Filtering device

Abstract: The invention provides a filtering device of the transmission-reception switched type which can be constructed in a form with a reduced size at a low cost without having to use circuit elements such as a capacitor, a coil, and a transmission line forming a phase shift circuit which are not essential to the filtering device. Inner conductors (2a - 2f) serving as distributed-parameter resonance lines are formed in a dielectric block. There is provided a coupling line (3b) coupled with particular inner conductors (2c, 2d). The open-circuited ends of these particular inner conductors (2c, 2d) are connected to an outer conductor via corresponding diode switches (D1, D2) so that transmission and reception filters are switched from each other when either diode (D1, D2) switch is selectively turned on.

Description: 1. Field of the Invention The present invention relates to a filtering device used in a high-frequency device for use in a mobile communication system or the like. 2. Description of the Related Art As a result of recent introduction of the TMDA technique into portable telephone systems, the communication scheme of intermittent transmission/reception in units of time slots has become widely used instead of the concurrent transmission/reception technique. As a result of the change in the communication scheme, the microwave filter which is located at the first stage of a radio communication device and which is used in common in transmission and reception has been changed from a combination of transmission and reception filters to a switching type filter in which a transmission filter and a reception filter are switched from time to time. In general, when a transmission filter and a reception filter are switched from each other by a switch, isolation of the switching circuit makes it possible to reduce signal leakage from a transmission circuit to a reception circuit to a lower level than can be achieved by a single filter. Therefore, requirement of the attenuation characteristic for a filter of the transmission-reception switched type is less severe than that for a filter of the combined transmission-reception type. This makes it possible to realize a smaller-sized filter at a lower cost. The article "Broadbanding Microwave Diode Switches" of R.E. Fischer (IEEE Trans on MTT, vol. MTT-13, Sept, 1965, page 706) and the patent specification US-A-4467296 describe filter networks with diode switches connected to resonant transmission lines Fig. 31 illustrates a typical transmission-reception switched type filter. In Fig. 31, diodes Dl and D2 are used as switching devices for switching a transmission filter and a reception filter from each other. If a switching control current is applied so as to turn on both diodes D1 and D2 into a closed state, a transmission signal is passed through the transmission filter to an ANT terminal. However, because the transmission signal is shunted to ground by the diode D2, the transmission signal cannot reach the reception filter. On the other hand, when the switching control signal is given in such a manner as to turn off both diodes D1 and D2 into an open state, a reception signal is passed through the reception filter, In Fig. 31, L3 is a high-frequency choke coil and C2 is a high-frequency signal shunting capacitor. The combination of L3 and C2 prevents ingress of the RF signal to a control circuit which generates the switching control signal. To improve the isolation of the switching circuit using diodes, it is more desirable to dispose the diodes in a shunted fashion. If the diodes are disposed in a series fashion, leakage of signal occurs due to residual capacitance when the diodes are in an off-state, which results in degradation in isolation between reception and transmission filters. However, in the switching circuit of the type in which a switching device is turned on into a closed state so as to shunt the circuit, it is required that the impedance of the switching device seen from the antenna terminal should be as high as can be regarded as open-circuited thereby eliminating the influence of the closed switching device on the filter used. One known technique of achieving the above requirement is to add an LC phase shift circuit consisting of L1, L2, and C1 to the switching device as shown in Fig. 31. Another technique is to insert a λ/4 transmission line so that the impedance seen from the transmission filter becomes as high as can be regarded as substantially open-circuited. Thus, it is an object of the present invention to provide a filtering device of the transmission-reception switched type which can be constructed in a form with a reduced size at a low cost without having to use circuit elements such as a capacitor and a coil forming a phase shift circuit which are not essential to the filtering device. SUMMARY OF THE INVENTION To achieve the above requirement of reducing the device size and the production cost without using a conventional phase shift circuit, the present invention provides a filtering device according to any aspect described below. According to a first aspect of the present invention, there is provided a