JPH08321743A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE: To obtain the surface acoustic wave filter in which generation of spurious radiation at a high frequency attenuation region while ensuring a characteristic required for insertion loss and out-band suppression and eliminating generation of harmonic ripple components in a pass band in the case of a broad band filter specification. CONSTITUTION: The filter is a composite surface acoustic wave filter having one filter function unit consisting of a resonator connecting serially electrically to a signal line and other resonator connecting electrically in parallel with the signal line each comprising an interdigital electrode and a grating reflector 2b are provided onto a piezoelectric substrate. A duty ratio of the grating reflector 2b for the resonator connected in series with the signal line and/or the resonator connecting in parallel is selected to be 40-48%.

Description

Detailed Description of the Invention

[0001]

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.

[0002]

2. Description of the Related Art With the widespread use of mobile phones and car phones in recent years, there is an increasing need for compact high-performance high frequency filters. As a high-frequency filter, a dielectric filter has been conventionally known, but a surface acoustic wave filter is more suitable for miniaturization and higher performance, and further, it has low loss and does not require a matching circuit. Composite surface acoustic wave filters are receiving attention. This composite surface acoustic wave filter
As shown in FIG. 10, one filter function unit 3 is provided with two one-port resonators 2 each including a comb-shaped electrode 2a and grating reflectors 2b and 2b on a piezoelectric substrate (LiNbO ₃ etc.). The 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 one-port resonator 2 is electrically parallel to the signal line. (This is referred to as a parallel arm resonator 2P).

This composite type surface acoustic wave filter realizes a bandpass filter by utilizing the difference in impedance between the series arm resonator 2S and the parallel arm resonator 2P.
The principle will be briefly described below 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 antiresonance frequency of the parallel arm resonator 2P is f _ap , the resonance frequency thereof is f _rP , and the antiresonance of the series arm resonator 2S. _Assuming that the frequency is f _aS and the resonance frequency thereof is f _rS , as shown in FIG.
_{By making ap} and the resonance frequency f _rS substantially coincident with each other, a filter characteristic having the antiresonance frequency f _aS and the resonance frequency f _rP as the poles is obtained with the coincidence point as the center, as shown in FIG. .

The condition for the pass band is | X _S | <
│X _P │, and the condition for the attenuation range is │X _S │> │X
_P | Further, the bandwidth of the filter is determined by the difference between the resonance frequency f _rP and the anti-resonance frequency f _{aS. Therefore} , in order to realize the bandwidth based on the specifications, the resonance frequency f _rP and the anti-resonance frequency are required in designing the filter. The difference in f _aS may be adjusted appropriately.

FIG. 12 is a longitudinal sectional view showing a portion of the grating reflector 2b of the one-port resonator 2. The grating reflector 2b has a reflection element 2b ₁ ...
The reflective elements 2b ₁ are arranged in a wavelength cycle.
The ratio (duty ratio) of the space portion 2b ₂ to the space portion 2b ₂ was set to 50% from the viewpoint of reducing insertion loss and suppressing out-of-band.

[0006]

However, in the composite type surface acoustic wave filter including the 1-port resonator 2 having the above-mentioned conventional grating reflector 2b, the conventional surface acoustic wave filter shown in FIG.
As shown in (b), spurious 100 is
Occurs. Further, in the case of the wide band filter specification, ripple 101 is further generated in the high frequency part in the pass band,
It had a drawback that signal disturbance occurred. The cause of the spurious 100 is that the pass characteristic of the series arm resonator 2S causes the spurious 102 on the high frequency side of the anti-resonance point, as shown in FIG. 101 is caused by the parallel arm resonator 2P
This is because, as shown in (a) of the same figure, the spurious characteristic of (3) causes spurious 103 on the high frequency side of the anti-resonance point in the pass band of the series arm resonator 2S.

In view of the above circumstances, the present invention suppresses the occurrence of spurious in the high frequency attenuation region while ensuring the characteristics required for insertion loss and out-of-band suppression, and when a wide band filter specification is adopted. An object of the present invention is to provide a surface acoustic wave filter capable of eliminating the occurrence of ripples in the high frequency part in the pass band.

[0008]

In order to solve the above-mentioned problems, a surface acoustic wave filter of the present invention comprises two resonators each having a comb-shaped electrode and a grating reflector on a piezoelectric substrate. Connect the resonator electrically to the signal line in series,
A composite type surface acoustic wave filter comprising one or more filter functional units in which the other resonator is electrically connected in parallel to a signal line, wherein the resonator connected in series and / or
Alternatively, the duty ratio of the grating reflectors of the resonators connected in parallel is set to 40 to 48%.

