Abstract:
Disclosed is a radio frequency filter used for a mobile communication apparatus such as a mobile phone, having an inductor component, a capacitor component, and a resistor component formed of a conductor pattern. The filter includes the conductor pattern formed on the surface of dielectric layer, and an adjusting layer for adjusting the frequency characteristic of the filter formed so as to cover at least a part of the conductor pattern. In the radio frequency filter, the conductor pattern is formed and sintered, followed by adjusting the frequency characteristic of the filter. Accordingly, in the radio frequency filter, the yield of the manufacture is improved. Further, the electrode layer can be single-layered by using extra-thin lines for the conductor pattern and interdigital electrodes for the capacitor. As a result, it is possible to make the filter smaller and thinner by using intaglio-printing technique and thin-film forming technique.

Description:
FIELD OF THE INVENTION 
     The present invention mainly relates to a radio frequency filter used for mobile communication equipment such as a mobile phone. 
     BACKGROUND OF THE INVENTION 
     Recently, with the increase in needs for mobile communications equipment such as a mobile phone, the frequency band used for them has become insufficient in band width, giving rise to a trend of shifting to a higher frequency band. Accordingly, it is necessary for mobile communication equipment such as mobile phones to be compatible with a higher frequency. Above all, radio frequency filters used for them are becoming less in relative band width, and their component circuit elements are required to ensure higher dimensional accuracy. 
     A conventional radio frequency filter generally has a structure as shown in FIG.  5 . The structure is as follows: 
     (1) First ground electrode  502  is formed on the top surface of first dielectric layer  501 , and second dielectric layer  503  is laminated on ground electrode  502 . 
     (2) Two resonator electrodes  504   a  and  504   b  are formed on the top surface of dielectric layer  503 , and third dielectric layer  505  is laminated on resonators  504   a  and  504   b.    
     (3) First transmission electrode  506   a , second transmission electrode  506   b , third transmission electrode  506   c , first capacitor electrode  507   a  and second capacitor electrode  507   b  are formed on the top surface of dielectric layer  505 , and fourth dielectric layer  508  is laminated on these electrodes. 
     (4) Second ground electrode  509  is formed on the top surface of dielectric layer  508 , and fifth dielectric layer  510  is laminated on ground electrode  509 . 
     (5) Terminal electrodes  511   a ,  511   b ,  511   c ,  511   d  are formed on the side surface of dielectric layer  510 . 
     Here, resonator electrode  504   a  and capacitor electrode  507   a , and resonator electrode  504   b  and capacitor electrode  507   b  are respectively disposed so as to be at least partly opposed to each other via dielectric layer  505 . As shown in FIG. 5, transmission electrode  506   a , capacitor electrode  507   a , transmission electrode  506   b , capacitor electrode  507   b  and transmission electrode  506   c  are formed in order between terminal electrodes  511   c  and  511   d , thereby forming a transmission line. In addition, terminal electrode  511   a  is connected to ground electrodes  502  and  509  for the purpose of grounding. Further, terminal electrode  511   b  is connected to ground electrodes  502  and  509 , and to each end of resonator electrodes  504   a  and  504   b  for the purpose of grounding. 
     In a conventional radio frequency filter having a structure as described above, resonator electrodes  504   a  and  504   b  are of triplate structure sandwiched between two ground electrodes  502  and  509 . Resonator electrodes  504   a  and  504   b , one end of each electrode being grounded, operate as a quarter-wavelength resonator, that is, as a serial resonator. Moreover, these resonators are partly opposed to capacitors  507   a  and  507   b  respectively via dielectric layer  505 , and operate as a parallel-plate capacitor. More specifically, two serial resonators with one end grounded are connected in shunt to the transmission line between terminal electrodes  511   c  and  511   d  via the parallel-plate capacitor. Accordingly, this filter operates as a band-stop filter having terminal electrodes  511   c  and  511   d  as input terminal and output terminal respectively. 