filtering device corresponding to Claim 1, using a plurality of filters each including a distributed-parameter resonance line . both ends of which are short-circuited, in such a manner that a switch is connected to a substantially central part of the distributed-parameter resonance line so that the substantially central part is selectively short-circuited when the switch is operated. In this configuration, when the switch is in an open state, the distributed-parameter resonance line acts as a λ/2 resonator both ends of which are short-circuited. When the switch is turned on into a closed state, the center of the distributed-parameter resonance line is short-circuited, and, as a result, the effective length of the resonance line becomes half the original length. As a result, the resonance frequency becomes twice the original resonance frequency, and the distributed-parameter resonance line behaves as a very high impedance at frequencies in the signal frequency band. According to a second aspect of the invention, there is provided a filtering device including a plurality of filters each composed of a distributed-parameter resonance line, wherein a switch is connected to one of the distributed-parameter resonance lines located at the first stage counted from a coupling line, coupling electrode, or coupling element, so that when the switch is operated a predetermined filter becomes negligible or comes to behave as merely a reactance seen from the coupling line or coupling electrode coupled to the distributed-parameter resonance lines of each filter. The structure of the filtering device is not limited to an integral structure such as that described above, but it may also be constructed in such a manner that a plurality of filters constructed in a separate fashion are connected to a common port via a transmission line such as a microstrip line. In this case, a switch may be connected to a distributed-parameter resonance line at the first stage counted from that common port. The number of coupling lines or coupling electrodes sharing the input/output terminal it not limited to one. For example, in the case where an antenna terminal ANT1 is used in common in both transmission and reception, and an RX terminal is used in common to output a reception signal which is received by either of two antenna terminals ANT1 and ANT2 and is transferred to the RX terminal after being passed through either of two RX filters, switches D1 and D2 may be connected to distributed-parameter resonance lines R13 and R21, respectively, at the first stage counted from the terminal ANT1, and switches D3 and D4 may be connected to distributed-parameter resonance lines R22 and R32, respectively, at the first stage counted from the terminal RX. In this configuration, when a signal is transmitted, the switch D2 is turned on so that the signal to be transmitted is prevented from reaching RX or ANT2. When a signal is received, the switch D3 is turned on so that the signal received by ANT2 is transferred to the terminal RX via the RX filter 2 or otherwise the switch D4 is turned on so that the signal received by ANT1 is transferred to the terminal RX via the RX filter 1. By properly controlling the above switching operation, antenna diversity can be achieved. Furthermore, the above technique of the invention may also be applied to a filtering device in which one port is used in common as an input/output port by three or more filters as shown in Fig. 4. In this case, switches D1, D2, and D3 are connected to distributed-parameter resonance lines R11, R21, and R31, respectively, at the first stage counted from port 4. In the case where a filter at a certain location relative to a coupling line or coupling electrode is isolated so that it does not act as a filter as is the case in the above-described examples, a switch is connected to a distributed-parameter resonance line located at the first stage counted from the coupling line or coupling electrode. Referring now to Figs. 7(A), 7(B), 8(A) and 8(B), examples of circuits for supplying a bias voltage to diode switches will be described below. In the example of a bias voltage supply circuit shown in Fig. 7(A), a DC blocking capacitor Cc is connected in series to a diode switch D and both ends of the diode switch D are connected to respective RF choke circuits each consisting of an inductor L and a capacitor C B . If a bias voltage is applied between terminals T B and T B so that the diode D is biased in a forward direction, then the diode D is turned on into a closed state and thus the path between terminals T1 and T2 becomes conductive for a high-frequency signal. In the example shown in Fig. 7(B), a DC blocking capacitor Cc is connected to one end of a diode switch D and the other end of the diode switch is grounded. Furthermore, an RF choke circuit consisting of an inductor L and a capacitor C B is also connected to the one end of the diode D. If a bias voltage is applied to the diode D via a terminal T B , a terminal T is grounded (short-circuited) for a high-frequency signal. In the example shown in Fig. 8(A), a bias voltage is applied selectively to either one of terminals T B1 and