[0009]

According to the above construction, the maximum effect of reducing the insertion loss when the duty ratio of the reflecting element is 50%,
Or although the maximum effect of improving out-of-band suppression cannot be obtained,
Since spurious in the resonator connected in series and / or ripple in the resonator connected in parallel is suppressed more than when the duty ratio of the reflective element is set to 50%, the signal disturbance is reduced. Further, since the lower limit value of the duty ratio is set to 40%, the insertion loss and the out-of-band suppression are not extremely deteriorated.

[0010]

The present invention will be described below with reference to the drawings showing the embodiments thereof.

The feature of the present invention lies in the duty ratio of the reflecting element of the grating reflector, and the basic structure of the surface acoustic wave filter is the same as that of the conventional example shown in FIG. Therefore, the description of the basic configuration will be omitted, and the same functional parts as the functional parts shown in the conventional example will be denoted by the same reference numerals and will be briefly described.

FIG. 1 is a longitudinal sectional view showing a grating reflector 2b portion of a one-port resonator 2 in a surface acoustic wave filter of the present invention. Grating reflector 2b
Are the reflective elements 2b ₁ ... Arranged in a 1/2 wavelength period, and the reflective elements 2b ₁ ...
The ratio (duty ratio) of b ₂ is set to 40 to 48%.

FIG. 2A shows a series arm in which the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 50% and the duty ratio of the grating reflector 2b of the series arm resonator 2S is 44%. The pass characteristics of the resonator 2S and the parallel arm resonator 2P are shown respectively. Then, FIG. 6B shows the pass characteristics of the composite surface acoustic wave filter configured by using the series arm resonator 2S and the parallel arm resonator 2P having the pass characteristics shown in FIG. There is.

The surface acoustic wave filter has a center frequency of 902.5 MHz and a bandwidth of 25 MHz. Therefore, the width of the reflective element (electrode portion) 2b ₁ of the grating reflector 2b of the series arm resonator 2S is 1.05.
μm, and the width of the space portion 2b ₂ is 1.33 μm. Moreover, as the piezoelectric substrate 1 was used LiNbO ₃ (substrate acoustic velocity = 4419M) substrate 64 ° Y-X (Y-cut X-propagation).

As is clear from FIG. 2 described above, since the spurious 102 of the series arm resonator 2S which causes the spurious 100 of the surface acoustic wave filter is reduced, the spurious 100 of the surface acoustic wave filter is reduced. Further, in the case of FIG. 2, the specifications are such that the bandwidth is narrowed, and the spurious 103 in the parallel arm resonator 2P is located outside the pass band of the filter, so no ripple occurs.

FIG. 3A shows the pass characteristics of the series arm resonator 2S and the parallel arm resonator 2P when the duty ratio of the grating reflector 2b of the parallel arm resonator 2P and the series arm resonator 2S is 44%. Are shown respectively. And
FIG. 11B shows the pass characteristic of the composite surface acoustic wave filter configured by using the series arm resonator 2S and the parallel arm resonator 2P having the pass characteristics shown in FIG.

The surface acoustic wave filter has a center frequency of 933.5 MHz and a bandwidth of 33 MHz.
In addition, the piezoelectric substrate 1 is 64 ° Y-X as in the case of FIG.
(Y-cut X-axis propagation) LiNbO ₃ (Substrate sound velocity = 4419
m) Substrate.

As is apparent from FIG. 3 described above, since the spurious 102 of the series arm resonator 2S that causes the spurious 100 of the surface acoustic wave filter is reduced, the spurious 100 of the surface acoustic wave filter is reduced. on the other hand,
In the case of FIG. 3, since the bandwidth is as large as 33 MHz,
Spurious 103 of parallel arm resonator 2P is ripple 101
However, since the spurious 103 of the parallel arm resonator 2P, which causes the ripple 101, is also reduced, the ripple 101 hardly occurs.

Next, the upper and lower limits of the duty ratio of the grating reflector 2b of the series arm resonator 2S will be considered.

In FIG. 4A, the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 50%, and the duty ratio of the grating reflector 2b of the series arm resonator 2S is 40% (lower limit of the present invention). The respective pass characteristics of the series arm resonator 2S and the parallel arm resonator 2P in the case of are shown. Then, FIG. 6B shows the pass characteristics of the composite surface acoustic wave filter configured by using the series arm resonator 2S and the parallel arm resonator 2P having the pass characteristics shown in FIG. There is. The other conditions are the same as in the case of FIG.

