JP2003347895A - Saw filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a means for flattening ripples in a passing band of a cascade-connected SAW (surface acoustic wave) filter, in which dual mode SAW filters are connected in cascade, and input and output terminals are of a balanced type. <P>SOLUTION: In the cascade-connected SAW filter, a first dual mode SAW filter, provided with three IDT (interdigital transducer) electrodes on a piezoelectric substrate and reflectors on both the sides of the IDT electrodes, and a second dual mode SAW filter, constituted in the same way as the first filter are connected in cascade, and the input and output terminals are each a balanced type. The SAW filter is constituted, in such a way that interdigital electrodes near an end of the piezoelectric substrate of the IDT electrodes on both the outer sides of the first and second SAW filters and interdigital electrodes near the center are connected with each other, and the connection portions are protected against grounding. <P>COPYRIGHT: (C)2004,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cascade connection type SAW.
The present invention relates to a filter, and more particularly to a SAW filter with improved ripple in a bass band.

[0002]

2. Description of the Related Art In recent years, SAW filters have been widely used in the field of communications, and have excellent characteristics such as high performance, small size, and mass productivity.
Among them, a resonator type SAW filter has a feature that insertion loss is small, and thus is widely used for wireless devices. FIG. 6A shows one of the resonator type filters of a primary-tertiary longitudinally coupled dual-mode SAW filter (hereinafter, referred to as a dual-mode SAW filter) using first- and third-order longitudinal modes. FIG. 3 is a plan view showing a configuration. That is, the IDT electrodes 21 are disposed on the main surface of the piezoelectric substrate 20 along the propagation direction of the surface wave.
22 and 23 are arranged close to each other, and grating reflectors (hereinafter, referred to as reflectors) 24a and 24b are arranged on both sides of the IDT electrodes 21, 22 and 23. IDT
Each of the electrodes 21, 22, and 23 is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween.
And one of the IDT electrodes 21 at the center,
The input terminal IN is connected to the input terminal IN using, for example, wire bonding or the like, and the other interdigital electrode is grounded. Further, one of the IDT electrodes 22 and 23 at both ends is connected to each other by a lead electrode formed on the piezoelectric substrate 20, and this is connected to the output terminal OUT, and the other comb electrode is grounded. By doing so, a dual mode SAW filter is configured.

[0003] The pass bandwidth of the dual mode SAW filter depends on the difference (f1-f3) between the resonance frequency f1 of the first mode and the resonance frequency f3 of the third mode. Here, FIG.
As shown in (a), the widening of the outermost electrode finger of the central IDT electrode 21 is an effective means for increasing the pass bandwidth of the dual mode SAW filter.

The input / output balanced cascade-connected SAW filter shown in FIG. 6 (b) increases the steepness of the filter characteristics and increases the guaranteed attenuation, as shown in FIG. 6 (a). The double mode SAW filters are cascaded in two stages, and two comb-shaped electrodes constituting the central IDT electrode of each filter are connected to the input terminals IN1, 2 and the output terminals OUT1, OUT2.
Thus, a balanced filter is obtained. That is,
The IDTs 25, 26, 27 are arranged close to each other, and the reflectors 28a, 28
b to form a dual mode SAW filter F1 (filter F1). Then, IDTs 25 ', 26', 27 'formed in the same manner as the filter F1 and a reflector 28'
a, and a filter F2 provided with 28'b is formed at a predetermined interval. Further, each comb-shaped electrode of the center IDT electrode 25 of the filter F1 and two input terminals IN
1 and 2 are connected by means such as wire bonding, and the respective comb-shaped electrodes of the center IDT electrode 25 'of the filter F2 are connected to the two output terminals OUT1 and OUT2. Further, the piezoelectric substrates 2 of the outer IDT electrodes 26, 27 and 26 ', 27' of the filters F1, 2 respectively.
Of the comb-shaped electrodes closer to the end with respect to the piezoelectric substrate 20.
Connect each with the lead electrode formed above,
The gap side (center toward the piezoelectric substrate 20)
Comb-shaped electrodes are connected by lead electrodes L1 and L2,
Each of them is grounded to form an input / output balanced cascade-connected SAW filter.