T B2 so as to turn on either one of switches Dl and D2. In the example shown in Fig. 8(B), if a positive bias voltage is applied to a common terminal T B , then a switch D1 is turned on. Conversely, if a negative bias voltage is applied to the common terminal T B , then a switch D2 is turned on. According to a third aspect of the invention, microstrip lines formed on a dielectric plate are employed as the distributed-parameter resonance lines, and a switch is disposed on the dielectric plate. This makes it possible to realize a filtering device on which the switch is integrated. According to a fourth aspect of the invention, an element for supplying a bias voltage to the switch is disposed on the dielectric plate. This makes it possible to realize a filtering device on which the bias voltage supply circuit is also integrated. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram illustrating an example of the configuration of a filtering device; Fig. 2 is a diagram illustrating another example of the configuration of a filtering device; Fig. 3 is a diagram illustrating still another example of the configuration of a filtering device; Fig. 4 is a diagram illustrating a further example of the configuration of a filtering device; Fig. 5 is a diagram illustrating an example of the configuration of a filtering device; Fig. 6 is a diagram illustrating an example of the configuration of a filtering device; Fig. 7 is a diagram illustrating an example of the configuration of a circuit for supplying a bias voltage to a diode switch; Fig. 8(A) and 8(B) are diagrams illustrating another example of the configuration of a circuit for supplying a bias voltage to a diode switch; Fig. 9 is a perspective view of an example of a filtering device; Fig. 10(A), 10(B) and 10(C) are an equivalent circuit diagrams of the filtering device shown in Fig. 9; Fig. 11(A) and 11(B) are representations, in the form of an equivalent circuit, of distributed coupling associated with a coupling line; Fig. 12 is a perspective view of a further example of a filtering device; Fig. 13 is an equivalent circuit diagram of the filtering device shown in Fig. 12; Fig. 14 is a perspective view of a further example of a filtering device; Fig. 15 is a perspective view of a further example of a filtering device; Fig. 16 is an equivalent circuit diagram of the filtering device shown in Fig. 15; Fig. 17 is a cross-sectional view of a further example of a filtering device; Fig. 18 is a cross-sectional view of a further example of a filtering device; Fig. 19 is a cross-sectional view of a further example of a filtering device according to the invention; Fig. 20 is a perspective view of a further example of a filtering device; Fig. 21 is a perspective view of a further example of a filtering device; Fig. 22(A), 22(B) and 22(C) are equivalent circuit diagrams of the filtering device shown in Fig. 21; Fig. 23 is a perspective view of a further example of a filtering device; Fig. 24 is an equivalent circuit diagram of the filtering device shown in Fig. 23; Fig. 25 is a perspective view of a second embodiment of a filtering device according to the invention; Fig. 26 is an equivalent circuit diagram of the filtering device according to the second embodiment of the invention; Fig. 27 is a perspective view of a third embodiment of a filtering device according to the invention; Fig. 28 is an equivalent circuit diagram of the filtering device according to the third embodiment of the invention; Fig. 29 is a perspective view of a further embodiment of a filtering device; Fig. 30 is an equivalent circuit diagram of the filtering device shown in Fig. 29; and Fig. 31 is a diagram illustrating an example of a filter switching circuit according to a conventional technique. DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a filtering device useful for understanding the present invention will be described below with reference to Figs. 9 to 11. Fig. 9 is a perspective view of the filtering device. As shown in Fig. 9, inner conductor holes 2a, 2b, 2c, 2d, 2e, and 2f and coupling line holes 3a, 3b, and 3c are formed in a hexahedron-shaped dielectric block 1. The inner surfaces of the inner conductor holes 2a, 2b, 2c, 2d, 2e, and 2f are covered with inner conductors 4a, 4b, 4c, 4d, 4e, and 4f, respectively, and coupling lines 5a, 5b, and 5c are formed in the coupling line holes 3a, 3b, and 3c, respectively. Input/output terminals 6a, 6b, and 6c extending from the coupling lines 5a, 5b, and 5c are formed on the outer surface of the dielectric block 1. Nearly all areas of the outer surface, except for those areas where the input/output terminals are formed, are covered with an outer conductor 7. A non-conducting portion is formed in each inner conductor 4a-4f at a location near one end thereof so that one open end of each inner conductor hole acts as an short-circuited end and the non-conducting portion near the opposite open end acts as an open-circuited end of the corresponding distributed-parameter resonance line and thus each distributed-parameter resonance line acts as a λ /4 resonator. These distributed-parameter resonance lines are disposed in an interdigital