MULTIPLEXER/BRANCHING FILTER

Abstract: A branching filter used for mobile communication equipment; specifically, a multiplexer/branching filter small in size and loss, characterized in that a first resonance circuit having first to third ports each terminating with a pure resistance at the outside and series-resonating at frequencies within a first frequency band in the first frequency band and a second frequency band different from the first frequency band is connected between the first port and ground, an induction element is series-connected between the first port and the second port, a second resonance circuit series-resonating at frequencies within the second frequency band is parallel-connected between the third port and ground, and a capacitive element is series-connected between the third port and the second port with the second port used as a common port.

Description: The present invention relates to a multiplexer/demultiplexer for use in mobile communication devices such as portable telephone. BACKGROUND OF THE INVENTION FIG. 14 represents a circuit construction of a conventional multiplexer/demultiplexer. In FIG. 14, the circuit is comprised of a low-pass filter 101, a high-pass filter 102, phase shifters 103, 104, input terminals 105, 106, and an output terminal 107. Here, low-pass filter 101 and high-pass filter 102 are configured as π-shaped 3-stage circuits. In a filter of this configuration, as the filter functions closer to short-circuiting as the signal frequency goes away from the pass band, it will not function as a multiplexer/demultiplexer by simply connecting each filter as it is. Accordingly, by connecting phase shifters 103, 104 as shown in FIG. 14 on the side the filters are mutually connected, the phase of a signal outside of the pass band is shifted by about 180 degrees thereby to open the circuit. Phase shifters 103 and 104 are comprised of a transmission line such as a strip line. However, this configuration sufferes serious problems of difficulty in miniaturization because of the larger size of transmission lines composing phase shifters 103, 104 compared with other components and of a large loss due to longer line lengths. The article "HF-Weichen: Ein Überblick über Probleme und Schaltungen, Teil II", by M. Borchert, Radio Mentor, no.5, May 1960, pp.378-382, gives an overview of high freguency filters comprising resonant circuits. SUMMARY OF THE INVENTION The present invention is directed to providing an easy-to-miniaturize, low loss, high-performance multiplexer/demultiplexer. In order to achieve the above purpose, the multiplexer/demultiplexer of the present invention has a structure as defined in claim 1. In this way, it is made possible to easily obtain a small and high-performance multiplexer/demultiplexer with a simple circuit with an exceptionally small number of elements without using a transmission line which tends to cause a loss. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a multiplexer/demultiplexer in a first example. FIG. 2 is an equivalent circuit diagram showing a circuit operation of the multiplexer/demultiplexer. FIG. 3 is an equivalent circuit diagram showing a circuit operation of the multiplexer/demultiplexer. FIG. 4 is a circuit diagram of a multiplexer/demultiplexer in a second example. FIG. 5 is circuit diagram of a multiplexer/demultiplexer in a third example. FIG. 6 is a circuit diagram of a multiplexer/demultiplexer in an examplary embodiment of the present invention. FIG. 7 is an exploded perspective view of a multiplexer/demultiplexer of the present invention. FIG. 8 is a perspective view of the multiplexer/demultiplexer. FIG. 9 is a graphical representation of the characteristics of the multiplexer/demultiplexer. FIG. 10 is a circuit diagram of a multiplexer/demultiplexer in a fourth example. FIG. 11 is a circuit diagram illustrating parasitic capacitance of a capacitive element and an inductive element in a high frequency region. FIG. 12 is an equivalent circuit in which the parasitic capacitance in a high frequency region is taken into account in the fourth example. FIG. 13 is an equivalent circuit in which the parasitic capacitance in a high frequency region is taken into account in the first example. FIG. 14 is a circuit diagram of a prior art multiplexer/demultiplexer. First Example: Referring now to the drawings, a description of a first example will be given in the following. FIG. 1 is a circuit diagram of a multiplexer/demultiplexer in the first example. In the drawing, numerals 1, 2, 3 are an input-output port (first port) for a low-frequency band, an input-output port (second port) for a high-frequency band, and a common input-output port (third port), respectively. Numerals 4, 6, 9 are capacitive elements, and numerals 5, 7, 8 are inductive elements. Here, capacitive element 4 and inductive element 5, and capacitive element 6 and inductive element 7 are respectively connected in series forming respective serial resonant circuits 30, 31. Circuit constants are so chosen that serial resonant circuit 30 resonates in the pass band from second port 2 to third port 3, and serial resonant circuit 31 resonates in the pass band from first port 1 to third port 3. When the circuit constants are chosen as above, at a frequency in the pass band from first port 1 which is in the low-frequency band to port 3, the circuit functions as an equivalent circuit represented by FIG. 2, namely, a π-type