JPH1079641A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce insertion losses based on the improvement of Q of each surface acoustic wave(SAW) resonator by arranging electrodes in respective unit SAW resonators constituting a ladder-type band pass filter(BPF) as respectively different film thickness. SOLUTION: Each of series arm SAW resonators 10 and parallel arm SAW resonators 20 constituting the ladder-type BPF is composed of forming 1st and 2nd interdigital electrodes on a piezo-electric substrate consisting of crystal, LiTaO3 or the like and arranging a reflector on the outside of these electrodes. The film thickness of the resonator 10 is thinned and the Q of the resonator 10 is improved. The film thickness of the resonator 20 is made larger than that of the resonator 10 and the influence of spurious is reduced to constitute a low loss SAW filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ladder-type band-pass filter using a surface acoustic wave resonator applicable to a high-frequency section (RF section) of a small mobile communication device such as a mobile phone and a mobile phone. is there.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, there are the following. FIG. 9 is an equivalent circuit diagram of a ladder-type bandpass filter using such a conventional surface acoustic wave resonator.

In FIG. 9, a series arm (R _s1 , R _s2 ,
R _s3 ) and the surface acoustic wave resonators of the parallel arms (R _P1 , R _P2 , R _P3 ) have comb-shaped electrodes formed on a piezoelectric substrate made of quartz, LiTaO ₅ or the like. Are formed to have the same thickness.

[0004]

However, the SAW filter
In order to further reduce the insertion loss of the_s1, R
_s2, R_s3) And parallel arms (R_P1, R_P2, R_P3) Surface elasticity
Attempts have been made to reduce the electrode thickness of the wave resonator,
When the electrode thickness is reduced in such a manner, the electrode thickness is reduced.
Spurious due to the pass band
Near the passband or passband and satisfy the required standards.
There is a problem that disappears.

Further, with the recent improvement in performance of SAW filters for mobile phones and the like (particularly, the same characteristics as those of dielectric filters using a quarter-wave dielectric coaxial resonator with a short-circuited termination), the SAW filters have become even lower. Insertion loss is required. The present invention achieves a low insertion loss based on a higher Q of each surface acoustic wave resonator by changing the electrode thickness of each surface elastic resonator of the series arm and the parallel arm constituting the SAW filter. It is an object to provide a surface acoustic wave filter that can be obtained.

[0006]

According to the present invention, there is provided a ladder-type band-pass filter using a surface acoustic wave resonator, wherein each unit surface constituting the ladder-type band-pass filter is provided. The electrodes of the elastic wave resonator are arranged with different thicknesses.

(2) The surface acoustic wave filter according to (1), wherein the film thickness of the electrodes of the serial arm surface acoustic wave resonator is smaller than the film thickness of the electrodes of the parallel arm surface acoustic wave resonator. It is. By the way, in a resonator type SAW filter, it is known that the series arm has an attenuation pole frequency in a higher attenuation band of a pass band, and the parallel arm has an attenuation pole number in a lower attenuation band of a pass band. . Therefore, the insertion loss is most affected by the series arm in the higher passband of the passband, and is most affected by the parallel arm in the lower passband of the passband.

In the serial arm surface acoustic wave resonator, Q depends on the film thickness of the electrode forming the serial arm. In the parallel arm surface acoustic wave resonator, Q depends on the film thickness of the electrode forming the parallel arm. Is known to be small. Therefore, the present invention focuses on the fact that Q depends on the film thickness of the electrodes of the serial arm surface acoustic wave resonator, and that Q has a small amount depending on the film thickness of the electrodes of the parallel arm surface acoustic wave resonator. By making the electrode thickness of the series arm surface acoustic wave resonator different from the electrode thickness of the parallel arm surface acoustic wave resonator to increase the Q of the series arm surface acoustic wave resonator, a high-performance resonance type surface acoustic wave filter is provided. To achieve.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a ladder-type bandpass filter using a surface acoustic wave resonator according to an embodiment of the present invention, in which three series arm surface acoustic wave resonators and three parallel arm surface acoustic wave resonators are used. 5 shows a ladder-type bandpass filter composed of five stages. FIG. 2 is a lumped parameter equivalent circuit diagram of the ladder type bandpass filter, FIG. 3 is a circuit diagram of the series arm surface acoustic wave resonator, and FIG. 4 is a lumped parameter equivalent circuit diagram of the series arm surface acoustic wave resonator. 5 is a plan view showing a pattern of the serial arm surface acoustic wave resonator, FIG. 6 is a circuit diagram of the parallel arm surface acoustic wave resonator, FIG. 7 is a lumped constant equivalent circuit diagram of the parallel arm surface acoustic wave resonator, FIG. 8 is a plan view showing a pattern of the parallel arm surface acoustic wave resonator.

As shown in FIG. 5, a series-arm surface acoustic wave resonator 10 has a first comb-shaped electrode 11 and a second comb-shaped electrode 12 on a piezoelectric substrate 1 made of quartz, LiTaO ₃ or the like. A reflector 1 is formed outside these comb-shaped electrodes 11 and 12.
3 is arranged. As shown in FIG. 8, the parallel arm surface acoustic wave resonator 20 forms a first comb-shaped electrode 21 and a second comb-shaped electrode 22 on the same piezoelectric substrate 1, and these comb-shaped electrodes 21 The reflector 23 is arranged outside the and 22. These electrodes are, for example, Al-Cu
It is realized by a membrane.

