Composite filter comprising a surface acoustic wave filter and an LC filter

The present invention relates to a composite filter to be used in communication devices such as mobile phones.

A conventional composite filter used in portable wireless communication devices such as mobile phones is described with reference to FIG. 9 , which shows its perspective exploded view. In FIG. 9 , the conventional composite filter is formed of a combination of an elastic surface-wave filter and a low-pass filter.

The low-pass filter is formed by combining an inductor with a capacitor, and is embedded in laminated substrate 2 made of low temperature co-fired ceramics and formed by layering a plurality of dielectric layers 1 . The inductor is formed of a plurality of dielectric layers 1 with arc-shaped conductors 3 formed thereon and coupled to each other. The capacitor is formed of planar capacitor electrodes 4 sandwiching dielectric layer 1 .

The elastic surface-wave filter is formed by combining elastic surface-wave resonators 5 in a ladder shape as shown in FIG. 10 , and placing resonators 5 on laminated substrate 2 .

The low-pass filter is coupled to a transmission output side of the foregoing filter in order to attenuate the doubled or tripled harmonics in the pass band of the filter. The composite filter discussed above, however; is formed by embedding the low-pass filter in laminated substrate 2 , so that the inductor and the capacitor are obliged to occupy a large area, and the capacitor, in particular, takes a greater portion of the occupied area because its capacitance is proportionate to its area. As a result, the filter cannot be further downsized.

The art related to the present invention is listed here as patent documents 1 and 2.

Patent Document 1: Unexamined Japanese Patent Publication No. 2004-254257

Patent Document 2: Unexamined Japanese Patent Publication No. 2005-184773

The present invention addresses the foregoing problem and aims to provide a composite filter downsized from the conventional one without degrading the characteristics. To achieve this objective, the present invention employs the following structure: at least one of capacitors is placed such that its comb-shaped electrodes are oppositely placed to each other on a piezoelectric substrate, and the opposing direction of the comb-shaped electrodes differs from that of comb-shaped electrodes of a surface acoustic wave resonator.

The foregoing structure allows for a decrease in the area occupied by the capacitors because at least one of the capacitors is formed such that its comb-shaped electrodes oppose each other on the piezoelectric substrate. On top of that, unlike the comb-shaped electrodes of the surface acoustic wave resonator, the structure allows the comb-shaped electrodes of the capacitors to not be excited because its opposing direction differs from that of the comb-shaped electrodes of the surface acoustic wave resonator, so that no loss is produced. As a result, a downsized composite filter can be obtained without degrading the characteristics.

FIG. 1 shows a top view of a composite filter in accordance with an exemplary embodiment of the present invention.

FIG. 2 shows an exploded perspective view of the composite filter in accordance with an embodiment of the present invention.

FIG. 3 shows an equivalent circuit diagram of the composite filter in accordance with an embodiment of the present invention.

FIG. 4 shows frequency characteristics of the composite filter in accordance with an embodiment of the present invention.

FIG. 5 shows a top view of a composite filter in accordance with another exemplary embodiment of the present invention.

FIG. 6 shows an equivalent circuit diagram of a surface acoustic wave resonator.

FIG. 7 shows an equivalent circuit diagram of a composite filter in accordance with another embodiment of the present invention.

FIG. 8 shows frequency characteristics of a composite filter in accordance with another embodiment of the present invention.

FIG. 9 shows an exploded perspective view of a conventional composite filter.

FIG. 10 shows an equivalent circuit diagram of a surface acoustic wave filter used in the conventional composite filter.

An exemplary embodiment of the present invention is demonstrated hereinafter with reference to the accompanying drawings. FIG. 1 shows a top view of a composite filter in accordance with an embodiment of the present invention. FIG. 2 shows an exploded perspective view of the composite filter. FIG. 3 shows an equivalent circuit diagram of the composite filter. FIG. 4 shows frequency characteristics of the composite filter.

In FIG. 1-FIG . 3 , the composite filter comprises the following elements:

piezoelectric substrate 10 made from LiTaO 3 ; laminated substrate 11 made from ceramic baked at a low temperature, and on which substrate 11 piezoelectric substrate 10 is layered; surface acoustic wave filter 12 ; and LC filter 13 coupled to surface acoustic wave filter 12 .

Elastic surface-wave filter 12 is formed by combining surface acoustic wave resonators 14 placed on piezoelectric substrate 10 such that their comb-shaped electrodes oppose to each other. LC filter 13 is formed by combining π-shaped LC filter 17 with capacitor 15 c, where filter 17 is formed of capacitors 15 a and 15 b coupled to inductor 16 a in a π-shape, and capacitor 15 c is coupled in parallel to this inductor 16 a.

This π-shaped LC filter 17 is configured by connecting respective first ends of capacitors 15 a and 15 b to inductor 16 a, and respective second ends thereof are grounded. Each one of two capacitors 15 a and 15 b is configured by placing its comb-shaped electrodes opposing each other on piezoelectric substrate 10 , and the opposing direction (A) of the comb-shaped electrodes differs orthogonally from the opposing direction (B) of the comb-shaped electrodes of surface acoustic wave resonator 14 . Inductor 16 a is embedded in laminated substrate 11 in a spiral shape. Capacitor 15 c placed in parallel with inductor 16 a is also configured by placing its electrodes opposing each other on piezoelectric substrate 10 , and the opposing direction (A) of its electrodes orthogonally differs from the foregoing opposing direction (B).