In the case of FIG. 4, the duty ratio of the grating reflector 2b of the series arm resonator 2S is smaller than the duty ratio (44%) of the grating reflector 2b of the series arm resonator 2S of FIG. 2 (44%). %), The spurious 102 of the series arm resonator 2S, which causes the spurious 100 of the surface acoustic wave filter, is further reduced as compared with the case of FIG. However, the minimum insertion loss is 2 dB in the case of FIG. 2, whereas it is 5 dB in the case of FIG. When the surface acoustic wave filter is used as a high frequency filter, it cannot be said to be practical if the minimum insertion loss is less than 5 dB. Therefore, the duty ratio of the grating reflector 2b of the series arm resonator 2S is set to 40% or more from the viewpoint of the insertion loss.

In FIG. 5A, the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 50%, and the duty ratio of the grating reflector 2b of the series arm resonator 2S is 48% (the upper limit of the present invention). The respective pass characteristics of the series arm resonator 2S and the parallel arm resonator 2P in the case of are shown. Then, FIG. 6B shows the pass characteristics of the composite surface acoustic wave filter configured by using the series arm resonator 2S and the parallel arm resonator 2P having the pass characteristics shown in FIG. There is. The other conditions are the same as in the case of FIG.

In the case of FIG. 5, the duty ratio of the grating reflector 2b of the series arm resonator 2S is made larger than the duty ratio (44%) of the grating reflector 2b of the series arm resonator 2S in the case of FIG. (48%), so
The degree of reduction of the spurious 102 of the series arm resonator 2S that causes the spurious 100 of the surface acoustic wave filter is shown in FIG.
2 is 20 dB in the case of FIG. 2, whereas it is 15 dB in the case of FIG. Spurious 10
When 2 is larger than this, there is a problem that noise is picked up when it is used in a device. Therefore, for the grating reflector 2b of the series arm resonator 2S, the duty ratio is set to 48% or less from the viewpoint of the insertion loss. did.

Similar to FIGS. 4 and 5, FIG. 8 is a graph focusing on the duty ratio of the grating reflector 2b of the series arm resonator 2S, and was created to find an appropriate range of the duty ratio. It is a thing. In this graph, the horizontal axis represents the duty ratio, the left vertical axis represents the insertion loss, and the right vertical axis represents the spurious. As can be seen from this graph, the spurious can be suppressed to 15 dB or less by setting the duty ratio to 48% or less. On the other hand, by setting the duty ratio to 40% or more, the insertion loss can be suppressed to 5 dB or less. Therefore,
Duty ratio is 40 to 48% in series arm resonator 2S
Within the range, it is possible to simultaneously satisfy the predetermined criteria in both insertion loss and spurious.

Next, the upper and lower limits of the duty ratio of the grating reflector 2b of the parallel arm resonator 2P will be considered.

FIG. 6A shows a series arm in which the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 40% and the duty ratio of the grating reflector 2b of the series arm resonator 2S is 44%. The pass characteristics of the resonator 2S and the parallel arm resonator 2P are shown respectively. Then, FIG. 6B shows the pass characteristics of the composite surface acoustic wave filter configured by using the series arm resonator 2S and the parallel arm resonator 2P having the pass characteristics shown in FIG. There is. The other conditions are the same as in the case of FIG. 3 (bandwidth 93
3.5 MHz).

In the case of FIG. 6, the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is smaller than the duty ratio (44%) of the grating reflector 2b of the parallel arm resonator 2P in the case of FIG. (40%), so
Although the degree of reduction of the ripple 101 of the surface acoustic wave filter is better, the performance of out-of-band suppression is worse than that in the case of FIG. When the surface acoustic wave filter is a high frequency filter, the out-of-band suppression of 20 dB needs to be ensured at a minimum, so that the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 40% or more. did.

FIG. 7A shows a series arm in which the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 48% and the duty ratio of the grating reflector 2b of the series arm resonator 2S is 44%. The pass characteristics of the resonator 2S and the parallel arm resonator 2P are shown respectively. Then, FIG. 6B shows the pass characteristics of the composite surface acoustic wave filter configured by using the series arm resonator 2S and the parallel arm resonator 2P having the pass characteristics shown in FIG. There is. The other conditions are the same as in the case of FIG. 3 (bandwidth 93
3.5 MHz).

In the case of FIG. 7, the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is made larger than the duty ratio (44%) of the grating reflector 2b of the parallel arm resonator 2P in the case of FIG. (48%), so
Although the performance of out-of-band suppression is secured, the degree of reduction of the ripple 101 of the surface acoustic wave filter becomes poor. Since the ripple 101 is preferably 1.0 dB or less, the duty ratio of the grating reflector 2b of the parallel arm resonator 2P is 48% or less.