fashion. The open-circuited ends of the inner conductors 4c and 4d are connected to the outer conductor 7 via switches D1 and D2, respectively. The direction of the switches Dl and D2 is not limited to that shown in Fig. 1, but the direction may be determined in different manners depending on the configuration of the bias circuit used to a bias voltage to the switches D1 and D2. The coupling line 5a has distributed coupling with the inner conductor 4a. Similarly, the coupling line 5c has distributed coupling with the inner conductor 4f. The coupling line 5b has distributed coupling with the inner conductors 4c and 4d. In this configuration, the part between the input/output terminals 6a and 6b serves as a bandpass filter consisting of three stages of resonators realized by the inner conductors 4a, 4b, and 4c, respectively. The part between the input/output terminals 6b and 6c serves as a bandpass filter consisting of three stages of resonators realized by the inner conductors 4d, 4e, and 4f, respectively. Namely, a duplexer is provided as a whole. If the part between the input/output terminals 6a and 6b is served as a transmission filter and the part between the input/output terminals 6b and 6c is served as a reception filter, the duplexer can be used as a antenna duplexer in which the input/output terminal 6b is connected to an antenna, the input/output terminal 6a is connected to an output of a transmission circuit and the input/output terminal 6c is connected to an input of a reception circuit Fig 10(A), 10(B) and 10(C) illustrate an equivalent circuit of the filtering device shown in Fig. 9. The equivalent circuit for the case where both switches D1 and D2 are in an open state is shown in Fig. 10(A). In these figures, Ra, Rb, Rc, Rd, Re, and Rf correspond to the inner conductors 4a, 4b, 4c, 4d, 4e, and 4f acting as resonators shown in Fig. 1. If the switch D1 is turned on, the resonators Ra, Rb, and Rc are isolated from the circuit, and thus the circuit becomes equivalent to that shown in Fig. 10(B). That is, in Fig. 9, if the switch D1 is turned on, the inner conductor 4c comes to act as merely a ground conductor (shielding conductor) connected between the upper and lower portions of the outer conductor formed on the outer surface of the dielectric block 1. In this state, there is substantially no coupling between the inner conductor 4c and the coupling line 5b. Conversely, if the switch D2 is turned on, the resonators Rd, Re, and Rf are isolated from the circuit as shown in Fig. 10(C). Fig. 11(A) is a representation, in the form of an equivalent circuit, of the distributed coupling between the coupling line 5c and the inner conductors 4c and 4d shown in Fig. 9. If the switch Dl is turned on, the distributed coupling will be represented by the equivalent circuit shown in Fig. 11(B). However, the part surrounded by a broken line in Fig. 11(B) is merely an equivalent representation, and such an element is not present in the actual circuit. In reality, the inner conductor 4c shown in Fig. 9 acts as a ground conductor, and the characteristic impedance seen from the coupling line 5b to the ground conductor is equivalently represented by the part surrounded by the broken line in Fig. 11(B). Figs. 12 and 13 illustrate the exemplary structure of a filtering device useful for understanding the invention. In this filtering device, inner conductor holes 2a, 2b, 2c, 2d, 2e, and 2f are formed in a dielectric block 1, and the inner surfaces thereof are covered with inner conductors 4a, 4b, 4c, 4d, 4e, and 4f, respectively. Input/output terminals 6a, 6b, and 6c are formed on the outer surface of the dielectric block 1. Nearly all areas of the outer surface, except for those areas where the input/output terminals are formed, are covered with an outer conductor 7. A non-conducting portion is formed in each inner conductor 4a-4f at a location near one end thereof so that one open end of each inner conductor hole acts as an short-circuited end and the non-conducting portion near the opposite open end acts as an open-circuited end of the corresponding distributed-parameter resonance line and thus each distributed-parameter resonance line acts as a λ /4 resonator. These distributed-parameter resonance lines are disposed in a comb-line form in which the non-conducting portion in each inner conductor is located on the same side. In this structure, the input/output terminals 6a and 6c are capacitively coupled with the inner conductors 4a and 4f, respectively, at locations near their open-circuited ends, and the input/output terminal 6b is capacitively coupled with the inner conductors 4c and 4d at locations near their open-circuited ends. The open-circuited ends of the inner conductors 4c and 4d are connected to the outer conductor 7 via switches D1 and D2, respectively. Fig. 13 illustrates an equivalent circuit of the filtering device shown in Fig. 12. In Fig. 13, Ra to Rf correspond to the inner conductors 4a to 4f acting as resonators shown in Fig. 12. Adjacent resonators are coupled with each other in a comb-line fashion, and input/output terminals are capacitively coupled with resonators adjacent to them. When