JP2003283297A - Surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave filter which outwardly makes bandwidth of a surface acoustic wave resonator narrow and obtains desired filter characteristics. <P>SOLUTION: In a ladder type surface acoustic wave filter having the 1st surface acoustic wave resonator connected in series to the input end and the output end, and the 2nd surface acoustic wave resonator connected in parallel to the input end and the output end, a capacitor element is added in series for at least one of the 1st or the 2nd surface acoustic wave resonator. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to RF (high frequency) used in small mobile communication devices such as car phones and mobile phones.
Surface acoustic wave filter for automobiles.

[0002]

2. Description of the Related Art SAW (Surface Acoustic Wave) devices, which are small and lightweight and have excellent mass productivity, are being widely used in the field of mobile communication, which is rapidly advancing in recent years. As a SAW device, a SAW filter, which is used as a duplexer in an RF (Radio Frequency) part of a mobile communication system, is particularly attracting attention.

This SAW filter is composed of a plurality of SAW resonators formed on a piezoelectric substrate.
It is used in MPS (Advanced Mobile Phone System). In the following, for ease of explanation, a case where the SAW filter is used as a transmission filter of the duplexer will be considered.

The SAW filter used for AMPS is AM
Since the PS transmission pass band is 824 to 849 MHz and the reception pass band is 869 to 894 MHz, the transmission pass band is formed to have a center frequency of 837 MHz and a transmission stop band of 869 to 894 MHz. In other words, when the center frequency 837 MHz of the transmission pass band is the resonance frequency fr and the transmission stop band 869 to 894 MHz is the anti-resonance frequency far, the bandwidth Δf defined by the equation (1) = about 3.8. A SAW resonator of (%) and a SAW resonator of bandwidth Δf = approximately 6.8 (%) are required to form a SAW filter. Δf = (far-fr) / fr (%) (1)

However, the bandwidth Δf depends on the substrate material on which the SAW resonator is formed. For example, when an LTO (lithium tantalate) substrate which is widely used at present is used, the bandwidth Δf = about 3. The SAW resonator was 8%, and the degree of freedom of the bandwidth Δf was small (the bandwidth Δf was limited). Therefore, in the past, as shown in FIG.
By adding the inductance element L in series to the resonator S, the impedance is increased by ωL compared to before the inductance element L is added, and the resonance frequency Fr is set to Fr ′ (Fr ′ <Fr) as shown in FIG. Change the bandwidth Δf
A method of apparently expanding has been proposed (Patent No. 280).
0905). With this method, the bandwidth Δf =
The SAW resonator of about 6.8 (%) is obtained to form the SAW filter.

By the way, recently, the SAW filter is set to P
There is a demand for adoption in next-generation mobile communication systems such as CS (Personal Communication Services). P
In CS, the center frequency of the transmission pass band is 1880 MH
Since the z and the stop band at the time of transmission are 1930 to 1990 MHz, a SAW resonator having a bandwidth Δf = about 2.7 (%) and a SAW resonator having a bandwidth Δf = about 5.8 (%) are required. That is, contrary to the case of the conventional AMPS, a SAW resonator having a narrow bandwidth Δf is required.

However, since the technique for narrowing the bandwidth Δf of the SAW resonator has not been developed so far, SA
The W filter could not be applied to PCS. Of course, it is conceivable to change the substrate material so that a resonator having a narrow bandwidth Δf can be obtained, but this is not preferable in view of the time and cost for newly selecting the substrate material, and the LTO substrate currently widely used. Contrary to the desire to use as is.

[0008]

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a surface acoustic wave filter that apparently narrows the bandwidth of the surface acoustic wave resonator to obtain desired filter characteristics.

[0009]

SUMMARY OF THE INVENTION According to the present invention, when a capacitance element is added in series to a surface acoustic wave resonator, the impedance becomes smaller and the resonance frequency is apparently higher than when the capacitance element C is not added. That is, it was made by paying attention to the narrowing of the bandwidth of the surface acoustic wave resonator. That is, the surface acoustic wave filter of the present invention is characterized by having a surface acoustic wave resonator and a capacitance element connected in series to the surface acoustic wave resonator.

According to another aspect of the present invention, a first surface acoustic wave resonator connected in series to an input end and an output end and a second surface acoustic wave resonator connected in parallel to the input end and the output end. In the ladder-type surface acoustic wave filter including the surface acoustic wave resonator, the capacitance element is serially added to at least one of the first and second surface acoustic wave resonators. When the surface acoustic wave filter of this aspect is applied to a transmission filter or a reception filter of a duplexer, for example, the band to be blocked by the transmission filter or the reception filter is only the reception band or the transmission band. A capacitance element may be added to either the wave resonator (series side) or the second surface acoustic wave resonator (parallel side).