low-pass filter. Here, serial resonant circuit 30 functions equivalently to a capacitive element 28 as the frequency is lower than its resonant frequency. On the other hand, at a frequency in the pass band from second port 2 which is in the high-frequency band to third port 3, the circuit functions as an equivalent circuit represented by FIG. 3, namely, a π-type high-pass filter. Here, serial resonant circuit 31 functions equivalently to an inductive element 29 as the frequency is higher than its resonant frequency. Consequently, by employing the above circuit construction, it is possible to implement a multiplexer/demultiplexer with an exceptionally small number of elements thus realizing a small, high performance device. Second Example: Referring to the drawings, a description of a second example will be given in the following. FIG. 4 is a circuit diagram of a multiplexer/demultiplexer in a second example. The circuit of this example is one in which a serial resonant circuit 32 comprising a capacitive element 10 and an inductive element 11 has been added between serial resonant circuit 31 and second port 2 of the first example. Here, the resonant frequency of serial resonant circuit 32 is set at a frequency about twice the pass band frequency from second port 2 to third port 3 thus enabling efficient elimination of harmonics from an amplifier and the like to be connected to second port 2 (providing an attenuating pole to eliminate high-frequency spurious components) and providing a multiplexer/demultiplexer with further enhanced performance. Incidentally, by connecting serial resonant circuit 32 between serial resonant circuit 31 and second port 2 as illustrated in FIG. 4, it is possible to restrain deterioration of the pass characteristics from first port 1 to third port 3 due to the influence of serial resonant circuit 32. Third Example: Referring to the drawings, a description of a third example will be given in the following. FIG. 5 is a circuit diagram of a multiplexer/demultiplexer in the third example. The circuit of this example is one in which a capacitive element 12 has been connected in parallel with inductive element 7 of the first example and furthermore the serial resonant frequency of a circuit 33 comprising capacitive elements 6, 12 and inductive element 7 has been set to be in the pass band from first port 1 to third port 3. With this configuration, it is possible to realize a high performance multiplexer/demultiplexer function with an exceptionally small number of elements as well as to efficiently eliminate harmonics generated by an amplifier and the like to be connected to second port 2 because capacitive element 12 becomes a bypass passage for high-frequency signals. Exemplary Embodiment: Referring now to the drawings, a description of an exemplary embodiment of the present invention will be given in the following. FIG. 6 is a circuit diagram of a multiplexer/demultiplexer in the exemplary embodiment of the present invention. The circuit of this embodiment is one in which an inductive element 13 and a capacitive element 14 have been respectively connected in series and in parallel between serial resonant circuit 31 and second port 2 of the first example. With this configuration, it is easy to match the impedance at each port even when the frequency at one port is as high as twice that at the other port, for example the pass band of first port 1 to third port 3 is 900 MHz while that from second port 2 to third port 3 is 1.8 GHz. At the same time, a 2-stage low-pass characteristics can be obtained in the passage from second port 2 to third port 3, and harmonics of an amplifier and the like to be connected to second port 2 can be efficiently eliminated. Electrical characteristics of the present embodiment are as shown in Table 1 and FIG. 9. Element 4 5 6 7 8 9 13 14 Value 3.4pF 2.4nH 29.7pF 1.1nH 8.2nH 4.6pF 0.1nH 9.0pF By the way, the first to third examples and the exemplary embodiment can be implemented by laminating dielectric sheets on which electrodes have been printed as illustrated in FIG. 7 and FIG. 8. FIG. 7 illustrates internal construction of the present embodiment where numeral 19 is a dielectric sheet, numerals 20, 22, 24 are grounding layers printed with a grounding electrode 25, numeral 21 is a group of sheets inside which a capacitive element 4 and inductive elements 5, 8 have been printed and laminated, numeral 23 is a group of sheets inside which a capacitive element and an inductive element have been printed and laminated. In other words, these are groups of sheets on which capacitive elements 6, 9 and an inductive element 7 have been printed and laminated in the first example and capacitive elements 6, 9, 10 and inductive elements 7, 11 have been printed and laminated in the second example, capacitive elements 6, 9, 12 and an inductive element 7 have been printed and laminated in the third example and capacitive elements 6, 9, 14 and inductive elements 7, 13 have been printed and laminated in the exemplary embodiment, respectively. Numeral 26 is a lead electrode of inductive element 8 on the side of port 3 while numeral 27 is a lead electrode of capacitive element 9 on the side of port 3. Lead electrodes 26, 27 are so disposed as will come one on top of the other when stacked. By employing this configuration, it is possible to maximize the amount of isolation of signals passing each filter as the portion through which two different