JPH0661783A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To decrease the insertion loss, to extend the pass band, to easily take input output impedance matching and to increase the attenuation of non- pass band by selecting both the resonance and the anti-resonance frequency of one surface acoustic wave resonator within the non-pass band. CONSTITUTION:When a transmission antenna and a reception antenna are used in common and transmission reception are implemented simultaneously at different frequencies, it is required for the transmission filter that the attenuation at the outside of the reception band is large. The frequency band corresponds just to the vicinity of the resonance point of resonators 16, 17 located at the outermost side. Thus, the resonators 11-15 are used to form the pass band and the resonators 16, 17 are used to increase the attenuation at the outside of the reception band. Furthermore, since the resonators 16, 17 have no resonance and anti-resonance point in the vicinity of the usual band but act like almost a constant capacitive component, the filter whose input output impedance is matched with 5Oohms as a whole is generated by designing the resonators 11-15 by taking the capacitance into account.

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 using a surface acoustic wave resonator.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave devices have been actively researched for application to oscillators and filters. In particular, due to recent developments in mobile communication and higher frequencies, it has become necessary to increase the frequencies of surface acoustic wave devices.

As a conventional surface acoustic wave filter, a so-called multi-electrode filter has been mainly used. As shown in FIG. 28, at least one or more comb-shaped electrodes for exciting surface acoustic waves are formed on the piezoelectric substrate (9 pairs in FIG. 28), and the surface acoustic wave converter on one side is input to the other side. A surface acoustic wave converter is used as an output and is used as a filter by utilizing the frequency characteristic of conversion of electric signal-surface acoustic wave-electric signal.
Since there is a loss due to the bidirectionality of the comb-shaped electrodes when there is one pair of inputs and outputs, the input and output are alternately arranged as shown in FIG. I am calling. Typical characteristics are shown in FIG. This is a structure with 7 electrodes for input and 6 electrodes for output.
° This is an example using YX-LiTaO3.

As is clear from FIG. 29, even in the multi-electrode filter, the insertion loss of the filter due to the bidirectionality and the propagation loss of the surface acoustic wave is 3 dB or less when the band is 900 MHz and the band is 25 MHz. It was difficult.

Another type of filter is a resonator type filter. The filter with such a structure has 19
It is published in detail in Proceedings A-393 of the 1992 IEICE Spring National Convention. This is a conventional ceramic ladder filter as shown in FIG.
The characteristics are shown in FIG. 31. This is a type that uses five surface acoustic wave resonators, and the substrate uses 36 ° YX-LiTaO3.

As is well known in the prior art, the basic principle of this filter is that resonators connected in series take a resonance point in the middle of the pass band and become anti-resonant at the upper limit of the pass band. The resonators connected in parallel resonate at the lower limit of the pass band, and take anti-resonance almost in the middle of the pass band.

[0007]

However, the above-mentioned multi-electrode filter has the problems that the insertion loss is large, the pass band of the filter cannot be widened, and the input / output impedance in the pass band is difficult to match to 50 ohms.
Insertion loss is, for example, about 3 dB when a 36 ° YX-LiTaO3 substrate is used and a filter with a band of 25 MHz in the 900 MHz band is created, and the input / output impedance is 50
Not in ohms. If the band is 33 MHz, the insertion loss becomes 4 dB or more, and the input / output impedance does not match 50 ohms.

On the other hand, the resonator type filter has a problem that the amount of attenuation in the non-pass band is small. Although it is possible to increase the amount of attenuation in the non-passband here,
At the same time, the input / output impedance in the pass band moves away from 50 ohms, and the amount of attenuation in the pass band also increases. For this reason, in the resonator type filter, the non-passband attenuation amount is about 20 dB when the appropriate attenuation amount in the passband is, for example, 2 dB. This is a bad value as compared with the multi-electrode type of 30 dB, which is a drawback in actual use.

In consideration of the above problems of the prior art, the present invention provides a surface acoustic wave filter having a small insertion loss, a wide pass band, easy matching of input / output impedances, and a large non-pass band attenuation. It is intended to be provided.

[0010]

According to the present invention, a plurality of surface acoustic wave resonators are provided between input and output terminals, and at least one surface acoustic wave resonator of the plurality of surface acoustic wave resonators has resonance and It is a surface acoustic wave filter in which both anti-resonance frequencies are in the non-pass band.

