Abstract:
The invention provides a filter, comprising: at least one inductance device, at least one resonator, and at least one switching device. The inductance device may be connected in parallel with said switching device, and the resonator may be connected in series with the switching device. The above filter is small in size that is capable of functioning for high-frequency signals of plural frequency bands which are relatively adjacent to each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a filter for use in mobile communication apparatuses, such as portable or mobile telephones, that conform to the use for plural high-frequency bands. 
     2. Description of the Related Art 
     Hitherto, when two high-frequency bands of mobile communications are relatively adjacent to each other as in a case of a 900-MHz GSM (global system for mobile communications) and a 1.8-GHz DCS (digital cellular system), two mobile communication systems share a single antenna. FIG. 10 is a block diagram showing a structure in which two communication systems share a single antenna. In FIG. 10,  51  refers to an antenna,  52  refers to a duplexer,  53  refers to a GSM-side switch, and  54  refers to a DCS-side switch. The antenna  51  is connected to a first terminal  53   a  of the GSM-side switch  53  through the duplexer  52 , and a GSM transmitter circuit Txgsm and a GSM receiver circuit Rxgsm are connected to a second terminal  53   b  and a third terminal  53   c , respectively. Furthermore, the antenna  51  is connected to a first terminal  54   a  of the DCS-side switch  54  through the duplexer  52 , and a DCS transmitter circuit Txdcs and a DCS receiver circuit Rxdcs are connected to a second terminal  54   b  and a third terminal  54   c , respectively. The frequency band is thus separated for the GSM and the DCS through the duplexer  52 , and transmit/receive switching is performed by the GSM-side switch  53  and the DCS-side switch  54 . By this arrangement, transmitting-receiving communication between the two mobile communication systems GSM and DCS, sharing a single antenna, can be achieved. 
     However, in such a conventional structure sharing a single antenna, the antenna is connected to the GSM switch and the DCS switch through the duplexer, and through these switches, the antenna is further connected to the transmitter circuits and the receiver circuits; in which a problem arises with the increased number of constituent members. This problem makes it difficult to reduce the size of the mobile communication apparatus mounting the aforementioned constituent members. 
     SUMMARY OF THE INVENTION 
     To overcome the problems described above, preferred embodiments of the present invention provide a smaller-sized filter that is capable of functioning for high-frequency signals of plural frequency bands which are relatively adjacent to each other. 
     One preferred embodiment of the present invention provides a filter, comprising: at least one inductance device, at least one resonator, and at least one switching device. The inductance device may be connected in parallel with said switching device, and the resonator may be connected in series with the switching device. 
     According to the above described structure and arrangement, since it comprises an inductance device, a resonator, and a switching device, an equivalent circuit of a circuit formed of the inductance device, the resonator, and the switching device can be changed by controlling the voltage applied to the switching device. This allows the frequency band of high-frequency signals which pass through the filter to be changed; in which a single filter is enabled to sufficiently function for two high-frequency signals having two different frequency bands. Accordingly, mobile communication apparatuses having this filter can be smaller. 
     Furthermore, an antenna and a transmitting high-output amplifier can be shared by use of this filter in mobile communication apparatuses of plural frequency bands that are relatively adjacent to each other. 
     In the above described filter, the inductance device, the resonator, and the switching device may be disposed in or mounted on a multilayered body comprising a plurality of dielectric layers. 
     According to the above described structure and arrangement, since it is formed of a multilayered body, an inductance device, capacitance device, and switching device can be disposed in or mounted on the multilayered body. This allows the filter to be formed smaller. 
     In the above described filter, the resonator may be an open-stub resonator. 
     According to the above described structure and arrangement, it is not influenced by parasitic inductance of the pin diode; therefore, a greater insertion-loss attenuation can be established. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a circuit diagram of a filter according to a first preferred embodiment of the present invention. 
     FIG. 2 is a perspective view of the filter in FIG.  1 . 
     FIGS. 3A to  3 H are upper-surface plan views of a first dielectric layer to an eighth dielectric layer which constitute a multilayered body of the filter in FIG.  2 . 
     FIGS. 4A to  4 E are upper-surface plan views of a ninth dielectric layer to a thirteenth dielectric layer which constitute the multilayered body of the filter in FIG.  2 . FIG. 4F is a lower-surface plan view of the thirteenth dielectric layer. 
     FIG. 5A is an equivalent circuit diagram when a pin diode of the filter in FIG. 1 is turned ON and FIG. 5B is an equivalent circuit diagram when the pin diode of the filter in FIG. 1 is turned OFF. 