the switch D1 is in a closed state, the part between the input/output terminals 6b and 6c serves as a bandpass filter consisting of three stages of resonators. Conversely, when the switch D2 is in a closed state, the part between the input/output terminals 6a and 6b serves as a bandpass filter consisting of three stages of resonators. Fig. 14 is a perspective view illustrating another exemplary filtering device useful for understanding the invention. In this example, inner conductor holes 2a to 2f are formed in a dielectric block 1 and the inner surfaces of the these inner conductor holes are covered with an inner conductor. Open-circuited end electrodes 8a to 8f extending from the corresponding inner conductors are formed on the upper surface of the dielectric block 1 as shown in Fig. 14. Furthermore, coupling electrodes 9a, 9b, and 9c are formed on the upper surface of the dielectric block 1, and input/output terminals 6a, 6b, and 6c extending from the corresponding coupling electrodes are formed as shown in the figure. The side walls and the bottom surface of the dielectric block 1 are covered with an outer conductor 7. The open-circuited end electrodes 8c and 8d are connected to the outer conductor via switches Dl and D2, respectively. In this example, the resonators realized by the respective inner conductors are coupled with one another via capacitances between adjacent open-circuited end electrodes. Similarly, the input/output terminals are coupled with the resonators adjacent to the input/output terminals via capacitances between the corresponding open-circuited end electrodes and coupling electrodes. If the switch D1 is turned on, the inner conductor hole 2c acts as merely a ground electrode to the coupling electrode 9b and the input/output terminal 6b, and three stages of resonators between the input/output terminals 6b and 6c act as a bandpass filter. Conversely, when the switch D2 is turned on, the inner conductor hole 2d acts as merely a ground electrode to the coupling electrode 9b and the input/output terminal 6b, and three stages of resonators between the input/output terminals 6a and 6b act as a bandpass filter. Although in the example shown in Fig. 14, coupling capacitors are formed on the dielectric block, coupling elements such as chip capacitors may be attached directly to the dielectric block. Fig. 15 is a cross-sectional view illustrating another exemplary filtering device useful for understanding the invention. In contrast to the above mentioned examples in which each distributed-parameter resonance line acts as a λ /4 resonator, each distributed-parameter resonance line in this example acts as a λ /2 resonator both ends of which are open-circuited. In this example, as shown in Fig. 15, inner conductor holes and coupling line holes are formed in a dielectric block 1, and the inner surfaces of the inner conductor holes are covered with inner conductors 4a to 4f while coupling lines 5a, 5b, and 5c are formed in the coupling line holes. Non-conducting portions are formed in each inner conductor 4a-4f at locations near both ends so that open-circuited ends are formed at the non-conducting portions. Each coupling line 5a, 5b, and 5c has a similar non-conducting portion formed near its one end. One end of each inner conductor 4c and 4d is connected to the outer conductor 7 via a switch D1 or D2. Fig. 16 illustrates an equivalent circuit of the filtering device shown in Fig. 15. In Fig. 16, Ra to Rf correspond to the resonators realized by the inner conductors 4a to 4f shown in Fig. 15. When the switch D1 is in a closed state, the resonator Rc acts as a λ /4 resonator one end of which is open-circuited and the other end of which is short-circuited, and has a resonance frequency 1/2 times the resonance frequency of the other resonators. When seen from the coupling line 5b, therefore, the resonator Rc behaves as a very high impedance at frequencies in the signal frequency band. As a result, the resonators Ra to Rc do not operate as a filter. Conversely, when the switch D2 is in a closed state, the resonator Rd behaves as a very high impedance or a very low admittance at frequencies in the signal frequency band when seen from the coupling line 5b. As a result, the resonators Rd to Rf do not operate as a filter. In the following examples, techniques of mounting diode switches will be described with reference to Figs. 17 to 19. In the example shown in Fig. 17, a DC blocking capacitor Cc is attached to the inner conductor 4 at a location near its open-circuited end so that one end of the DC blocking capacitor Cc is connected to the inner conductor 4, and a diode switch D is disposed across the non-conducting portion in the inner conductor 4 so that the diode switch D is located between the open end of the inner conductor hole 2 and the other end of the DC blocking capacitor Cc. A bias voltage is applied to the node at which the diode switch D and the DC blocking capacitor Cc are connected to each other, via an RF choke circuit consisting of L and C B disposed between that node and the outer conductor 7 (ground). In the example shown in Fig. 18, an open-circuited end of the inner