As shown in FIG. 1 , grounding conductors 21 are formed on piezoelectric substrate 10 at places other than where conductors 21 face a top face or an underside of inductor 16 a embedded in laminated substrate 11 ; in other words, conductors 21 are placed anywhere on substrate 10 except just above or under inductor 16 a. Capacitors 15 a and 15 b of π-shaped LC filter 17 are coupled to grounding conductors 21 .

The structure discussed above allows for decreasing of a parasitic capacitance generated between inductor 16 a and grounding conductor 21 , and suppressing ripples to be generated in the pass band.

The composite filter further includes antenna terminal 18 disposed at a first end of surface acoustic wave filter 12 , and transmission terminal 19 disposed at a first portion of a second end thereof via LC filter 13 , and reception terminal 20 disposed at a second portion of the second end of surface acoustic wave filter 12 . If the reception side wants to attenuate a higher frequency band than the pass band, LC filter 13 connected between the reception terminal and the terminal of the surface acoustic wave filter will achieve this task. If both of the reception side and the transmission side want to attenuate a higher frequency band than the pass band, LC filter 13 connected between the antenna terminal and the terminal of the surface acoustic wave filter will achieve this task.

The composite filter discussed above has the frequency characteristics as shown in FIG. 4 , so that the filter can attenuate a large amount outside the pass band without degrading the waveform of the pass band. Assuming that the pass band is 2 GHz, the harmonics can be attenuated by approx. 35 dB at 4 GHz (twice of 2 GHz) and 20 dB at 6 GHz (triple of 2 GHz).

Unlike the conventional composite filter formed of planar capacitor electrodes opposing each other, capacitors 15 a - 15 c can be formed on a highly dielectric piezoelectric-substrate by using a fine patterning similar to the surface acoustic wave resonator, so that capacitors 15 a - 15 c can be placed to occupy a smaller area. Particularly the opposing direction (A) of comb-shaped electrodes of capacitors 15 a - 15 c is different from that (B) of comb-shaped electrodes of resonator 14 , so that the comb-shaped electrodes of capacitors 15 a - 15 c do not become excited unlike resonator 14 , of which comb-shaped electrodes become excited. Each one of capacitors 15 a - 15 c thus can always work as a capacitor regardless of frequencies.

For instance, use of the foregoing composite filter in UMTS (Universal Mobile Telecommunications Systems), a kind of mobile telephones and the third generation of mobile communication system in Europe, allows the outer dimensions of the filter to be approx. 30 μm×220 μm. Use of the conventional composite filter embedded in the laminated substrate, of which relative dielectric constant is 7.4 makes its outer dimensions approx. 350 μm×330 μm. The occupied area of the composite filter of the present invention is thus as small as approx. 1/18 of the conventional one.

As shown in FIG. 5 , when capacitor 15 c is replaced with surface acoustic wave resonator 14 , the resonator can be expressed as an equivalent circuit shown in FIG. 6 , namely, capacitor 24 is connected in parallel with a series circuit formed of capacitor 22 and inductor 23 . Therefore, an anti-resonant frequency can be created on both the low frequency band and the high frequency band with inductor 16 a, which is coupled in parallel to resonator 14 , without changing a capacitance of surface acoustic wave resonator 15 while the capacitance of an equivalent circuit of resonator 14 is kept equivalent to that of capacitor 15 . At the same time, the anti-resonant frequency can be changed with a value of the inductor without changing the resonance frequency. The equivalent circuit of resonator 14 thus can produce a similar effect to that of capacitor 15 c in the vicinity of the pass band. On top of that, signals in a lower band than the pass band can be attenuated by an anti-resonant frequency created by the inductor on the lower band side.

Laminated substrate 11 includes another grounding conductor 26 embedded therein other than grounding conductor 21 formed on piezoelectric substrate 10 (see FIG. 2 ), and connecting of these two grounding conductors to each other allows for producing of inductors 16 b and 16 c between these conductors as shown in FIG. 7 . Resonance between inductors 16 b, 16 c and capacitors 15 a, 15 b thus can be positively used, so that the values of capacitors 15 a, 15 b and capacitance that resonator 14 has, and respective values of inductors 16 a - 16 c can be designed for these elements to form a polarized low-pass filter.

The composite filter discussed above has the frequency characteristics as shown in FIG. 8 , and can attenuate a large amount of signals outside the pass band without degrading the waveform of the frequency characteristics in the pass band. For instance, assuming that the pass band is 2 GHz, compared with a typical surface acoustic wave filter, the foregoing composite filter can attenuate the signals outside the pass band by approx. 40 dB at 4 GHz (twice of 2 GHz), and approx. 30 dB at 6 GHz (triple of 2 GHz).

From the standpoint of capacitance, the comb-shaped electrodes of capacitors 15 a - 15 c can be covered with dielectric such as SiO 2 or SiN for increasing their capacitances, and this preparation can also suppress the dispersion of the capacitances due to changes in temperature.

Inductor 16 a can be formed of strip lines having equivalent dielectric to that of the equivalent circuit of resonator 14 at a given frequency, or formed of chip inductors for obtaining a higher Q value, or formed on piezoelectric substrate 10 instead of embedded in laminated substrate 11 in order to downsize the filter.

The composite filter of the present invention can be used in communication devices such as mobile phones.