The capacitance element can be formed on the piezoelectric substrate on which the first and second surface acoustic wave resonators are formed. Alternatively, it may be formed (embedded) in an exterior package that seals the piezoelectric substrate on which the first and second surface acoustic wave resonators are formed and the first and second surface acoustic wave resonators.

The surface acoustic wave resonator has a comb-shaped electrode (ID
T). In this case, it is preferable to use, as the capacitance element, a counter electrode formed in a direction different from the direction of the comb electrode by 90 °.
By forming the surface acoustic wave resonator and the capacitance element by the comb-shaped electrodes as described above, the surface acoustic wave resonator and the capacitance element can be formed at the same time, and the manufacturing process can be simplified.

[0013]

1 is a schematic external view of a SAW filter 10 according to the present invention. SAW filter 1
Reference numeral 0 is a high frequency filter used in the RF (high frequency) part of PCS (Personal Communication Services), and functions as a transmission filter of the duplexer.

The SAW filter 10 includes a piezoelectric substrate 11 made of LTO (lithium tantalate), a plurality of SAW resonators (FIG. 2) formed on the piezoelectric substrate 11, the piezoelectric substrate 11 and the SAW. And a package 12 for sealing the resonator. 2 shows an example of a circuit pattern formed on the piezoelectric substrate 11, and FIG. 3 shows an equivalent circuit diagram of FIG.

On the piezoelectric substrate 11, four SAW resonators S (first to fourth SAW resonators S) having a ladder type structure are provided.
1 to S4) are formed. Each of the first to fourth SAW resonators S1 to S4 constitutes an LC resonance circuit as shown in FIG. 5, and is formed using an IDT (Interdigital Transducer; comb-shaped electrode). The piezoelectric substrate 11 and the package 12 are electrically connected via the bumps 13,
The bump 13 is connected to the input end 14 and the output end 15 on the piezoelectric substrate 11, for example.

First and third SAW resonators S1 and S3
Are connected in series with the input end 14 and the output end 15. These first and third SAW resonators S1 and S3
Resonance frequency fpr is 1850 MHz, anti-resonance frequency f
pa is set to 1880 MHz. In FIG. 4B, the first
Also, the reactance-frequency characteristics of the third SAW resonators S1 and S3 are shown by solid lines. On the other hand, the second and fourth SAW resonators S2 and S4 are connected in parallel to the input end 14 and the output end 15. The resonance frequency fsr and the antiresonance frequency fpa of the second and fourth SAW resonators S2 and S4 are set to 1880 MHz and 1910 MHz, respectively. Figure 4
In (b), the reactance-frequency characteristics of the second and fourth SAW resonators S2 and S4 are indicated by a chain line. The SAW filter including the first to fourth SAW resonators S1 to S4 has the filter characteristics shown in FIG. In FIG. 4A, the vertical axis represents the attenuation amount (dB).

In the SAW filter 10 described above, a capacitance element C connected in series with the first SAW resonator S1 is further formed on the piezoelectric substrate 11.
If the capacitance element C is added in series to the SAW resonator in this way (FIG. 5), the impedance can be reduced by 1 / (ωC) as compared with the case where the capacitance element C is not added. As shown in 6, the resonance frequency fr can be changed (increased) to fr '(fr'> fr). And the resonance frequency fr
If S becomes larger, as can be seen from the above equation (1), S
The bandwidth Δf of the AW resonator is apparently narrowed. In the present embodiment, after the addition of the capacitance element C, the first SA
The capacitance of the capacitance element C is appropriately set so that the bandwidth Δf of the W resonator S1 is about 2.7 (%) (FIG. 7). The bandwidth Δf of the first SAW resonator S1 when the capacitance element C is not added is
Since the piezoelectric substrate 11 made of LTO is used, it is about 3.8 (%).

In the present embodiment, as the capacitance element C, a counter electrode formed in a direction different from the direction of the comb-shaped electrode forming the SAW resonator S by 90 ° is used.

Further, a third SA is formed on the piezoelectric substrate 11.
An inductance element L connected in series with the W resonator S3 is formed. Bandwidth Δf of the third SAW resonator S3
Is apparently expanded by the inductance element L. In this embodiment, after the inductance element L is added, the bandwidth Δf (= Δf of the third SAW resonator S3).
The size of the inductance element L is appropriately selected so that 4) becomes about 5.8 (%) (FIG. 7).

As described above, by adding the capacitance element in series to the SAW resonator, the capacitance element can apparently narrow the bandwidth Δf of the SAW resonator. Thus, S having the desired bandwidth Δf
An AW resonator is obtained. Therefore, a SAW filter having a filter characteristic adapted to PCS can be formed even if a piezoelectric substrate made of LTO is used as in the conventional case.