conductor 4 is formed on one open end of the inner conductor hole 2. A DC blocking capacitor Cc and a diode switch D are connected in series between the open-circuited end of the inner conductor 4 and the outer conductor 7. Furthermore, as in the example shown in Fig. 17, a bias voltage is applied across the diode switch D via an RF choke circuit. In the example shown in Fig. 19, an open-circuited end of the inner conductor 4 is formed on one open end of the inner conductor hole 2. A DC blocking capacitor Cc is disposed near the open end of the inner conductor hole 2 so that one end of the DC blocking capacitor Cc is connected to the inner conductor 4, and a diode switch D is disposed between the outer conductor 7 and the other end of the DC blocking capacitor Cc. Fig. 20 is a perspective view illustrating another exemplary filtering device useful for understanding the invention. As shown in Fig. 20, this filtering device includes two mono-block dielectric filters 11 and 12 each having two inner conductor holes formed in a dielectric block wherein each dielectric filter is surface-mounted on a dielectric plate 13. Microstrips 14, 15, and 16 are formed on the upper surface of the dielectric plate (microstrip substrate) 13, and a ground conductor 17 is formed on the back surface of the dielectric plate 13. The microstrip 15 is connected to the input/output terminals of the respective dielectric filters 11 and 12 so that the input/output terminals are connected to an antenna terminal via the microstrip 15. The microstrips 14 ad 16 are connected to the other input/output terminals of the respective dielectric filters 11 and 12 so that they are connected to RX and TX terminals, respectively. The open-circuited ends of the inner conductors in the inner conductor holes forming antenna-side resonators of the respective dielectric filters 11 and 12 are connected to the ground conductor 17 via switches D1 and D2, respectively. In Fig. 20, some elements such as DC blocking capacitors are not shown for simplicity. Figs. 21, 22(A), 22(B) and 22(C) illustrate another exemplary filtering device using dielectric coaxial resonators. In Fig. 21, reference numerals 21 to 26 denote dielectric coaxial resonators. Lead terminals 27 to 32 are inserted into the inner conductor holes of the respective dielectric coaxial resonators 21 to 26. Reference numeral 33 denotes a coupling substrate. Coupling electrodes 34 to 39 and input/output electrodes 40, 41, and 42 are formed on the upper surface of the coupling substrate 33, and the back surface thereof is covered with a ground electrode 43. The lead terminals 27 to 32 of the dielectric coaxial resonators are connected to the corresponding coupling electrodes 34 to 39 by means of soldering or the like. The lead terminals 29 and 30 are connected to the outer conductor of the corresponding dielectric coaxial resonators via switches D1 and D2, respectively. Fig. 22(A), 22(B), 22(C) indicate an equivalent circuit of the filtering device shown in Fig. 21. In these figures, k11 to k14 and k21 to k24 are coupling reactances (capacitors) present on the coupling substrate shown in Fig. 21. Adjacent resonators are capacitively coupled with each other via these coupling reactances. If the switch D1 is turned on, the end of the capacitor k14 opposite to the end connected to the ANT terminal is grounded as shown in the equivalent circuit of Fig. 22(B), and thus the part between the ANT terminal and the RX terminal acts as a reception filter. Conversely, if the switch D2 is turned on, the end of the capacitor k21 opposite to the end connected to the ANT terminal is grounded as shown in the equivalent circuit of Fig. 22(C), and thus the part between the ANT terminal and the TX terminal acts as a transmission filter. Unlike the filtering device shown in Fig. 9 in which both reception filter and transmission filter are formed in a single dielectric block, reactances k14 and k21 are realized by actual external devices. In the example shown in Fig. 21, capacitors are formed on the coupling substrate 33. Alternatively, chip capacitors serving as coupling elements may be mounted on a coupling substrate or directly on dielectric coaxial resonators so that resonates are coupled via these chip capacitors. Figs. 23 and 24 illustratea another exemplary filtering device using a dielectric plate. As shown in the perspective view of Fig. 23, resonance electrodes 52a to 52f and input/output electrodes 53a, 53b, and 53c are formed on the upper surface of the dielectric plate 51. A ground electrode 54 is formed in such a manner that it extends from the upper surface of the dielectric plate 51 to the lower surface via a side face as shown in Fig. 23. In this structure, comb-line microstrips form two bandpass filters which share the input/output electrode 53b. Through-hole electrodes 55a and 55b electrically connected to the ground electrode formed on the lower surface of the dielectric plate 51, and bias electrodes 56a and 56b are formed on the upper surface of the dielectric plate 51. Furthermore, auxiliary electrodes are formed on the upper surface of the dielectric plate 51 at locations between the