[0011]

According to the present invention, both the resonance and anti-resonance frequencies of at least one surface acoustic wave resonator are in the non-pass band of the filter. Therefore, the attenuation amount in the non-pass band can be increased without increasing the attenuation amount in the pass band. To increase.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings showing its embodiments.

(Embodiment 1) FIG. 1 is a block diagram of a surface acoustic wave filter according to a first embodiment of the present invention. As a substrate
It is a filter that uses 36 ° YX-LiTaO3 and has a center frequency of 881 MHz and a band of 25 MHz, that is, a transmission filter for car phones for North America. The frequency characteristic of this filter is shown in FIG. The impedance characteristics of the resonators 11, 12 and 13 in the series arm of this structure are shown in FIG. 3, and the impedance characteristics of the resonators 14 and 15 in the parallel arm are shown in FIG. Further, the impedance characteristics of the resonators 16 and 17 connected to the outermost side are shown in FIG. As shown in the conventional example, the parallel arm resonator 14,
The loss in the pass band and the loss in the non-pass band are determined by the ratio between the parallel capacitance of the equivalent circuit 15 and the parallel capacitance of the resonators 11, 12, and 13 in the series arm. However, in the case of this example, in particular, because of the circuit configuration of the car telephone, the transmission / reception antenna is shared, and transmission / reception is performed simultaneously at different frequencies. Therefore, it is necessary for the transmission filter to have a large amount of attenuation in the reception band. The frequency band is just near the resonance points of the outermost resonators 16 and 17. Since the resonators 16 and 17 are connected in parallel, the pass characteristic of the filter has an attenuation pole when the impedance becomes 0. In other words, the roles of the respective resonators are that the resonators 11, 12, 13, 14, and 15 form a pass band, and the resonators 16 and 17 represent the out-of-band attenuation of the reception band in this example. It is to increase. Further, since the resonators 16 and 17 have no resonance or anti-resonance point in the vicinity of the pass band and work as a substantially constant capacitance component, the resonator 11 and
By designing 12, 13, 14, and 15, it is possible to obtain a filter whose input / output impedance is matched to 50 ohms as a whole. With this configuration, the passband attenuation 2d
A filter with B, non-passband attenuation of 20 dB, and reception band attenuation of 30 dB could be created.

(Embodiment 2) Next, another embodiment of the surface acoustic wave filter according to the present invention will be described. The configuration diagram is similar to that of FIG. 1, and for convenience, the resonator 16 is referred to as an input side and the resonator 17 is referred to as an output side. Resonators 11 and 1 in series arm
The impedance characteristics of the resonators 2 and 13 are shown in FIG. 2, the impedance characteristics of the resonators 14 and 15 in the parallel arm are shown in FIG. 3, the impedance characteristics of the resonator 16 are shown in FIG. 6, and the impedance characteristics of the resonator 17 are shown in FIG. The characteristics as a filter are shown in FIG. This example is an example when the attenuation of the image frequency at the intermediate frequency is requested at the same time as the attenuation in the reception frequency band in the transmission frequency filter, and the resonance and anti-resonance frequency of the surface acoustic wave resonator that creates the attenuation pole are set. This is an example in which the attenuation pole can be freely set by appropriately setting it. Also in the case of the second embodiment, as in the case of the first embodiment, the resonators 16 and 17 do not have resonance and antiresonance points in the pass band and act as capacitive elements, so that the resonators 11 and 12 are provided. , 13, 14 and 15 can be appropriately set to match the input / output impedance to 50 ohms.

In this example, the attenuation pole on the low frequency side is set on the input side and the attenuation pole on the high frequency side is set on the output side.
Of course, this positional relationship may be opposite.