     In a conventional radio frequency filter having the structure and operation as described above, each electrode is formed by a plurality of electrode layers having predetermined dimensions inside dielectric layers. After the electrode layer forming process followed by a dielectric layer sintering process, a filter having a predetermined frequency characteristic is selected by measurement. However, as recent equipment becomes higher in frequency, causing the relative bandwidth of frequency characteristic of the filter to become narrower, electrode layers are required to ensure higher dimensional accuracy. Thus it is difficult for such conventional forming process to ensure the required characteristics. Moreover, since electrode layers have been formed by using screen printing technique in most cases, there has been a problem of worsening of the dimensional accuracy such as “blurring of printing.” Accordingly, the conventional method is unable to reduce the electrode dimensions while maintaining high accuracy and is limited with respect to reduction in size of the filter. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a small-sized radio frequency filter with which the desired frequency characteristic can be obtained with simple configuration even in higher frequency band. 
     The radio frequency filter of the present invention includes an inductor component, capacitor component, and resistor component, and has a configuration as follows. 
     A conductor pattern is formed on the surface of a dielectric layer, forming at least one of the inductor component, capacitance component and resistor component. 
     An adjusting layer is formed so as to cover at least a part of the conductor pattern and serves to adjust the frequency characteristic of the filter. 
     The conductor pattern is usually formed on the surface of dielectric layer, and sintered thereafter. 
     The present invention has the following features. 
     (1) Forming an adjusting layer with a dielectric material, it is possible to increase the frequency adjusting range by changing the effective dielectric constant. 
     (2) Forming an adjusting layer with a magnetic material, it is possible to increase the frequency adjusting range by changing the effective permeability. 
     (3) Forming a conductor on the top surface of adjusting layer formed of the dielectric material or magnetic material, it is possible to enhance the change of the effective dielectric constant or effective permeability. 
     (4) Grounding the adjusting layer in (3), it is possible to further enhance the change of the effective dielectric constant or effective permeability. 
     (5) Forming a dielectric layer between the conductor pattern and the adjusting layer, it is possible to prevent the breakdown or damage of electrodes not to be adjusted. 
     (6) The adjustment can be regulated by changing the area of the adjusting layer for each conductor pattern of the adjusting object. 
     (7) The frequency can be adjusted higher by removing a part of the adjusting layer. 
     (8) The frequency can be adjusted lower by adding a part of the adjusting layer thereon. 
     (9) The capacity value of the capacitor can be adjusted by forming at least a pair of interdigital electrodes in the conductor pattern. 
     (10) The inductance value of the inductor can be adjusted by forming the conductor pattern, making at least one of the line width and the line space thereof not more than 60 μm. 
     (11) Using intaglio-printing technique for forming the conductor pattern, it is possible to make the filter small-sized and adjustable. 
     (12) Using thin-film forming technique for forming the conductor pattern, it is possible to make the filter small-sized and adjustable. 
     The present invention having a configuration as described above may provide a small-sized radio frequency filter by which the desired frequency characteristic can be easily obtained with simple configuration even in a higher frequency band. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a configuration diagram of a radio frequency filter in the embodiment 1 of the present invention. 
     FIG. 2 is a configuration diagram of a radio frequency filter in the embodiment 2 of the present invention. 
     FIG. 3 is a configuration diagram of a radio frequency filter in the embodiment 3 of the present invention. 
     FIG. 4 is a configuration diagram of a radio frequency filter in the embodiment 4 of the present invention. 
     FIG. 5 is a configuration diagram of a radio frequency filter in a conventional example. 
     FIG. 6 is a configuration diagram of a radio frequency filter in the embodiment 5 of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of the present invention will be described in the following with reference to the drawings. 