In this embodiment, the input end 14 and the output end 1
5, the capacitance element C is added to the series side, but the capacitance element can be added to the parallel side, and the connection position of the capacitance element (series side or (and) depending on the desired filter characteristic). Parallel side) is set appropriately. In the duplexer's transmission filter or reception filter, since the band to be blocked by the transmission filter or reception filter is only the reception band or the transmission band, a capacitance element is added to either the serial side or the parallel side. do it.

The SAW filter can of course be applied to the reception filter of the PCS duplexer. P
When applied to the reception filter of the CS duplexer, for example, the resonance frequency fpr of the first and third SAW resonators S1 and S3 is set near 1960 MHz, the anti-resonance frequency fpa is set to 1990 MHz, and the second and fourth SAW resonators S1 and S3 are set. SA
The resonance frequency fsr of the W resonators S2 and S4 is set to 1910 MH
z and anti-resonance frequency fpa are set to 1960 MHz.
Then, as shown in FIG. 10, a capacitance element C is provided in series with the second SAW resonator S2 to apparently narrow the bandwidth Δf (Δf2) of the second SAW resonator S2 to about 2.
Set to about 6 (%). Further, an inductance element L is provided in series with the third SAW resonator S3, and the bandwidth Δf (Δf3) of the third SAW resonator S3 is apparently widened to about 5.6.
(%)

The capacitance element C is a piezoelectric substrate 11.
It is possible to form (embed) in the outer package 12 instead of the above. In FIG. 11, the capacitance element C is provided on the outer package 12.
FIG. 6 shows a circuit pattern diagram (an example) in the case of forming the. In FIG. 11, the capacitance element C is the bump 1
3 is connected in series to the first SAW resonator S1.

The present SAW filter described above can be formed not only by PCS but also by adapting to various communication systems such as AMPS and W-CDMA.

[0025]

According to the present invention, it is possible to provide a surface acoustic wave filter which apparently narrows the bandwidth of the surface acoustic wave resonator to obtain desired filter characteristics.

[Brief description of drawings]

FIG. 1 is a schematic external view showing a SAW filter according to the present invention.

FIG. 2 is a schematic diagram showing an example of a circuit pattern formed on the piezoelectric substrate of FIG.

FIG. 3 is an equivalent circuit diagram of FIG.

FIG. 4 is a reactance-frequency characteristic diagram of the SAW resonator shown in FIG.

FIG. 5 is an equivalent circuit diagram showing a series body including a SAW resonator and a capacitance element.

6 is a frequency characteristic diagram showing only the SAW resonator of FIG. 5 and the entire circuit of FIG. 5 in comparison.

FIG. 7 is a diagram illustrating the bandwidth of each surface acoustic wave resonator shown in FIG.

FIG. 8 is an equivalent circuit diagram showing a series body including a SAW resonator and an inductance element.

9 is a frequency characteristic diagram showing only the SAW resonator of FIG. 8 and the entire circuit of FIG. 8 in comparison.

FIG. 10 is an equivalent circuit diagram of an embodiment in which the present SAW filter is applied to a reception filter of a duplexer for PCS.

FIG. 11 is a circuit pattern diagram when a capacitance element is formed on an outer package.

[Explanation of symbols]

10 SAW filter 11 Piezoelectric substrate 12 Exterior 13 bumps 14 Input end 15 Output end S1 First SAW resonator S2 Second SAW resonator S3 Third SAW resonator S4 Fourth SAW resonator C capacitance element L inductance element

Claims (6)

[Claims] 1. A surface acoustic wave filter comprising a surface acoustic wave resonator and a capacitance element connected in series to the surface acoustic wave resonator. 2. A first surface acoustic wave resonator connected in series to an input end and an output end, and a second surface acoustic wave resonator connected in parallel to an input end and an output end. A ladder-type surface acoustic wave filter having: a surface acoustic wave filter, wherein a capacitance element is serially added to at least one of the first or second surface acoustic wave resonators. 3. The surface acoustic wave filter according to claim 2, wherein the capacitance element is formed on a piezoelectric substrate on which the first and second surface acoustic wave resonators are formed. 4. The surface acoustic wave filter according to claim 2, wherein a piezoelectric substrate on which the first and second surface acoustic wave resonators are formed, the piezoelectric substrate and the first and second surface acoustic waves. A surface acoustic wave filter, comprising: a package for enclosing a resonator, wherein the capacitance element is formed in the package. 5. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave resonator comprises a comb-shaped electrode. 6. The surface acoustic wave filter according to claim 5, wherein the capacitance element is composed of counter electrodes formed in a direction different from the direction of the comb-shaped electrode by 90 °.