FIG. 7 shows a piezoelectric filter using the electrode pattern of the input / output balanced cascade connection SAW filter shown in FIG. 6 (b) with a center frequency of 810 MHz, a bandwidth of 17 MHz, and an input / output impedance of 200Ω. IDT electrode 25 (25 ') using 39 ° YX LiTaO ₃ for the substrate
Are 27.5 pairs, and the IDT electrodes 26 and 27 (26 ′,
27 ') is 18.5 pairs, and the reflectors 28a, 28b
The number of (28'a, 28'b) is 140 and the crossover length is 1
The filter characteristic when 6λ (λ is the electrode period of the IDT electrode) is set. The broken line is the pass band characteristic, and the solid line is the characteristic in the attenuation region.

[0006]

However, FIG.
The filter characteristic of the input / output balanced cascaded SAW filter as shown in FIG. 7B has a problem that a ripple occurs in the pass band as shown in FIG. 7 and the communication quality of the wireless device is largely deteriorated. Was. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an input / output balanced cascaded SAW filter having a flat characteristic in a pass band.

[0007]

According to a first aspect of the present invention, there is provided a SAW filter according to the present invention.
A first double mode SAW filter having two IDT electrodes and grating reflectors on both sides of the IDT electrodes is juxtaposed with a predetermined gap therebetween, and the first double mode SW filter is provided.
While the outer comb electrodes of the comb electrodes constituting the IDT electrodes on both outer sides of the AW filter are respectively connected to those of the second dual mode SAW filter by lead electrodes,
Comb-shaped electrodes on the gap side (close to the center) are connected to the lead electrodes L1, L
2, the two comb-shaped electrodes constituting the central IDT electrode of the first dual-mode SAW filter are respectively used as balanced input terminals, and the central IDT electrode of the second dual-mode SAW filter is constituted. This is a cascade connection type dual mode SAW filter using two comb electrodes as balanced output terminals, wherein the lead electrodes L1 and L2 are not grounded. The invention according to claim 2 is the cascade connection SAW filter according to claim 1, wherein the lead electrodes L1 and L2 are connected to each other by a lead electrode L formed on the piezoelectric substrate. According to a third aspect of the present invention, there are provided three IDT electrodes on the main surface of the piezoelectric substrate along the propagation direction of the surface wave, and
A first type having a grating reflector on both sides of a DT electrode
And the second dual-mode SAW filter are juxtaposed with a predetermined gap therebetween, and the outer comb-shaped electrodes constituting the IDT electrodes on both outer sides of the first dual-mode SAW filter are replaced with the second comb-shaped electrodes. Of the dual mode SAW filter of FIG.
The IDT electrodes on both sides of the second dual mode SAW filter are formed by using the lead electrode R1 to connect the comb electrodes on the gap side (close to the center) of the IDT electrodes on both sides of the W filter. The interdigital electrodes near the center of the interdigital electrodes are connected to each other by a lead electrode R2, and the two interdigital electrodes constituting the central IDT electrode of the first dual mode SAW filter are respectively balanced input terminals. A cascaded double-mode SAW filter in which two comb electrodes constituting a central IDT electrode of a second dual-mode SAW filter have balanced output terminals, respectively, wherein the lead electrodes R1 and R2 are not grounded. Characteristic SAW
Filter. The invention according to claim 4 is the cascade-connected SAW filter according to any one of claims 1 to 3, wherein one of the input / output terminals is an unbalanced type.

[0008]

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 an input / output balanced cascade connection SAW filter according to a first embodiment of the present invention. That is, the IDT electrodes 2, 3, and 4 are arranged close to each other on the main surface of the piezoelectric substrate 1 along the propagation direction of the surface wave, and the reflectors 5a and 5b are arranged on both sides of the IDT electrodes 2, 3, and 4. Set up dual mode SAW
A filter F1 (filter F1) is formed. Further, the IDT electrodes 2, 3, 4 and the reflector 5 are formed on the same piezoelectric substrate 1.
IDT electrodes 2 ′, 3 ′ having substantially the same configuration as a and 5b,
4 ′ and a filter F2 composed of reflectors 5′a and 5′b
Is formed, and is juxtaposed with the filter F1 with a predetermined gap. IDT electrodes 2, 3, 4 of filter F1 (F2)
(2 ′, 3 ′, 4 ′) is composed of a pair of interdigital electrodes each having a plurality of electrode fingers interposed therebetween, and one of the interdigital electrodes of the central IDT electrode 2 (2 ′) and the input terminal. IN1 (output terminal OUT1) is connected to the other comb-shaped electrode and input terminal IN2 (output terminal OUT2) by means such as wire bonding. Further, the IDT electrodes 3, 4, 3 ', 4' on both outer sides of the filters F1, F2.
Comb-shaped electrodes located near the end of the piezoelectric substrate 1 are connected to each other, and the IDT electrodes 3, 4, 3 ', and 4' are located on the gap side (close to the center with respect to the piezoelectric substrate 1). The comb-shaped electrodes are connected by lead electrodes formed on the same piezoelectric substrate 1 to form an input / output balanced cascaded SAW filter.