frequency bands sharingly pass can be minimized and spurious coupling due to stray capacitance and the like can also be minimized. By configuring grounding layers 20, 24 on both of the top and bottom ends of the multiplexer/demultiplexer as illustrated in FIG. 7, a shielding effect against the outside can be obtained. Furthermore, by inserting a grounding layer 22 between the circuit section connecting capacitive element 4 and inductive elements 5, 8, inductive elements 7, 11, 13, and the circuit section connecting capacitive elements 6, 9, 10, 12, 14 and inductive elements 7, 11, 13, the electromagnetic interaction between each filter components can be eliminated, thus realizing a high level of isolation between first port 1 and second port 2. Furthermore, by laminating, an extremely small multiplexer/demultiplexer that measures, for example, 3.2 mm deep, 2.5 mm wide, and 1.3 mm high can be implemented. FIG. 8 is a perspective view of the laminated multiplexer/demultiplexer illustrated in FIG. 7. In FIG. 8, numerals 15, 16 are input terminals, numeral 17 is a grounding terminal, and numeral 18 is an output terminal. Here, numeral 18 is a terminal electrode corresponding to third port 3. As has been described above, it forms an external terminal at the point it is taken out. External terminal 18 is also a connecting point for inductive element 8 and capacitive element 9. By employing this configuration, the configuration can be made simpler and, at the same time, it becomes possible to form the above described grounding electrode for isolation up to immediately before the connecting point. By the way, as a ceramic material for lamination, a glass ceramic material consisting of a mixture of 55-65 % by weight of glass composition powders consisting of 40-50 % by weight SiO 2 , 30-40 % by weight BaO, 3-8 % by weight Al 2 O 3 , 8-12% by weight La 2 O 3 , and 3-6 % by weight B 2 O 3 , and 45-35 % by weight forsterite (Mg 2 SiO 4 ); or a glass ceramic material consisting of a mixture of 50-65 % by weight glass composition powders consisting of 40-50 % by weight SiO 2 , 30-40 % by weight BaO, 3-8 % by weight Al 2 O 3 , 8-12 % by weight La 2 O 3 , and 3-6 % by weight B 2 O 3 , and 50-35 % by weight forsterite (Mg 2 SiO 4 ) can be used after adding 0.2-5 % by weight CuO equivalent of copper oxide as a sub-component. By using these materials, the values of elements (on the order of 2 to 10 nH for inductive elements, and 2 to 10 pF for capacitive elements) as used in the exemplary embodiment can be efficiently realized with superior high-frequency characteristics and a low loss. Regarding also the mechanical characteristics, use of these materials makes it possible to realize a multiplexer/demultiplexer which is superior in mounting reliability especially onto resin substrates because of a high anti-breakage strength and an adequately high thermal expansion coefficient, easy manufacturing because the content of boron oxides in the glass powders is small, and the electrical characteristics at high frequencies are superior. Fourth Example Referring to the drawings, a description of a fourth example will be given in the following. FIG. 10 is a circuit diagram of a multiplexer/demultiplexer in the fourth example. The circuit of this example is one in which connecting relationships in the first example of capacitive element 4 and inductive element 5, and capacitive element 6 and inductive element 7 have been reversed, and in which more pronounced effect is obtained when applied to a high-frequency circuit in the GHz range or above. In actually fabricating a circuit, neither of the inductive elements and capacitive elements are difficult to exist as single elements from the view point of high-frequency circuits. In practice, as parasitic capacitances (stray capacitances) 34(a) - 34(d) exist at both ends of each of the inductive elements and the capacitive elements as illustrated in FIG. 11 (a) and (b), they have to be taken into account when designing a circuit. In the case of each element of the present example, the parasitic capacitances are as shown in FIG. 12(a), which can be further summarized as in FIG. 12(b). Similarly, when parasitic capacitances of the first example are considered for the purpose of comparison, they are as shown in FIG. 13(a), which can be further summarized as in FIG. 13(b). As shown in the drawing, in the first example, unnecessary parallel resonant circuits 36, 37 are actually formed by inductive elements 5, 7 and a parasitic capacitance 35. As the parallel resonant circuits 36, 37 comprise parasitic capacitance 35, they are uncontrollable and cause a discrepancy between the designed characteristics and the characteristics that are actually realized by fabrication, presenting a serious difficulty in taking measures. In this example, as it is possible to minimize the influence of parasitic capacitances on filter characteristics by simply changing the connecting relationships of inductive elements and capacitive elements, this approach is very effective in that it enables design and example of the design with ease. INDUSTRIAL APPLICATION As is clear from the above description, as a multiplexer/demultiplexer can be configured with an exceptionally small number of elements and with a simple circuit without using transmission lines which tend to cause a loss, the present invention provides a small and high performance multiplexer/demultiplexer with ease.