resonance electrodes 52c and 52d and the through-hole electrodes 55a and 55b, and the resonance electrodes 52c and 52d are connected to the corresponding auxiliary electrodes via DC blocking capacitors C C1 and C C2 , respectively. Furthermore, auxiliary electrodes are connected to the bias electrodes 56a and 56b via RF choke coils (chip coils) L1 and L2, respectively. Fig. 24 illustrates an equivalent circuit of the filtering device described above. In Fig. 24, Ra to Rf correspond to resonance electrodes 52a to 52f acting as resonators shown in Fig. 23. If a positive bias voltage is applied to the bias electrode 56a thereby turning on the switch D1, the resonance electrode 52c comes to behave as a resonance electrode both ends of which are short-circuited. As a result, the part between the input/output electrodes 53b and 53a does not operate as a bandpass filter, and thus it is possible to selectively use the part between the input/output electrodes 53b and 53c as a bandpass filter. Conversely, if a positive bias voltage is applied to the bias electrode 56b thereby turning on the switch D2, the resonance electrode 52d comes to behave as a resonance electrode both ends of which are short-circuited. As a result, the part between the input/output electrodes 53b and 53c does not operate as a bandpass filter, and thus it is possible to selectively use the part between the input/output electrodes 53a and 53b as a bandpass filter. In the construction shown in Fig. 24, capacitors used in the RF choke circuits may also be mounted on the dielectric plate 51. Fig. 25 is a perspective view illustrating another exemplary filtering device useful for understanding the invention. Resonance electrodes 52a to 52d, input/output electrodes 53a-53c, through-hole electrodes 55a and 55b, and bias electrodes 56a and 56b are formed on the upper surface of the dielectric plate 51. The lower surface of the dielectric plate 51 is covered with a ground electrode 54. One end of each resonance electrode 52b and 52c is connected to the through-hole electrode 55a or 55b via a diode switch D1 or D2. The opposite end of each resonance electrode 52b and 52c is connected to the bias electrode 56a or 56b via an RF choke coil (chip coil) L1 or L2. Fig. 26 illustrates an equivalent circuit of the filtering device shown in Fig. 25. In Fig. 26, Ra to Rd correspond to resonance electrodes 52a to 52d acting as resonators shown in Fig. 25. Each of these resonators behaves as a λ /2 resonator wherein these resonators are disposed so that there is a phase shift of λ /4 between adjacent resonators thereby achieving coupling between adjacent resonators. If a positive bias voltage is applied to the bias electrode 56a thereby turning on the switch D1, the resonator Rb as a whole behaves as a λ /4 resonator. As a result, the impedance of the resonator Rb seen from the input/output electrode 53b becomes very high at frequencies in the signal frequency band, and thus only the part between the input/output electrodes 53b to 53c operates as a bandpass filter. Conversely, if a positive bias voltage is applied to the bias electrode 56b thereby turning on the switch D2, the resonator Rc as a whole behaves as a λ /4 resonator. As a result, the impedance of the resonator Rc seen from the input/output electrode 53b becomes very high at frequencies in the signal frequency band, and thus only the part between the input/output electrodes 53b to 53a operates as a bandpass filter. Figs. 27 and 28 are a perspective view and an equivalent circuit diagram of another exemplary filtering device useful for understanding the invention. Resonance electrodes 52a to 52f, input/output electrodes 53a to 53c, through-hole electrodes 55a and 55b, and bias electrodes 56a and 56b are formed on the upper surface of the dielectric plate 51. The lower surface of the dielectric plate 51 is covered with a ground electrode 54. Through-holes are formed in the dielectric plate 51 at locations on both ends of each resonance electrode so that both ends are short-circuited. The equivalent circuit of this filtering device is shown in Fig. 28. Each resonator Ra, Rb, Re, and Rf acts as a λ /2 resonator both ends of which are short-circuited. When both switches D1 and D2 are in an open state, the resonators Rc and Rd act as a λ /4 resonator, while they act as a λ /2 resonator when both switches are in a closed state. Therefore, if a positive bias voltage is applied to the bias electrode 56a, the resonators Ra to Rc each behave as a λ /2 resonator, and the part between the input/output terminals 53a and 53b operates as a bandpass filter consisting of three stages of resonators. Conversely, if a positive bias voltage is applied to the bias electrode 56b, the resonators Rd to Rf each behave as a λ /2 resonator, and the part between the input/output terminals 53b and 53c operates as a bandpass filter consisting of three stages of resonators. Figs. 29 and 30 are a perspective view and an equivalent circuit diagram of a filtering device according to a first embodiment of the invention. As shown in Fig. 29, resonance electrodes 52a to 52d, input/output electrodes 53a to 53c, a through-hole electrode 