An input / output parallel cascade connection SA according to the present invention
The W filter is similar to the configuration of the conventional two-stage cascaded SAW filter, but has two IDT electrodes 3 and 4 on both sides of the dual mode SAW filter F1 and IDT electrodes on both sides of the dual mode SAW filter F2. When connecting the comb-shaped electrodes of the electrodes 3 'and 4' with the lead electrodes formed on the piezoelectric substrate, each of the lead electrodes is configured without grounding.

FIG. 3 is a plan view showing a configuration of a modified example of the embodiment of the present invention, and different points from FIG. 1 will be described. The IDT electrodes 3, 3 'located outside the cascaded SAW filter of the input / output balanced type, and the IDT electrodes 4, 4', the comb-shaped electrodes located closer to the center with respect to the piezoelectric substrate 1, have the same busbar. Lead electrode L formed on piezoelectric substrate 1
1 and L2, respectively, but without connecting any of them, the lead electrodes L1 and L2 are connected by a lead electrode L formed on a piezoelectric substrate. FIG. 4 shows a filter having a center frequency of 810 MHz, a bandwidth of 17 MHz, and an input / output impedance of 200 Ω, and a piezoelectric substrate of 39 ° YX LiTaO ₃ using the electrode pattern of FIG.
And the logarithm of the IDT electrode 25 (25 ') is 27.5.
18.5 pairs of IDT electrodes 26, 27 (26 ', 27') and reflectors 28a, 28b (28'a, 28 ').
In b), filter characteristics when a prototype is manufactured with the number of lines set to 140 and the crossover length set to 16λ, where the broken line is the passband characteristic and the solid line is the characteristic in the attenuation region. Although it is clear from the passband characteristic shown by the broken line in this figure that the ripple in the passband has been greatly improved, the guaranteed attenuation on the higher frequency side than the center frequency is slightly smaller than the characteristic shown in FIG. It turned out to be inferior.

FIG. 5 is a plan view showing the configuration of another modification of the embodiment according to the present invention, and the points different from the configuration of FIG. 1 will be described. That is, as shown in FIG.
The IDT electrodes 3 and 4 on both outer sides of 1 (F2) and the IDT electrodes 3 ′ and 4 ′ are formed on the same piezoelectric substrate 1. Although the connection is made by the lead electrodes R1 and R2, the bus bars of the comb-shaped electrodes of the filters F1 and F2 are not connected at the center of the piezoelectric substrate 1. With such a configuration, it has been found that although the guaranteed attenuation is slightly inferior to that of FIG. 2, the passband characteristics, particularly the ripple, of the cascaded SAW filter are greatly improved.

[0012] As described above, the I that has conventionally been considered common sense
When the lead electrodes L1 and L2, which connect the bus bars of the comb-shaped electrodes of the DT electrodes 3, 3 'and the IDT electrodes 4, 4' near the center of the piezoelectric substrate 1 and L2, are electrically disconnected from the ground, FIGS. It has been found that the ripple in the pass band is also greatly improved as shown in FIG. It has been found that the guaranteed attenuation slightly differs between the respective configurations of FIGS.

In the above, an example in which both input and output terminals are of a balanced type has been described. However, in FIGS.
IN2 is grounded, and output terminals OUT1, O2
The present invention can also be applied to an input / output unbalanced cascaded SAW filter in which one of the UTs 2 is grounded. Also, FIG.
An input unbalanced-output balanced filter in which one of the input terminals IN1 and IN2 of the input terminals 3 and 5 is grounded, or the output terminal OU
The present invention is also applicable to the case of an input balanced-output unbalanced filter in which one of T1 and T2 is grounded. In addition, although lithium tantalate has been described as the piezoelectric substrate, it is needless to say that the present invention can be applied to piezoelectric substrates such as lithium niobate, lithium tetraborate, and langasite.

