JP2004135322A - Saw filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a SAW (surface acoustic wave) filter having a high design freedom, capable of being easily manufactured to have desired characteristics, and being adjusted filter characteristics in its manufacturing process. <P>SOLUTION: In a ladder type SAW filter which uses a plurality of SAW resonators as parallel arms and serial arms, the SAW filter is designed to be adjusted a frequency of a resonance point and an anti-resonance point in response to values of an inductance connected parallel to at least one of the SAW resonators. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a SAW filter capable of adjusting a filter characteristic in an assembling process and obtaining a wide pass bandwidth and a large amount of attenuation.

In recent years, small communication devices such as mobile phones have been frequently used.
In such devices, many filters are used as frequency selection elements. In particular, SAW filters are relatively inexpensive and suitable for miniaturization, are easily mass-produced, and are provided with many products.

However, in the SAW filter, filter characteristics such as a pass band width and an attenuation amount are determined in principle by an electrode pattern adopted at the time of design.
Therefore, if the characteristics of the completed SAW filter do not satisfy the desired specifications, it is necessary to design a new one, and even with such efforts, a wide pass band and a large amount of attenuation can be generally obtained. Is difficult.

On the other hand, an inductance is inserted in series with a parallel arm and / or a serial arm of a ladder-type filter using a SAW resonator, and the resonance point and / or the anti-resonance point is moved according to the value of the inductance. Discloses a configuration in which a large degree of suppression is obtained in a wider pass band and out of the band.
JP-A-5-183380

(Problems of the prior art) However, in such a device, since the inductance is inserted in series with the SAW resonator, the degree of freedom of design is low, and when a ladder-type filter is actually manufactured, it is required many times. It is necessary to redo the design and trial production, and there is a problem of low productivity.

(Object of the Invention) The present invention has been made in view of the above circumstances, and has a high degree of freedom in design, can easily assemble a SAW filter having desired characteristics, and can adjust filter characteristics in an assembling process. It is an object to provide a SAW filter.

According to a first aspect of the present invention, in a ladder type SAW filter using a SAW resonator for a parallel arm and a series arm, a resonance point or an anti-resonance is determined according to an inductance value connected in parallel to at least one SAW resonator. This is a SAW filter characterized by adjusting the frequency of a point.

According to a second aspect of the present invention, in the first aspect, an inductance is connected in parallel to the SAW resonator of the parallel arm, so that the inductance value is adjusted with respect to a resonance point in the attenuation pole of the approximate filter characteristic in advance. According to a third aspect of the present invention, there is provided a SAW filter in which an anti-resonance point is set to a pass band of a filter characteristic according to the present invention, wherein an inductance is connected in parallel to the SAW resonator of the series arm. The SAW filter is characterized in that the resonance point is set to the attenuation pole of the approximate filter characteristic in accordance with the value of the inductance with respect to the anti-resonance point in the pass band of the approximate filter characteristic in advance.

According to the present invention having such a configuration, a wide pass band and a large amount of attenuation can be obtained, an insertion loss is small, and an industrially useful SAW filter can be provided.

Hereinafter, embodiments of the present invention will be described in detail with reference to the block diagram shown in FIG.
In FIG. 1, reference numerals 1 and 2 denote SAW resonators having a serial arm. Reference numerals 3 and 4 denote SAW resonators having parallel arms.

Then, inductances 5 and 6 are connected in parallel to the SAW resonators 1 and 2 of the series arm, respectively.
When the inductance 8 is connected in parallel to the SAW resonator 7 of the serial arm as shown in FIG. 2A, the anti-resonance point fr remains constant as shown by the S11 characteristic (broken line in FIG. 2) shown in FIG. 2B. Thus, the frequency of the resonance point fp of the S21 characteristic (solid line in the drawing) can be changed according to the value of the inductance 7.

Therefore, in S21 characteristic (shown solid line) of the filter characteristic shown in FIG. 2 (c), positioned substantially attenuation pole 10 of the frequency of the resonance point fp1, fp2 by SAW resonators 1 and 2 and the inductance 5 and 6 of the series arm And the frequencies of the anti-resonance points fr1 and fr2 of the S11 characteristic (broken line in the figure) are positioned substantially in the pass band 9 .

Similarly, the frequencies of the resonance points fp3 and fp4 of the SAW resonators 3 and 4 of the parallel arm are also positioned substantially at the attenuation pole 10 , and the frequencies of the anti-resonance points fr3 and fr4 are positioned substantially at the passband 9 .

Here, it is impossible for the SAW resonator alone to keep the frequency fr at the anti-resonance point more than a certain frequency with respect to the frequency fp at the resonance point.
However, as shown in FIG. 1, when the inductances 5 and 6 are connected in parallel to the SAW resonators 1 and 2 to change the frequency of the resonance point, the frequency of the resonance point is changed to the anti-resonance point fp. The frequency fr can be very far apart .

Therefore, a wide pass bandwidth can be realized by dispersing and arranging the frequencies fr of the anti-resonance points of the respective SAW resonators according to a desired pass bandwidth.
By arranging the frequencies of the resonance points as close as possible, a large amount of attenuation can be obtained.