     FIG. 6 is a graph showing frequency characteristics of the filter in FIG.  1 . 
     FIG. 7 is a block diagram showing a structure in which the filter in FIG. 1 is used and an antenna is shared by mobile communication apparatuses of different frequency bands. 
     FIG. 8 is a circuit diagram showing a filter according to a second preferred embodiment of the present invention. 
     FIG. 9 is a graph showing frequency characteristics of the filter in FIG.  8 . 
     FIG. 10 is a block diagram showing a structure in which an antenna is shared by conventional mobile communication apparatuses of different bands. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a circuit diagram of a filter of a first preferred embodiment in accordance with the present invention. A filter  10  is a low-pass filter and is formed of a transmission line L 11  which is an inductance device, an open-stub resonator RES 11 , a pin diode PD 11  which is a switching device, a capacitor C 11 , a choke coil CC 11 , and a resistor R 11 . 
     The pin diode PD 11  and the open-stub resonator RES 11  are T-connected between a first port P 1  and a second port P 2 , and a series circuit formed of the transmission line L 11  and the capacitor C 11  are parallel-connected to the pin diode PD 11 . 
     A control terminal Vcc 1  is connected to a junction of the transmission line L 11  and an anode of the pin diode PD 11  through the choke coil CC 11 . Further, a control terminal Vcc 2  is connected to a junction of the capacitor C 11  and a cathode of the pin diode PD 11  through the resistor R 11 . 
     In this case, the capacitor C 11  prevents a direct current from flowing into the transmission line L 11 . Furthermore, the choke coil CC 11  and the resistor R 11  prevent a high-frequency signal from leaking to the control terminals Vcc 1  and Vcc 2  when voltage is applied to the pin diode PD 11 . it is especially notable that the resistor R 11  also reduces power consumption. 
     FIG. 2 shows a perspective view of the filter  10  shown in FIG.  1 . This filter  10  comprises a multilayered body  11 , inside which are provided a strip conductor (not shown) forming the transmission line L 11 , a capacitor electrode (not shown) forming the capacitor C 11 , a strip conductor (not shown) forming the resonator RES 11 , and a strip conductor (not shown) forming the choke coil CC 11 . On an upper surface, which is one of the principal planes of the multilayered body  11 , the pin diode PD 11  and the resistor R 11  are mounted. 
     Furthermore, six external electrodes Ta to Tf are provided such that they reach from the upper surface to the lower surface through the lateral surfaces of the multilayered body  11 . Of these external electrodes Ta to Tf, the three external electrodes Ta to Tc are formed through the one lateral face of the multilayered body  11 , and the other three external electrodes Td to Tf are formed through the other lateral surface of the multilayered body  11 . The external electrode Ta functions as the first port P 1 , the external electrodes Tb and Te function as grounding terminals, the external electrode Tc functions as the second port P 2 , and the external electrodes Td and Tf respectively function as the control terminals Vcc 1  and Vcc 2  that control the voltage applied to the pin diode PD. 
     FIGS. 3A to  3 H and FIGS. 4A to  4 F are upper-surface plan views and lower-surface plan views of dielectric layers that constitute the multilayered body  11  of the filter  10 . The multilayered body  11  is formed in such a manner that first to thirteenth dielectric layers  11   a  to  11   m  are layered sequentially from the top toward the bottom. 
     On the first dielectric layer  11  a are print-formed lands La 1  to La 4  on which the pin diode PD 11  and the resistor R 11  are mounted. On respective upper surfaces of the second to fifth dielectric layers  11   b  to  11   e , capacitor electrodes Cp 1  to Cp 4  which are formed of conducting layers are print-formed. Further, on respective upper surfaces of the sixth to the eighth dielectric layers  11   f  to  11   h , the tenth dielectric layer  11   j , the eleventh dielectric layer  11   k , and the twelfth dielectric layer  11  strip electrodes Lp 1  to Lp 9  which are each formed of conducting layers are print-formed. 
     On respective upper surfaces of the ninth and the thirteenth dielectric layers  11   i  and  11   m , grounding electrodes Gp 1  and Gp 2  which are formed of conducting layers are print-formed. On a lower surface of the thirteenth dielectric layer  11   m  (FIG.  4 F), are print-formed the external terminals Ta and Tc (FIG. 2) which respectively function as the first port P 1  and the second port P 2 , the external terminals Tb and Te (FIG. 2) which each function as the grounding terminals, and the external terminals Td and Tf (FIG. 2) which each function as the control terminals. 