     Embodiment 1 
     FIG. 1 is a configuration diagram of a radio frequency filter in the embodiment 1 of the present invention. In the radio frequency filter of the present embodiment, ground electrode  102  is formed on the under surface of dielectric  101 , and terminal electrodes  103   a ,  103   b ,  103   c ,  103   d ,  103   e ,  103   f  are formed so as to go round the side of dielectric  101  and partly be routed to the top thereof. In addition, on the top surface of dielectric  101  are formed first transmission electrode  104 , second transmission electrode  105 , third transmission electrode  106 , first interdigital electrode  107   a / 107   b , and second interdigital electrode  108   a / 108   b . Further, dielectric layer  109  is laminated on the upper side of these electrodes, on which adjusting dielectric layers  110   a  and  110   b  are respectively formed so as to be at least partly overlapped with first interdigital electrode  107   a / 107   b  and second interdigital electrode  108   a / 108   b . One end of first transmission electrode  104  and one end of second transmission electrode  105  are connected to terminal electrode  103   a . Moreover, one end of first interdigital electrode  107   a  is connected to the other end of second transmission electrode  105 , and terminal electrode  103   d  is connected to the other electrode of first inter digital electrode  107   b . Similarly, the other end of first transmission electrode  104  and one end of third transmission electrode  106  are connected to terminal electrode  103   c . In addition, second interdigital electrode  108   a  is connected to the other end of third transmission electrode  106 , and terminal electrode  103   f  is connected to third interdigital electrode  108   b . Further, terminal electrodes  103   b ,  103   d ,  103   e  and  103   f  are all connected to ground electrode  102  for the purpose of grounding. 
     The operation of a radio frequency filter having a configuration as described above will be described in the following. 
     Interdigital electrodes  107   a / 107   b  are closely disposed without contacting each other. These operate as an interdigital capacitor whose capacity value is determined by the electrode dimension, the number of electrodes, the distance between electrodes, and the dielectric constant of dielectric  101 . Interdigital electrodes  108   a / 108   b  works in the same way. Moreover, transmission electrode  105  operates as an inductor whose inductance is proportional to the characteristic impedance determined by the shape and dimension of the transmission line, and the dielectric constant of dielectric  101 . Transmission electrode  106  works in the same constitution. More specifically, a serial resonator including transmission electrode  105  that operates as an inductor, and interdigital electrode  107   a / 107   b  that operates as a capacitor is connected in shunt to transmission line  104 . Transmission electrode  106  and interdigital electrodes  108   a / 108   b  work in the same way. The serial resonator connected in shunt to the transmission line has an attenuation pole. Accordingly, the radio frequency filter of the present embodiment operates as a band-stop filter having two attenuation poles in which terminal electrode  103   a  and terminal electrode  103   c  respectively function as input terminal and output terminal. 
     Dielectric layer  109  protects each electrode formed on the top surface of dielectric  101 , thereby preventing them from being broken down or damaged. Adjusting dielectric layers  110   a  and  110   b  are formed so as to cover interdigital electrodes  107   a / 107   b  and  108   a / 108   b  respectively. Accordingly, the effective dielectric constant of the portion covered becomes greater as compared with the portion not covered, causing their capacity values to become greater. Moreover, dielectric whose sintering temperature is lower than the sintering temperature for dielectric  101  and dielectric layer  109  is used as the material for adjusting dielectric layers  110   a  and  110   b . In this way, the adjusting dielectric layer can be formed in the final sintering process. Accordingly, it becomes possible to adjust the frequency characteristic after manufacturing the radio frequency filter. As an example, in the case of using thermosetting resin, it has been experimentally confirmed that the interdigital capacitor is increased by 20% in capacity. 
     Embodiment 2 
     FIG. 2 is a configuration diagram of the radio frequency filter in the embodiment 2 of the present invention. The difference from the embodiment 1 of the radio frequency filter in the present embodiment is the forming position of the adjusting dielectric layer. In the present embodiment, adjusting dielectric layers  210   a  and  210   b  are formed so as to be overlapped with at least a part of transmission electrodes  105  and  106  on the top surface of dielectric layer  109  as shown in FIG.  2 . In this case, the frequency characteristic of the radio frequency filter can be adjusted by changing the effective dielectric constant of these transmission electrodes, thereby adjusting the inductance value of the inductor. 
     In the case of the radio frequency filter of the present embodiment, an adjusting magnetic layer can be formed instead of an adjusting dielectric layer. In this case, it is possible to increase the inductance value adjustable range. In the case of the radio frequency filter of the embodiments 1 and 2, an adjusting conductor layer can be formed instead of an adjusting dielectric layer. Also in this case, it is possible to increase the capacity value or inductance value adjustable range. 