(Embodiment 3) Another embodiment of the surface acoustic wave filter according to the present invention will be described below. The configuration diagram is similar to that of FIG. 1, and for convenience, the resonator 16 is referred to as an input side and the resonator 17 is referred to as an output side. Resonators 11 and 1 in series arm
The impedance characteristics of Nos. 2 and 13 are shown in FIG. 2, the impedance characteristics of the resonators 14 and 15 in the parallel arm are shown in FIG. 3, the impedance characteristics of the resonator 16 are shown in FIG. 9, and the impedance characteristics of the resonator 17 are shown in FIG. The characteristics as a filter are shown in FIG. This example is an example when a wide range of attenuation is required in the reception frequency band, and the range of the attenuation pole can be freely set by slightly changing the resonance and anti-resonance frequencies of the surface acoustic wave resonators 16 and 17. Here is an example.
Also in the case of the third embodiment, as in the case of the first embodiment, the resonators 16 and 17 have no resonance or anti-resonance point in the pass band and act as capacitive elements.
By appropriately setting 2, 13, 14 and 15, the input / output impedance can be matched to 50 ohms.

(Embodiment 4) Next, an embodiment of the surface acoustic wave filter of the present invention having another structure will be described. The configuration is as shown in FIG. 12, and the resonator 22 is referred to as the input side for convenience. The pass loss characteristics of this filter are shown in Fig. 1.
3 shows. The impedance characteristic of the resonator 21 in the series arm is shown in FIG. 14, the impedance characteristic of the resonator 22 is shown in FIG. 15, and the impedance characteristic of the resonator 23 is shown in FIG. Further, FIG. 17 shows impedance characteristics of the resonators 24 and 25 included in the parallel arms. Thus, the surface acoustic wave resonator that determines the attenuation pole does not have to be in the parallel arm, but may be in the series arm. This is because the surface acoustic wave resonator has a phenomenon of resonance and antiresonance.

(Embodiment 5) Next, an embodiment of the surface acoustic wave filter of the present invention having another structure will be described. Figure 1
8 shows. That is, the resonators 31 having no resonance or anti-resonance point in the pass band are inserted in series on the input side and the output side, respectively. The pass loss characteristic of this filter is shown in FIG. In addition, the impedance characteristic of the resonator 31 is shown in FIG.
FIG. 21 shows impedance characteristics of the resonators 32, 33 and 34, and FIG. 2 shows impedance characteristics of the resonators 35 and 36.
2 shows.

(Sixth Embodiment) Next, as a sixth embodiment, a structure as shown in FIG. 23 was also manufactured. The pass loss characteristics of this filter are shown in FIG. 24, and the characteristics of each resonator are shown in FIG.
FIG. 25 shows impedance characteristics of the resonators 42, 43,
FIG. 26 shows the impedance characteristics of the resonator 44 and the resonators 45, 4
The impedance characteristic of No. 6 is shown in FIG. Here, as can be seen by comparing both Example 5 and Example 6, there is no significant difference in the in-band characteristics, but the levels outside the band, especially at the attenuation poles, are different. This is due to the difference in the level of resonance and anti-resonance of the surface acoustic wave resonator, and in general, the anti-resonance level is easier to take.

In the above embodiment, the surface acoustic wave resonator for determining the attenuation pole is placed at the position symmetrical with respect to the input and output sides, but the present invention is not limited to this and need not be symmetrical.

Further, in the above-mentioned embodiment, the surface acoustic wave resonator for determining the attenuation pole is placed on the outermost side of the input / output, but the present invention is not limited to this, and it may be located at any position inside the filter.

Further, in the above embodiment, two surface acoustic wave resonators for setting the attenuation pole are used, but this number is a value that can be changed by design, and the amount of out-of-band attenuation, bandwidth, and in-band It can be set freely depending on the balance of insertion loss.

In the above embodiment, the number of resonators connected between the input and the output is 5 or 7, but the number of resonators is not limited to this and may be any number of 2 or more. Good.

[0024]

As is apparent from the above description, the present invention has at least one surface acoustic wave resonator having both resonance and antiresonance frequencies in the non-pass band,
It has advantages that the insertion loss is small, the pass band can be widened, the input / output impedance can be easily matched, and the non-pass band attenuation is large.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a surface acoustic wave filter according to a first embodiment of the present invention.

FIG. 2 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the first embodiment.

FIG. 3 is a diagram showing impedance characteristics of the surface acoustic wave resonator of the series arm of the surface acoustic wave filter of the first embodiment.

FIG. 4 is a diagram showing impedance characteristics of surface acoustic wave resonators of parallel arms in a central portion of the surface acoustic wave filter of the first embodiment.