     The capacitor electrodes Cp 1  to Cp 4  form the capacitor C 11  (FIG.  1 ). The strip electrodes Lp 1 , Lp 3 , and Lp 5  form the transmission line L 11  (FIG.  1 ), the strip electrodes Lp 7  to Lp 9  form the resonator RES 11  (FIG.  1 ), and the strip electrodes Lp 2 , Lp 4 , and Lp 6  form the choke coil CC 11  (FIG.  1 ). 
     Hereinbelow, a description will be given of the performance of the filter  10  structured as described above. When the pin diode PD 11  is turned ON (Vcc 1 =3 V, Vcc 2 =0 V), it turns short; in which an equivalent circuit is as shown in FIG.  5 ( a ). Therefore, a circuit formed of the transmission line L 11 , the resonator RES 11 , and the pin diode PD 11  becomes a circuit formed of only the resonator RES 11 ; by which high-frequency signals having a wavelength of four times that of the resonator RES 11  can be prevented from flowing in. 
     On the other hand, when the pin diode PD 11  is turned OFF (Vcc 1 =0 V, Vcc 2 =3 V), it turns open; in which an equivalent circuit is as shown in FIG.  5 B. Therefore, a circuit formed of the transmission line L 11 , the resonator RES 11 , and pin diode PD 11  becomes an LC resonator circuit formed of the transmission line L 11  and the pin diode PD 11 ; by which high-frequency signals having a resonating frequency of this LC resonator circuit can be prevented from flowing in. 
     In such a structure, the frequency band of high-frequency signals passthrough the filter  10  can be changed by turning the pin diode PD 11  ON or OFF. 
     FIG. 6 shows the pass-through characteristics of the filter  10 . In FIG. 6, the solid line indicates the characteristics when the pin diode PD 11  is ON, and the broken line indicates the characteristics when the pin diode PD 11  is OFF. This figure apparently indicates that (A) when the pin diode PD 11  is ON, a high-frequency signal of 900 MHz can be passed through, and (B) when the pin diode PD 11  is OFF, a high-frequency signal of 1.8 GHz can be passed through. 
     An application example of an arrangement is shown in FIG. 7, in which the aforementioned filter  10  is used for a transmitter member Tx between two frequency bands which are relatively adjacent to each other, for example, between a GSM (global system for mobile communications), which is a 900-MHz band mobile phone system, and a DCS (dynamic cell system), which is a 1.8-GHz band mobile phone system. 
     An antenna  1  is connected to a first terminal  2   a  of a second switch comprised of a coaxial switch, a PIN diode switch, or the like. A transmitter member Tx and a receiver member Rx are connected to a second terminal  2   b  and a third terminal  2   c  of the second switch, which are a transmitting terminal and a receiving terminal, respectively. In the transmitter member Tx, one of the terminals of the filter  10  is connected to the second terminal  2   b  of a switch  2 , while the other terminal is connected to a high-output amplifier PA that is shared by the 900-MHz band GSM and the 1.8-GHz DCS. 
     In such a structure, in the case of two different frequency bands, for example, when the pin diode PD 11  of the filter  10  is ON, only a transmitting signal of a GSM frequency of 900 MHz is passed through, while when the pin diode PD 11  of the filter  10  is OFF. a transmitting signal of a DCS frequency of 1.8 GHz can be passed through. 
     The above indicates that high-frequency signals of the 900-MHz band GSM and the 1.8-MHz band DCS can be distributed and coupled, allowing communication in two high-frequency bands to be achieved. 
     Accordingly, an antenna and a transmitting-side high-output amplifier can be shared by the 900-MHz band GSM and 1.8-MHz band DCS, which are two high-frequency signal bands that are relatively adjacent to each other. 
     According to the filter  10  of the first preferred embodiment, as described above, the equivalent circuit of the circuit formed of the transmission line, the resonator, and the pin diode can be changed by changing the voltage applied to the pin diode to turn it ON or OFF. This allows the frequency band of high-frequency signals which pass through the filter, which is a low-pass filter, to be changed; in which a single filter is enabled to sufficiently function for two high-frequency signals having two different frequency bands. Accordingly, mobile communication apparatuses having this filter can be smaller. 
     Furthermore, since the resonator is an open-stub resonator, it is not influenced by parasitic inductance of the pin diode; therefore, an increased insertion-loss attenuation can be established. 
     Furthermore, as shown in FIG. 2, when the filter is formed of the multilayered body, the transmission line can be formed inside the multilayered body. This allows the filter to be formed smaller. 