Similar to FIGS. 6 and 7, FIG. 9 is a graph focusing on the duty ratio of the grating reflector 2b of the parallel arm resonator 2P, and was created to find an appropriate range of the duty ratio. It is a thing. In this graph, the horizontal axis represents the duty ratio, the left vertical axis represents out-of-band suppression, and the right vertical axis represents ripple. As can be seen from this graph, the ripple can be suppressed to 1.0 dB or less by setting the duty ratio to 48% or less. On the other hand, when the duty ratio is 40% or more, the out-of-band suppression can be 20 dB or more. Therefore,
In the parallel arm resonator 2P, the duty ratio is 40 to 48%
Within the range, the predetermined criteria can be satisfied at the same time for both out-of-band suppression and ripple.

[0031]

As described above, according to the present invention, spurious in series arm resonators and / or ripples in parallel arm resonators can be suppressed without incurring extreme deterioration of insertion loss and out-of-band suppression. It is possible to reduce the disturbance of signals and improve the total performance of the surface acoustic wave filter.

[Brief description of drawings]

FIG. 1 is an enlarged vertical sectional view showing a grating reflector of a surface acoustic wave filter of the present invention.

FIG. 2 (a) is a graph showing pass characteristics of a series arm resonator (duty ratio) and a parallel arm resonator (duty ratio) of the present invention, and FIG. It is a graph which shows the passage characteristic of the compound type surface acoustic wave filter comprised using the series arm resonator and the parallel arm resonator which have the passage characteristic shown to a).

FIG. 3 (a) is a graph showing pass characteristics of a series arm resonator (duty ratio) and a parallel arm resonator (duty ratio) of the present invention, and FIG. It is a graph which shows the passage characteristic of the compound type surface acoustic wave filter comprised using the series arm resonator and the parallel arm resonator which have the passage characteristic shown to a).

FIG. 4 (a) is a graph showing the pass characteristics of the series arm resonator (duty ratio) and the parallel arm resonator (duty ratio) of the present invention, and FIG. It is a graph which shows the passage characteristic of the compound type surface acoustic wave filter comprised using the series arm resonator and the parallel arm resonator which have the passage characteristic shown to a).

5A is a graph showing the pass characteristics of a series arm resonator (duty ratio) and a parallel arm resonator (duty ratio) of the present invention, and FIG. It is a graph which shows the passage characteristic of the compound type surface acoustic wave filter comprised using the series arm resonator and the parallel arm resonator which have the passage characteristic shown to a).

FIG. 6A is a graph showing the pass characteristics of the series arm resonator (duty ratio) and the parallel arm resonator (duty ratio) of the present invention, and FIG. It is a graph which shows the passage characteristic of the compound type surface acoustic wave filter comprised using the series arm resonator and the parallel arm resonator which have the passage characteristic shown to a).

FIG. 7A is a graph showing the pass characteristics of the series arm resonator (duty ratio) and the parallel arm resonator (duty ratio) of the present invention, and FIG. It is a graph which shows the passage characteristic of the compound type surface acoustic wave filter comprised using the series arm resonator and the parallel arm resonator which have the passage characteristic shown to a).

FIG. 8 is a graph prepared to find an appropriate range of the duty ratio of the grating reflector in the series arm resonator of the present invention.

FIG. 9 is a graph prepared to find an appropriate range of the duty ratio of the grating reflector in the parallel arm resonator of the present invention.

FIG. 10 is a schematic plan view showing a general structure of a composite surface acoustic wave filter having filter function units arranged in three stages.

FIG. 11 is a graph illustrating the filter action of the composite surface acoustic wave filter.

FIG. 12 is an enlarged vertical sectional view showing a grating reflector of a conventional surface acoustic wave filter.

FIG. 13A is a conventional series arm resonator (duty ratio 50%) and parallel arm resonator (duty ratio 50%).
3B is a graph showing the pass characteristics of the same, and FIG. 6B is a composite surface acoustic wave filter configured using the series arm resonator and the parallel arm resonator having the pass characteristics shown in FIG. It is a graph which shows the passage characteristic of.

[Explanation of symbols]

 1 Piezoelectric substrate 2 1-port resonator 2a Comb type electrode 2b Grating reflector 3 Filter function unit 4 Signal line

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Kosuke Takeuchi 2-5-5 Keihan Hondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Kenichi Shibata 2-chome, Keihan Hondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd.

Claims (1)

[Claims] 1. A piezoelectric substrate is provided with two resonators each including a comb-shaped electrode and a grating reflector, one resonator is electrically connected in series to a signal line, and the other resonator is connected to a signal line. A composite type surface acoustic wave filter having one or more filter functional units electrically connected in parallel, wherein the series-connected resonator and / or the parallel-connected resonator grating reflector The duty ratio of 40 to
It is characterized by being 48%.