FIG. 5 is a diagram showing impedance characteristics of the surface acoustic wave resonators of the outermost parallel arms of the surface acoustic wave filter of the first embodiment.

FIG. 6 is a diagram showing impedance characteristics of the surface acoustic wave resonator of the outermost parallel arm on the input side of the surface acoustic wave filter according to the second embodiment of the present invention.

FIG. 7 is a diagram showing impedance characteristics of a surface acoustic wave resonator of an output side outermost parallel arm of the surface acoustic wave filter of Example 2;

FIG. 8 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the second embodiment.

FIG. 9 is a diagram showing impedance characteristics of a surface acoustic wave resonator of an input side outermost parallel arm of a surface acoustic wave filter according to a third embodiment of the present invention.

FIG. 10 is a diagram showing impedance characteristics of a surface acoustic wave resonator of an output side outermost parallel arm of the surface acoustic wave filter of the third embodiment.

FIG. 11 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the third embodiment.

FIG. 12 is a configuration diagram of a surface acoustic wave filter according to a fourth embodiment of the present invention.

FIG. 13 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the fourth embodiment.

FIG. 14 is a diagram showing impedance characteristics of the surface acoustic wave resonator 21 of the series arm of the surface acoustic wave filter according to the fourth embodiment.

FIG. 15 is a diagram showing impedance characteristics of the surface acoustic wave resonator 22 of the series arm of the surface acoustic wave filter according to the fourth embodiment.

FIG. 16 is a diagram showing impedance characteristics of the surface acoustic wave resonator 23 of the series arm of the surface acoustic wave filter according to the fourth embodiment.

FIG. 17 is a diagram showing impedance characteristics of surface acoustic wave resonators of parallel arms of the surface acoustic wave filter of the fourth embodiment.

FIG. 18 is a configuration diagram of a surface acoustic wave filter according to a fifth embodiment of the present invention.

FIG. 19 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the fifth embodiment.

FIG. 20 is a diagram showing impedance characteristics of the surface acoustic wave resonator 31 of the surface acoustic wave filter according to the fifth embodiment.

FIG. 21 is a diagram showing impedance characteristics of surface acoustic wave resonators 32, 33, and 34 of the surface acoustic wave filter according to the fifth embodiment.

FIG. 22 is a diagram showing impedance characteristics of the surface acoustic wave resonators 35 and 36 of the surface acoustic wave filter according to the fifth embodiment.

FIG. 23 is a configuration diagram of a surface acoustic wave filter according to a sixth embodiment of the present invention.

FIG. 24 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the sixth embodiment.

FIG. 25 is a diagram showing impedance characteristics of the surface acoustic wave resonator 41 of the surface acoustic wave filter according to the sixth embodiment.

FIG. 26 is a diagram showing impedance characteristics of surface acoustic wave resonators 42, 43, and 44 of the surface acoustic wave filter according to the sixth embodiment.

FIG. 27 is a diagram showing impedance characteristics of surface acoustic wave resonators 45 and 46 of the surface acoustic wave filter according to the sixth embodiment.

FIG. 28 is a configuration diagram of a conventional multi-electrode filter.

FIG. 29 is a diagram showing frequency characteristics of a conventional multi-electrode filter.

FIG. 30 is a configuration diagram of a conventional resonator type filter.

FIG. 31 is a diagram showing frequency characteristics of a conventional resonator type filter.

[Explanation of symbols]

 11-13 surface acoustic wave resonator of series arm 14-17 parallel surface acoustic wave resonator 21-23 series arm surface acoustic wave resonator 24-25 parallel arm surface acoustic wave resonator 31-34 series arm Surface acoustic wave resonator 35-36 Parallel arm surface acoustic wave resonator 42-44 Series arm surface acoustic wave resonator 41, 45-46 Parallel arm surface acoustic wave resonator

Front page continuation (72) Inventor Shunichi Seki 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A plurality of surface acoustic wave resonators are provided between input and output terminals, and at least one surface acoustic wave resonator of the plurality of surface acoustic wave resonators has a non-resonance frequency. A surface acoustic wave filter characterized by being in a pass band. 2. The surface acoustic wave resonator having both resonance and anti-resonance frequencies in the non-pass band is a surface acoustic wave resonator connected in series between the input and output terminals. The surface acoustic wave filter according to claim 1.