     FIG. 8 is a circuit diagram of a filter in accordance with a second preferred embodiment of the present invention. A filter  20  is a low-pass filter and it is formed of transmission lines L 21  to L 23  which are inductance devices, open-stub resonators RES 21  and RES 22 , pin diodes PD 21  and PD 22  which are switching devices, capacitors C 21  and C 22 , choke coils CC 21  and CC 22 , and resistors R 21  and R 22 . 
     The pin diode PD 21  and the resonator RES 21 , and the pin diode PD 22  and the resonator RES 22  are π-connected to portions, respectively, between a first port P 1  and a second port P 2 . A series circuit, which is formed of the transmission line L 21  and the capacitor C 21 , and a series circuit, which is formed of the transmission line L 22  and the capacitor C 22 , are parallel-connected to the pin diode PD 21  and the pin diode PD 22 , respectively. 
     A control terminal Vcc 1  is connected to a junction of the transmission line L 21  and an anode of the pin diode PD 22  through the choke coils CC 21  and CC 22 . Further, a control terminal Vcc 2  is connected to a junction of the capacitor C 22  and a cathode of the pin diode PD 22  through the resistors R 21  and R 22 . 
     For reference, the anode of the pin diode PD 21  and the anode of the pin diode PD 22  are connected through the transmission line L 23 . That is, the filter  20  has a structure in which the filter  10  of the first embodiment (FIG. 1) is series-connected between the first port P 1  and the second port P 2  through the transmission line L 23 . 
     In this structure, the capacitors C 21  and C 22  each prevent a direct current from flowing into the transmission lines L 21  and L 22 . Furthermore, the choke coils CC 21  and CC 22  and the resistors R 21  and R 22  prevent high-frequency signals from leaking into the control terminals Vcc 1  and Vcc 2  when voltage is applied to the pin diodes PD 21  and PD 22 . It is notable that the resistors R 21  and R 22  also reduce power consumption. 
     FIG. 9 is a graph showing frequency characteristics of the filter  20 . In FIG. 9, a dot line indicates characteristics when the pin diodes PD 21  and PD 22  are ON (Vcc 1 =3 V, Vcc 2 =0 V), while a broken line indicates characteristics when the pin diodes PD 21  and PD 22  are OFF (Vcc 1 =0 V, Vcc 2 =3 V). This figure apparently indicates that (A) when the pin diodes PD 21  and PD 22  are ON, a high-frequency signal of 900 MHz can be passed through, and (B) when the pin diodes PD 21  and PD 22  are OFF, a high-frequency signal of 1.8 GHz can be passed through. 
     The above indicates that, with the structure of the filter  20  (FIG.  7 ), the frequency band of the high-frequency signals pass-through the filter  20  can be changed by turning the pin diodes PD 21  and PD 22  ON or OFF. 
     According to the aforementioned filter of the second preferred embodiment, two resonators are used to allow the insertion-loss attenuation to be greater than that of the filter of the first embodiment which has only a single resonator. That is, the larger the number of resonators used, the greater the insertion-loss attenuation will be. 
     For reference, in the aforementioned first and second embodiments, the filter is a low-pass filter; however, filters such as high-pass filters, band-pass filters, and band-rejection filters which have inductance devices, resonators, and switching devices are all within the scope of the present invention. 
     Furthermore, although arrangements in which pin diodes are used as switching devices are already described, varicap diodes, bipolar transistors, or field-effect transistors may be used to provide the same effect as in the aforementioned embodiments. 
     Furthermore, although arrangements in which a choke coil or a resistor through which the control terminal is connected is used are already described, the device may be any alternative device as long as it works to prevent high-frequency signals from flowing into the control terminal during the voltage application to the pin diode. 
     Furthermore, although arrangements in which inductance devices and capacitance devices are formed inside a multilayered body formed of dielectric layers are already described, chip devices may be mounted on the multilayered body. 
     Furthermore, arrangements in which switching devices are mounted on the multilayered body are already described, but an arrangement may be such that cavities or the like are formed inside the multilayered body and through the cavities or the like switching devices are formed inside the body. 
     Furthermore, although an arrangement in which the filter according to the present invention is used in the case of coupling between a GSM and a DCS is already described, the use of the filter is not restricted to the case of coupling between the systems of the GSM and the DCS, but the filter may be used in other cases of coupling between systems of, for example, the GSM and a DCS (digital cellular system), the GSM and a PCS (personal communication services), an AMPS (advanced mobile phone services) and the PCS, the GSM and a DECT (Digital European Cordless Telephone), and the PDC and a PHS (personal handy-phone system). 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the forgoing and other changes in form and details may be made therein without departing from the spirit of the invention.