55, and bias electrodes 56a and 56b are formed on the upper surface of the dielectric plate 51. The lower surface of the dielectric plate 51 is covered with a ground electrode 54. Through-holes are formed in the dielectric plate 51 at locations on both ends of each resonance electrode so that both ends are short-circuited. The equivalent circuit of this filtering device is shown in Fig. 30. Each resonator Ra to Rd acts as a λ /2 resonator both ends of which are short-circuited. When both switches D1 and D2 are turned on into a closed state, the center positions, which act equivalently as open-circuited terminals, of the resonance electrodes 52b and 52c are short-circuited, and the equivalent lengths of the resonators become half. Therefore, when a positive bias voltage is applied to the bias electrode 56a, the part between the input/output electrodes 53a and 53b does not operate as a filter, but the part between the input/output electrodes 53b and 53c operates as a bandpass filter consisting of two stages of resonators. Conversely, if a positive bias voltage is applied to the bias electrode 56b, the part between the input/output electrodes 53c and 53d does not operate as a filter, but the part between the input/output electrodes 53a and 53b operates as a bandpass filter consisting of two stages of resonators. In the above embodiment, the filtering device operating as a duplexer is disclosed. In the same manner, the filtering, device can also operate as a multipulexer by providing the filter between each of at least 4 input/output portion, as exemplary shown in Fig. 3 and 4. The filter device according to the present invention has various advantages as described below. In the filtering device according to the invention, elements such as a coil, a capacitor, and a transmission line which are required only to form a phase shift circuit in the conventional technique and which are not essential to the filter device are no longer necessary. This makes it possible to achieve a filtering device with a reduced size at a low cost. In the filtering device according to theinvention, the characteristics of the filter can be switched by means of controlling a switch. This makes it possible to realize a filtering device capable of functioning in various manners using a small number of components or elements. According to the invention, a filtering device is constructed in such a manner that a distributed-parameter resonance line is shared by a plurality of filters wherein either one of the plurality of filters can be used selectively. In the filtering device according to the invention, a switch element such as a diode switch is disposed on the filtering device in an integral fashion. This makes it easier to realize a filtering device with a reduced size. According to the invention, a switch element such as a diode switch is disposed in an integral fashion on a filtering device comprising a microstrip line. This makes it possible to realize a filtering device with a reduced total size.

Claims: A filtering device comprising: a plurality of filters each having a distributed-parameter resonance line (52a to 52d) both ends of which are short-circuited; and a coupling line (53b), a coupling electrode, or a coupling element coupled to at least one said distributed-parameter resonance line included in each filter, wherein a switch (D1, D2) is connected to a substantially central part of said at least one distributed-parameter resonance line so that said substantially central part is selectively short-circuited when said switch (D1, D2) is operated. A filtering device according to Claim 1, wherein said switch (D1, D2) is connected to one of said distributed-parameter resonance lines (52b, 52c) located at the first stage counted from said coupling line (53b), coupling electrode, or coupling element. A filtering device according to Claim 1 or 2, wherein at least one of said distributed-parameter resonance lines is shared by said plurality of filters, and said at least one distributed-parameter resonance line is coupled with said coupling line, coupling electrode, or coupling element. A filtering device according to any of Claims 1 to 3, wherein microstrip lines (52a-52f) formed on a dielectric plate (51) are employed as said distributed-parameter resonance lines, and said switch (D1, D2) is disposed on said dielectric plate (51). A filtering device according to Claim 4, wherein an element for supplying a bias voltage to said switch is disposed on said dielectric plate. Use of the filtering device of one of claims 1 to 5 for a duplexer, wherein said duplexer is associated with a shared input/output portion and two input/output portions, and said filtering device is provided between said shared input/output portion and said two input/output portions. Use of the filtering device of one of claims 1 to 6 for a multiplexer, wherein said multiplexer is associated with at least four input/output portions, and said filtering device is provided between each of said input/output portions.

Inventors: TADA HITOSHI, TSUNODA KIKUO, TADA, HITOSHI, TSUNODA, KIKUO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H01P1
IPC codes: H01P1