[0014]

As described above, the present invention is constructed as described above. According to the first aspect of the present invention, there is provided a filter having a balanced input / output terminal, a small ripple in a pass band, and a large guaranteed attenuation. To provide an excellent effect. Since one of the input / output terminals is unbalanced, the invention according to claim 2 can be applied to the case where the IF unit of the wireless device is a balanced-unbalanced type or an unbalanced-balanced type. The ripples within represent the excellent effect of providing a SAW filter.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration of an input / output balanced cascade connection SAW filter according to an embodiment of the present invention.

FIG. 2 is a diagram showing pass band characteristics and attenuation band characteristics of an input / output balanced cascade connection SAW filter according to the present invention.

FIG. 3 is a plan view showing a configuration of an input / output balanced cascade connection SAW filter which is a modification of the embodiment according to the present invention.

FIG. 4 is a diagram illustrating passband characteristics and attenuation region characteristics of a balanced cascade connection SAW filter configured using the electrode patterns of FIG. 3;

FIG. 5 is a plan view showing a configuration of an input / output balanced cascade connection SAW filter as another modification of the embodiment according to the present invention.

6A is a diagram illustrating a configuration of a conventional dual mode SAW filter, and FIG. 6B is a diagram illustrating a configuration of a conventional input / output balanced cascade connection SAW filter.

FIG. 7 is a diagram showing pass band characteristics and attenuation region characteristics of a conventional input / output balanced cascade connection SAW filter.

[Explanation of symbols]

1. Piezoelectric substrates 2, 3, 4, 2 ', 3', 4 'IDT electrodes 5a, 5b Grating reflectors F1, F2 Primary-tertiary longitudinally coupled dual mode SAW filter L, L1, L2, R1, R2 ... lead electrode

Claims (4)

[Claims] 1. A first dual mode SAW filter having three IDT electrodes on a main surface of a piezoelectric substrate along a propagation direction of a surface wave and grating reflectors on both sides of the IDT electrodes is provided. And the outer comb electrodes of the IDT electrodes on both outer sides of the first dual mode SAW filter are connected to those of the second dual mode SAW filter and the lead electrodes. While the comb-shaped electrodes on the gap side (close to the center) are connected by the lead electrodes L1 and L2, respectively.
The two comb-shaped electrodes constituting the central IDT electrode of the AW filter are respectively used as balanced input terminals, and the second double mode S
A cascaded dual mode SAW filter in which two comb electrodes constituting a central IDT electrode of an AW filter each have balanced output terminals, wherein the lead electrodes L1, L2
A SAW filter characterized by not grounding. 2. The cascaded SAW filter according to claim 1, wherein the lead electrodes L1 and L2 are connected to each other by a lead electrode L formed on the piezoelectric substrate. 3. A first and second dual mode SAW having three IDT electrodes on a main surface of a piezoelectric substrate along a propagation direction of a surface wave and grating reflectors on both sides of the IDT electrodes. Filters are juxtaposed with a predetermined gap,
The outer comb electrodes of the IDT electrodes on both outer sides of the first dual mode SAW filter are respectively connected to those of the second dual mode SAW filter by the lead electrodes, while the second comb electrodes are connected to the second double mode SAW filter. Of the comb electrodes constituting the IDT electrodes on both outer sides of the dual mode SAW filter, the comb electrodes on the gap side (closer to the center) are connected by the lead electrode R1.
Of the comb-shaped electrodes forming the IDT electrodes on both outer sides of the second dual-mode SAW filter, the comb-shaped electrodes closer to the center are connected by lead electrodes R2, respectively, and the center IDT of the first double-mode SAW filter is connected. A cascaded double-mode SAW in which two comb electrodes constituting electrodes are respectively balanced input terminals, and two comb electrodes constituting a central IDT electrode of the second double mode SAW filter are each balanced output terminals. A filter, wherein said lead electrode R1,
A SAW filter, wherein R2 is not grounded. 4. The cascaded SAW filter according to claim 1, wherein one of the input / output terminals is an unbalanced type.