Further, in the block diagram shown in FIG. 1, since the inductance is connected in parallel to the SAW resonators 1 and 2 of each series arm, the resistance of each inductance 5 and 6 does not become the insertion loss of the filter characteristic. .
For this reason, an extremely small insertion loss can be realized.

For example, when a simulation was performed on a filter having the configuration shown in the block diagram of FIG. 1, a result was obtained in which a filter with an insertion loss of 0.3 dB and an attenuation of 55 dB could be realized in a relative band of 9% or more.
Therefore, a filter having a wide pass band, a small insertion loss, and a large attenuation can be realized.

With such a configuration, it is possible to adjust the filter characteristics by selecting an inductance having an optimal value in the filter assembling process, thereby realizing a filter characteristic having a wide pass band and a large amount of attenuation. it can.
Also, the inductance is connected in parallel to the SAW resonator. Therefore , unlike the case where the inductance is connected in series to the SAW resonator, a loss is not caused by the resistance of the essential inductance, and a filter with a small insertion loss can be realized.

Particularly, in a configuration in which an inductance is connected in series to a SAW resonator, a large value of inductance needs to be used depending on the required characteristics of the filter. If the size needs to be further reduced, the resistance of the inductance necessarily increases. Become. Accordingly, the insertion loss using such a filter increased significantly, conceivable practical inability to and Mochiiruko.

The present invention is not limited to the above embodiment. For example, as shown in FIG. 3, the inductances 5 and 6 are connected in parallel to the serial arm SAW resonators 1 and 2 and the parallel arm SAW resonators are connected. An inductance 9 may be connected in series with 3.
In this case, when the inductance 11 is connected in series to the SAW resonator 10 of the parallel arm as shown in FIG. 4A, the frequency of the anti-resonance point of the S11 characteristic is constant as shown in FIG. Yes, the frequency of the resonance point fp of the S21 characteristic can be changed according to the value of the inductance 11.

Therefore, the number of elements that can be adjusted to obtain optimal filter characteristics increases, and adjustment can be easily performed to obtain desired characteristics.
Further, in the present invention, as in another embodiment shown in FIG. 5, inductances 12 and 6 are connected in series and in parallel to the SAW resonators 1 and 2 of the series arm.
Then, the inductances 9 and 13 may be connected in series and in parallel to the SAW resonators 3 and 4 of the parallel arm.
In this case, when an inductance 15 is connected in series to the SAW resonator 14 of the series arm as shown in FIG. 6A, the frequency fr of the anti-resonance point of the S11 characteristic becomes the value of the inductance 15 as shown in FIG. And the frequency of the resonance point fp of the S21 characteristic becomes a constant value.

When the inductance 17 is connected in parallel to the SAW resonator 16 of the parallel arm as shown in FIG. 7A, the frequency fr of the anti-resonance point of the S11 characteristic is changed to the inductance 17 as shown in FIG. The frequency at the resonance point fp of the S21 characteristic becomes a constant value.

Therefore, if a filter is assembled in such a combination, the number of adjustable elements is further increased, and a filter having desired characteristics can be easily realized.
Note that, in the above embodiment, the case where the inductance is connected in parallel or in series with the SAW resonator to change the frequency of the resonance point has been described. However, the present invention is not limited to such an example. The resonance frequency may be varied by connecting a capacitor in series.

よ う In this way, it is generally difficult to obtain a wide pass band, but there is an advantage that a steep attenuation characteristic can be obtained.

It is a block diagram of one embodiment of the present invention. (A) is a circuit diagram of a main part of the embodiment shown in FIG. 1, (b) is a diagram showing a change in S parameter, and (c) is a characteristic diagram of a filter. FIG. 4 is a block diagram of another embodiment of the present invention. (A) is a circuit diagram of a main part of the embodiment shown in FIG. 3, and (b) is a diagram showing a change in S parameter. FIG. 11 is a block diagram of still another embodiment of the present invention. (A) is a circuit diagram of a main part of the embodiment shown in FIG. 5, (b) is a diagram showing a change in S parameter. (A) is a circuit diagram of another main part of the embodiment shown in FIG. 5, and (b) is a diagram showing a change in S parameter.

Explanation of reference numerals

1, 2, 3, 4, 7, 10, 14, 16 SAW resonators 5, 6, 8, 9, 11, 12, 13, 15, 17 Inductance

Claims (3)

In a ladder-type SAW filter using a SAW resonator for a parallel arm and a series arm, the frequency of a resonance point or an anti-resonance point is adjusted according to the value of inductance connected in parallel to at least one SAW resonator. Characteristic SAW filter. 2. The filter according to claim 1, wherein an inductance is connected in parallel to the SAW resonator of the parallel arm, and an anti-resonance point is roughly determined according to an inductance value with respect to a resonance point at an attenuation pole of the approximate filter characteristic in advance. A SAW filter characterized in that it is set to a characteristic pass band . 2. The filter according to claim 1, wherein an inductance is connected in parallel to the SAW resonator of the series arm, and a resonance point is roughly filtered according to a value of the inductance with respect to an anti-resonance point in a pass band of the general filter characteristic in advance. A SAW filter characterized by having a characteristic attenuation pole .

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