attenuation region is formed in a wider frequency region.

  For example, when configuring a three-stage SAW filter including three series arm resonators, the gap length of the IDT of the first series arm resonator is 2λ, for example, The gap length of the IDT is 3λ, for example, and the IDT gap length of the third series arm resonator is 4λ, for example. The number of ladder filters (number of resonators), the size of each gap length (for example, 1.5λ, 2.4λ, etc., may not be an integral multiple of λ) and combinations are other than this. Also, various settings can be made, and the present invention is not limited to these.

  According to a third SAW filter of the present invention, in the first or second SAW filter, the crossing length of the electrode fingers in the IDT of the series arm resonator is set to a value in the range of 5λ to 20λ.

  As a result of the above experiment, it was found that when the crossing length of the electrode fingers is changed, the degree of contribution of the secondary resonance mode to the main resonance mode is changed with the change.

  That is, when the crossing length of the electrode fingers of the IDT is increased, the frequency of the secondary resonance mode does not change, but the amount of increase in impedance due to the secondary resonance mode is gradually reduced. In particular, when the crossing length exceeds 20λ, the amount of increase in impedance due to the secondary resonance mode is reduced, and the effect of improving the attenuation characteristics is reduced.

  On the other hand, if the crossing length of the electrode fingers is reduced, the anti-resonance impedance of the main resonance mode is lowered. In particular, when the crossing length is smaller than 5λ, the deterioration of the anti-resonance impedance in the main resonance mode becomes large.

  Therefore, in the third SAW filter of the present invention, the crossing length of the electrode fingers in the IDT of the series arm resonator is set to a value within the range of 5λ to 20λ, thereby improving the attenuation characteristics by the secondary resonance mode. Plan.

As a piezoelectric substrate constituting the first to third SAW filters, for example, a LiTaO ₃ substrate can be used.

  The duplexer of the present invention includes any one of the first to third SAW filters.

  By configuring the duplexer with any one of the first to third SAW filters, a duplexer having a flat and high attenuation characteristic in the passband of the counterpart filter can be formed.

  According to the present invention, it is possible to obtain a flat and larger attenuation characteristic outside the pass band without deteriorating the characteristic in the pass band.

  Other features and advantages of the present invention will become apparent from the following description of embodiments of the present invention.

  Hereinafter, an embodiment of the present invention (hereinafter referred to as the present embodiment) will be described with reference to the accompanying drawings.

FIG. 1 shows a ladder-type SAW filter according to an embodiment of the present invention.
As shown in the figure, this SAW filter 11 is provided with three SAW resonators 21, 22, and 23 on a series arm 14 that connects an input terminal 12 and an output terminal 13, and the series arm 14 and a reference potential 15 Two SAW resonators 24 and 25 are provided on the parallel arms 16 and 17 connecting the two, and the series arm resonators 21 to 23 and the parallel arm resonators 24 and 25 constitute a four-stage ladder type SAW filter. is there.

  In the filter 11, each SAW resonator 21 to 25, as shown in FIG. 2, is arranged on both sides of an IDT (Interdigital Transducer) 31 for exciting a surface acoustic wave formed on a piezoelectric substrate, and the IDT 31. And a single-terminal pair SAW resonator having a short-circuited grating reflector 41 (only one of the reflectors is shown in FIG. 2) for confining the surface acoustic wave excited in step S2.

  The IDT 31 is a regular IDT, and has a pair of bus bars 32a and 32b extending along the surface wave propagation direction. One of the bus bars 32a is connected to one end of a plurality of electrode fingers 33a, and each electrode finger 33a extends in a direction orthogonal to the surface wave propagation direction, that is, toward the other bus bar 32b. Similarly, one end of a plurality of electrode fingers 33b is connected to the other bus bar 32b, and each electrode finger 33b extends toward the one bus bar 32a. The electrode fingers 33a and 33b are arranged so as to be interleaved with each other, the number of electrode fingers is 80 in this example, and the cycle of the electrode fingers is λ. In FIG. 2, the number of pairs of electrode fingers is expressed less than the actual number.

The resonance frequencies of the series arm resonators 21 to 23 are substantially matched to the antiresonance frequencies of the parallel arm resonators 24 and 25. Further, as the piezoelectric substrate forming the SAW resonators 21 to 25 (IDT 31 and reflector 41), for example, a LiTaO ₃ substrate can be used, and electrode fingers 33a and 33b, bus bars 32a and 32b, and connection lines 34a, 34b is made of aluminum. For the reason described later, the crossing length w of the bus bars of the series arm resonators 21 to 23 is set to a value within the range of 5λ to 20λ, and the distance (gap length) Gap between the electrode finger tip and the bus bar facing the electrode fingers. Is a value larger than 1λ. For example, the gap length of the first series arm resonator 21 is 2λ, the gap length of the second series arm resonator 22 is 3λ, and the gap length of the third series arm resonator 23 is 4λ.