The above reflector has a closed structure, but may have an open structure.
Normally, the characteristic simulation of the ladder-type bandpass filter shown in FIG. 1 is evaluated by converting it into a lumped-constant equivalent circuit shown in FIG. With the recent increase in the performance of SAW filters for mobile phones and the like (particularly, the same characteristics as dielectric filters using a quarter-wave dielectric coaxial resonator with a short-circuited termination), SAW filters have been further reduced in insertion loss. Has been requested. This SAW
As measures to reduce the insertion loss of the filter, it is conceivable to increase the Q of each surface acoustic wave resonator and widen the pass band.

Table 1 shows a comparison of Q obtained from characteristic measurement by each of the single resonator circuits shown in FIGS. 3 to 5 and 6 to 8.

[0013]

[Table 1]

As is apparent from Table 1, the Q of the serial arm surface acoustic wave resonator increases as the film thickness decreases. That is, at 3000 °, Q = 1225.3, and at 5000 °, Q = 530, which is a difference of about 2.3 times. But,
The parallel arm surface acoustic wave resonator has Q = 617.4 at 4000 °.
And the highest. Furthermore, at 3000 °, Q = 541.7,5
At 000 °, Q = 579.3 and the difference in Q due to the difference in film thickness is small.

However, it is known that the presence of spurious components cannot be ignored when the thickness of the surface acoustic wave resonator is reduced. 3 to 5 and 6 to 6 as in the case of Table 1.
Table 2 shows the results of spurious measurement by the measurement circuit of FIG. 8 in the series arm surface acoustic wave resonator and the parallel arm surface acoustic wave resonator (1).
The relationship between the film thickness (20: 120 μm) and the position and size of the spurious is shown.

[0016]

[Table 2]

Since the spurious component of the 5000 ° serial arm surface acoustic wave resonator exists in the higher band of the attenuation pole, this band is negligible because it is an attenuation band. However, although the spurious of the parallel arm resonator exists in the higher band of the attenuation pole, it cannot be ignored since this band is a pass band. The present invention reduces the thickness of the serial arm surface acoustic wave resonator to increase the Q of the serial arm surface acoustic wave resonator, and adjusts the film thickness of the parallel arm surface acoustic wave resonator to that of the serial arm surface acoustic wave resonator. The low-loss SAW filter is configured to be thicker than the film thickness and to reduce the influence of spurious components.

As shown in FIG. 1, in the ladder type band pass SAW filter of the present invention, the film thickness of the three series arm surface acoustic wave resonators ( _Rs1 , _Rs2 , _Rs3 ) is selected to be 3000 °. , The thickness of the three parallel arm resonators (R _P1 , R _P2 , R _P3 ) is selected to be 4000 ° to reduce the size of spurious components and reduce SAW with low insertion loss.
Filter realized.

As described above, in a ladder-type band-pass filter using a surface acoustic wave resonator, by making the electrode thickness of each unit surface-acoustic-wave resonator constituting the ladder-type band-pass filter different, the spurious response is reduced. Low insertion loss of the surface acoustic wave filter can be reduced. Further, the present invention is not limited to the above-described embodiments, and various modifications are possible based on the gist of the present invention, and these are not excluded from the scope of the present invention.

[0020]

As described above, according to the present invention, the following effects can be obtained. (1) In a ladder-type bandpass filter using a surface acoustic wave resonator, by making the electrode thickness of each unit surface acoustic wave resonator constituting the ladder-type bandpass filter different, a surface acoustic wave with less spurious is provided. Low insertion loss of the filter can be achieved.

(2) By forming the film thickness of the serial arm surface acoustic wave resonator thinner than the film thickness of the parallel arm surface acoustic wave resonator, the size of spurious components is reduced, and a SAW filter having a low insertion loss is obtained. Can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a ladder-type bandpass filter using a surface acoustic wave resonator according to an embodiment of the present invention.

FIG. 2 is a lumped-constant equivalent circuit diagram of a ladder-type bandpass filter using a series surface acoustic wave resonator according to an embodiment of the present invention.

FIG. 3 is a circuit diagram of a series arm surface acoustic wave resonator showing an embodiment of the present invention.

FIG. 4 is a lumped constant equivalent circuit diagram of a series arm surface acoustic wave resonator showing an embodiment of the present invention.

FIG. 5 is a plan view showing a pattern of a serial arm surface acoustic wave resonator showing an embodiment of the present invention.

FIG. 6 is a circuit diagram of a parallel arm surface acoustic wave resonator showing an embodiment of the present invention.

FIG. 7 is a lumped constant equivalent circuit diagram of a parallel arm surface acoustic wave resonator showing an embodiment of the present invention.

FIG. 8 is a plan view showing a pattern of a parallel arm surface acoustic wave resonator showing an embodiment of the present invention.

FIG. 9 is an equivalent circuit diagram of a ladder-type bandpass filter using a conventional surface acoustic wave resonator.

[Explanation of symbols]

First piezoelectric substrate 10 series-arm surface acoustic wave resonator _{(R S1, R S2, R} S3) 11,21 first comb-like electrodes 12 and 22 the second comb-shaped electrodes 13, 23 reflector 20 parallel arm surface Elastic wave resonator ( _RP1 , _RP2 , _RP3 )

 ────────────────────────────────────────────────── ─── Continued from the front page (72) Inventor Tomokazu Komazaki 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd.

Claims (2)

[Claims] 1. A ladder-type bandpass filter using a surface acoustic wave resonator, wherein each unit surface acoustic wave resonator constituting the ladder-type bandpass filter has a different electrode thickness. Surface acoustic wave filter. 2. The surface acoustic wave filter according to claim 1, wherein the film thickness of the electrodes of the series arm surface acoustic wave resonator is smaller than the film thickness of the electrodes of the parallel arm surface acoustic wave resonator. Surface acoustic wave filter.