     Embodiment 3 
     FIG. 3 is a configuration diagram of the radio frequency filter in the embodiment 3 of the present invention. In the radio frequency filter of the present embodiment, as shown in FIG. 3, adjusting conductor layers  310   a  and  310   b  are provided instead of the adjusting dielectric layers in the embodiment 1 which are grounded by using terminal electrodes  303   d  and  303   f . Except the above, the configuration is in the same constitution as the embodiment 1. In FIG. 3, adjusting dielectric conductor layers  310   a  and  310   b  are formed on the interdigital electrodes, and the capacitances are added thereto in proportion to the area occupied by the interdigital electrode. Accordingly, it is possible to increase the capacitance value adjustable range. 
     Embodiment 4 
     FIG. 4 is a configuration diagram of the radio frequency filter in the embodiment 4 of the present invention. In the radio frequency filter of the present embodiment, as shown in FIG. 4, instead of adjusting dielectric layers in the embodiment 2, adjusting conductor layers  410   a  and  410   b  are disposed so as to be at least partly overlapped with transmission electrodes  105  and  106  on the top surface of dielectric layer  109 . Further, they are grounded by using grounding electrode  403   e  as shown in FIG.  4 . The configuration other than the above is in the same constitution as that in the embodiment 2. In this case, transmission electrodes  105  and  106  are of triplate structure, and their electromagnetic field is concentrated between ground electrode  102  and conductor layers  410   a  and  410   b . Accordingly, it is possible to increase the effective dielectric constant near transmission electrodes and to enlarge the inductance value adjustable range. 
     FIG. 6 is a configuration diagram of the radio frequency filter in the embodiment 5 of the present invention. In the radio frequency filter of the present embodiment, as shown in FIG. 6, adjusting layers  610   a  and  610   b  are disposed so as to be on the top surface of dielectric layer  109 . Adjusting layers  610   a  and  619   b  may be either a dielectric material of a magnetic material. A conductor  611  may be formed on top of the adjusting layer  610   b.  In an alternative embodiment, a conductor  611  may be formed on top of the adjusting layer  610   a.  This embodiment may enhance the change of the effective dielectric constant or the effective permeability of the adjusting layer. 
     In each of the embodiments described above, it is also preferable to adjust the frequency by increasing or decreasing the area of the dielectric layer formed on the top surface of each electrode in order to protect the electrode. In this case, it is advantageous in that the radio frequency filter manufacturing processes may be reduced. 
     In addition, since the above radio frequency filter includes transmission electrodes and interdigital electrodes, it is possible to make the electrode layer in single layer and to manufacture the radio frequency filters by using intaglio-printing technique or thin-film forming technique. Accordingly, in the radio frequency filter of the present invention, each electrode can be formed so as for the line width and the line space to be not more than 60 μm. As a result, it is possible to make the radio frequency filter smaller and thinner as compared with a conventional radio frequency filter. 
     Further, in the serial resonator in each of the above embodiments, one end is connected to the transmission line between the input and output terminals, and the other end is grounded. Accordingly, the radio frequency filter in each of the embodiments operates as a band-stop filter. Further, with the addition of the conditions mentioned below, it is possible to allow the filter function as a filter other than a band-stop filter. 
     Since each of the two serial resonators has an attenuation pole, the impedance is capacitive when the frequency is lower than the resonance frequency and is inductive when higher. Accordingly, when the frequency is between both resonance frequencies, the reactance components of the two resonators cancel each other. In that case, the shapes and dimensions of second transmission electrode  105  and first interdigital electrodes  107   a / 107   b , and third transmission electrode  106  and second interdigital electrodes  108   a / 108   b  are set so that the reactance components become zero or nearly zero. As a result, the radio frequency filter operates as a band-pass filter having a pass-band within the band between the two resonance frequencies. 
     In such a band-pass filter, it is possible to adjust the frequency characteristic of the filter after manufacturing the filter. Furthermore, in the manufacturing process of mobile communication equipment such as mobile phones, it is possible to adjust the frequency characteristic of the filter after assembling the circuit components. As a result, the yield can be improved with respect to the manufacture of the equipment. 
     The present invention having a configuration as described above provides a small-sized radio frequency filter by which the desired frequency characteristic can be obtained with a simple configuration even in higher frequency band.