  FIG. 3 shows frequency-impedance characteristics of a conventional one-terminal-pair SAW resonator, and FIG. 4 shows a case where a four-stage ladder-type SAW filter is configured by the conventional one-terminal-pair SAW resonator as in this embodiment. It is a conceptual diagram which shows an attenuation characteristic. In the one-terminal-pair SAW resonator, the electrode finger intersection length w is 25λ, and the gap length Gap is 0.2λ.

  On the other hand, FIG. 5 shows the frequency-impedance characteristics of the one-terminal-pair SAW resonator (FIG. 2) used in this embodiment, and FIG. 6 is a conceptual diagram showing the attenuation characteristics of the ladder-type SAW filter 11 according to this embodiment. It is. FIG. 7 is a diagram showing the attenuation characteristics (relationship between gap length Gap and attenuation) of the ladder-type SAW filter 11 according to the present embodiment.

  As can be seen from these figures, when the gap length Gap in the IDT 31 of the series arm resonators 21 to 23 is increased from 0.2λ to 2λ, 3λ, 4λ, and 5λ, the right side of the antiresonance point (high frequency side) A peak appears at this point, and this peak moves to a higher frequency as the gap length Gap increases. When the gap length is 1λ, the peak almost overlaps with the antiresonance point of the main resonance mode. Further, as the attenuation characteristics (FIGS. 6 and 7) of the filter, as the gap length Gap increases, a bulge protruding downward appears in the attenuation area on the high frequency side, the amount of attenuation increases, and the attenuation characteristics are improved. The

  The peak is due to the secondary resonance mode and is considered to be based on the following principle.

  FIG. 8 is a schematic diagram showing the propagation state of the surface wave of the waveguide mode (secondary resonance mode) in the one-terminal-pair SAW resonator of the present embodiment, showing the IDT, gap portion, and bus bar regions from the top. ing.

  Referring to the figure, the surface acoustic wave excited by the IDT is propagated mainly in the electrode finger arrangement direction (left-right direction in the figure), but in actuality, not only this direction but also the direction intersecting this ( (Hereinafter referred to as the horizontal direction). On the other hand, assuming that the velocity of the surface wave in each of the IDT region, the gap region, and the bus bar region is V1, V2, and V3, in the one-terminal pair SAW resonator of the present embodiment, the connection line, bus bar, and electrode finger If the material and film thickness are equal, V1 <V2 and V3 <V2. Therefore, the transverse component 51a of the surface acoustic wave excited in the IDT region is reflected by the boundary between the gap region and the bus bar region having different surface wave velocities, and propagates in the gap region 51b, 51c. Become. This is considered to be the secondary resonance mode used in the present invention.

  For example, when the material or film thickness of the connection line, bus bar, or electrode finger is changed due to other design requirements, for example, the electrode mass of the bus bar part is increased or the duty of the gap part is changed. The filter of the present invention may be configured so as to satisfy the confinement conditions (V1 <V2, V3 <V2) in the gap region of the secondary resonance mode.

  FIG. 9 is a diagram showing the relationship between the IDT intersection length w and the secondary resonance mode. The peak of the secondary resonance mode peak when the gap length is fixed to 2λ and the electrode finger crossing length is changed. It shows a change. The electrode capacity was adjusted logarithmically to be the same.

  As can be seen from the figure, as the crossing length w is increased, the frequency of the secondary resonance mode does not change, but the peak height (impedance) decreases. As a result of the experiment, when the crossing length w is 20λ or more, the secondary resonance mode becomes very small, and the attenuation improvement effect of the present invention by the secondary resonance mode becomes small.

  Furthermore, the impedance ratio between the secondary resonance mode and the main resonance mode was taken, and the relationship between this and the crossing length was examined. The results are as shown in Table 1 below and FIG. The impedance when the gap length is 0.2λ and the crossing length is 25λ is Zref, and the impedance of the secondary resonance mode at the same frequency is Zsub.

  As can be seen from Table 1 and FIG. 10, when the crossing length exceeds 20λ, the amount of increase in impedance due to the secondary resonance mode is as small as about 3%, and the effect of improving the attenuation characteristics is also reduced.

  On the other hand, when the gap length is smaller than 5λ, the anti-resonance impedance of the main resonance mode is deteriorated and the attenuation characteristic of the filter is impaired.