Claims: A multiplexer/demultiplexer including a first, a second and a third port each externally terminated with a pure resistance, wherein said third port (3) is made to be a common port by: connecting a first resonant circuit (30) which makes serial resonance at a frequency within a first frequency band or in its proximity between said first port (1) and the ground; further connecting an inductive element (8) in series between said first port (1) and third port (3); connecting a second resonant circuit (31) which makes serial resonance at a frequency within a second frequency band which is different from the first frequency band or in its proximity between said second port (2) and the ground; and further connecting a capacitive element (9) in series between said third port (3) and second port (2); characterized by comprising a further resonant circuit which is configured by connecting an inductive element (13) between one of said first port (1) and said second port (2) on the one hand and one of said second and first resonant circuits (31; 30) on the other hand, and a capacitive element (14) between said first or second port and the ground, and wherein each of said first resonant circuit, second resonant circuit, inductive elements, and capacitive elements are configured on laminated ceramic substrates. The multiplexer/demultiplexer of claim 1 wherein the first frequency band is chosen to be lower in frequency than the second frequency band. The multiplexer/demultiplexer of claim 1 including a third resonant circuit (32) in parallel to said second resonant circuit and in the proximity of said second port, said third resonant circuit being a notch circuit which makes serial resonance at about twice the frequency of the first frequency band. The multiplexer/demultiplexer of claim 1 wherein said second resonant circuit (31) is a resonant circuit comprising a capacitive element (6) and an inductive element (7) connected in series and another capacitive element (12) is additionally connected in parallel with said inductive element (7). The multiplexer/demultiplexer of claim 1 wherein a matching inductive element is connected in series between the second resonant circuit and the second port at a position closer to the second port, and further a matching capacitive element is connected between the second port side of said matching inductive element and the ground. The multiplexer/demultiplexer of claim 1 wherein shielding electrode layers are formed on the top and bottom surfaces of a first filter and on the top and bottom surfaces of a second filter. The multiplexer/demultiplexer of claim 6 wherein one of the input-output ports of each of said first and second filters is connected at the portion where these ports are taken out to the edges of the laminated ceramic substrates to form a common port. The multiplexer/demultiplexer of claim 1 wherein a glass ceramic material consisting of a mixture of 55-65 % by weight of glass composition powders consisting of 40-50 % by weight SiO2, 30-40 % by weight BaO, 3-8 % by weight Al2O3, 8-12% by weight La2O3, and 3-6 % by weight B2O3, and 45-35 % by weight forsterite (Mg2SiO4) is used as the laminating ceramic material. The multiplexer/demultiplexer of claim 1 wherein a glass ceramic material consisting of a mixture of 50-65 % by weight glass composition powders consisting of 40-50 % by weight SiO2, 30-40 % by weight BaO, 3-8 % by weight Al2O3, 8-12 % by weight La2O3, and 3-6 % by weight B2O3, and 50-35 % by weight forsterite (Mg2SiO4) is used as the laminating ceramic material after adding 0.2-5 % by weight CuO equivalent of copper oxide as a sub-component.

Inventors: KATSUMURA HIDENORI, MATSUMURA TSUTOMU, SATOH YUKI, YUDA NAOKI, KATSUMURA, HIDENORI, MATSUMURA, TSUTOMU, SATOH, YUKI, YUDA, NAOKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: H03H7
IPC codes: C03C14, H03H7, C03C10, C03C8