  Therefore, in the present invention, the crossing length of the electrode fingers is set within the range of 5λ to 20λ.

  As described above, in the SAW filter of the present embodiment, the IDT gap length Gap of the series arm resonators 21 to 23 in FIG. 1 is made larger than 1λ. Thereby, the amount of attenuation can be increased and the attenuation characteristic can be improved. The gap length in the first series arm resonator 21 is 2λ, the gap length in the second series arm resonator 22 is 3λ, and the gap length in the third series arm resonator 23 is 4λ. This makes it possible to flatten the attenuation region. In the present invention, the gap length may be other values as long as it is larger than 1λ and not more than 5λ. For the gap lengths of the first to third series arm resonators 21 to 23, other values may be used. A combination of numerical values is also possible.

  Further, FIG. 11 shows an example of a duplexer according to the present invention. The duplexer 71 includes two SAW filters 72 and 73 connected to a common terminal 74 and having different band center frequencies, that is, a low-frequency filter 72 and a high-frequency filter 73. Was formed by the SAW filter 11 according to the embodiment. The common terminal 74 is connected to the antenna terminal.

  In such a duplexer 71, it is possible to obtain a flat and high attenuation characteristic in the pass band of the high frequency side filter 73 with respect to the low frequency side filter 72.

  As mentioned above, although embodiment of this invention was described based on drawing, this invention is not limited to this, A various change can be made within the range as described in a claim.

  For example, the number of series arm resonators and parallel arm resonators (the number of connection stages) and the size / combination of each gap length can be variously set in addition to the above embodiment. In addition, the SAW filter of the present invention is suitable for use in a duplexer of a transmission / reception unit of a cellular phone, but is not limited to this, a PDA having a PHS or a communication function, a wireless LAN card, etc. It can be applied to various electronic communication devices.

It is a circuit diagram which shows the ladder type SAW filter which concerns on embodiment of this invention. It is a figure which shows the 1 terminal pair SAW resonator used with the SAW filter of this embodiment. It is a conceptual diagram which shows the frequency-impedance characteristic of the conventional one terminal pair SAW resonator. It is a conceptual diagram which shows the attenuation characteristic of the conventional ladder type SAW filter. It is a conceptual diagram which shows the frequency-impedance characteristic of the one terminal pair SAW resonator used by this embodiment. It is a conceptual diagram which shows the attenuation characteristic of the ladder type SAW filter which concerns on this embodiment. It is a diagram which shows the attenuation characteristic (relationship between gap length and attenuation amount) of the ladder type SAW filter which concerns on this embodiment. It is a schematic diagram which shows the propagation state of the surface wave of the secondary resonance mode in the one terminal pair SAW resonator of this embodiment. It is a diagram which shows the relationship between the crossing length of IDT, and a secondary resonance mode about this embodiment. It is a diagram which shows the relationship between the crossing length of IDT, and impedance ratio Zref / Zsub of a secondary resonance mode and a main resonance mode about this embodiment. It is a figure which shows an example of the duplexer which concerns on this invention.

Explanation of symbols

11 Ladder type SAW filter 12 Input terminal 13 Output terminal 21, 22, 23 Series arm resonator 24, 25 Parallel arm resonator 31 IDT
32a, 32b Bus bar 33a, 33b Electrode finger 34a, 34b Connection line 41 Reflector 71 Duplexer 72 Low frequency side filter 73 High frequency side filter 74 Common terminal

Claims (5)

A surface acoustic wave filter formed by connecting one or more series arm resonators and one or more parallel arm resonators in a ladder shape,
In the interdigital transducer of at least one series arm resonator, when the distance between one bus bar and the tip of the electrode finger connected to the other bus bar is λ, the period of the electrode finger pair is A surface acoustic wave filter having a large value of 5λ or less. Having two or more series arm resonators,
Among the two or more series arm resonators, the distance in the interdigital transducer of the first series arm resonator is a first value greater than 1λ and less than or equal to 5λ, and the interdigital transducer of the second series arm resonator The surface acoustic wave filter according to claim 1, wherein the distance at is a second value different from the first value greater than 1λ and less than or equal to 5λ. 3. The surface acoustic wave filter according to claim 1, wherein the crossing length of the electrode fingers in the interdigital transducer of the series arm resonator is set to a value in a range of 5λ to 20λ. The surface acoustic wave filter according to any one of claims 1 to 3, wherein the piezoelectric substrate constituting the filter is a LiTaO ₃ substrate.   A duplexer including the surface acoustic wave filter according to claim 1.

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