Patent Publication Number: US-6703912-B2

Title: Dielectric resonator devices, dielectric filters and dielectric duplexers

Description:
FIELD OF THE INVENTION 
     The present invention relates to dielectric resonator devices, dielectric filters and dielectric duplexers for use in communications devices, image devices, etc. 
     BACKGROUND OF THE INVENTION 
     Mobile communications systems using a frequency band of hundreds of megahertz to several gigahertz have terminal devices comprising a receiving circuit  52  and a transmitting circuit  62  which are connected in parallel with an antenna  7  via a duplexer  72  to use the single antenna  7  for both the receiving circuit  52  and the transmitting circuit  62  as shown in FIG.  35 . The duplexer  72  comprises a receiving filter  50  and a transmitting filter  60 , each of which is provided, for example, by a coaxial dielectric resonator  20  shown in FIG.  38 . 
     With reference to FIG. 38, the coaxial dielectric resonator  20  comprises a rectangular parallelepipedal dielectric block  21  having a bore  22  extending therethrough, an outer conductor layer  24  and an inner conductor layer  23  which are formed on the dielectric block  21  respectively over the outer peripheral surface thereof and the inner peripheral surface thereof defining the bore  22 , and a short-circuiting conductor layer  25  formed on the dielectric block  21  over an end face thereof where the bore  22  has an opening and providing a short circuit between the outer conductor layer  24  and the inner conductor layer  23 . 
     With reference to FIG.  39 ( a ) showing the coaxial dielectric resonator  20 , the outer conductor layer  24  is connected to the ground, and the inner conductor layer  23  to a signal input terminal S, whereby the coaxial dielectric resonator  20  is made equivalent to a circuit comprising an inductance element and a capacitance element which are connected in parallel with each other as shown in FIG.  39 ( b ), thus providing a trap filter having a resonance frequency which is determined by the inductance of the inductance element L and the capacitance of the capacitance element C. 
     Terminal devices which are usable for a plurality of communications systems of different frequency bands are required of mobile communications systems. Accordingly, it has been proposed to use a dielectric resonator device shown in FIG. 37 (see, for example, JP-A No. 7-147503/1995) for the receiving filter  50  and transmitting filter  60 . 
     The dielectric resonator device is provided by connecting a switch SW to the point of connection between the inner conductor layer  23  of the coaxial dielectric resonator  20  and the signal input terminal S, via an external capacitor element C 0 , such that the capacitance C 0  of the external capacitor element can be connected to or disconnected from a capacitance C provided between the outer conductor layer  24  of the resonator  20  and the inner conductor layer  23  thereof by operating the switch SW. The resonance frequency of the resonator  20  alters with the variation of capacity effected by switching. 
     FIG. 36 shows an arrangement of a receiving filter  50  and a transmitting filter  60  each comprising such a dielectric resonator device. As illustrated, the receiving filter  50  has a signal line extending from a receiving connection terminal  51  to an antenna terminal  71 , and a plurality of capacitance elements C 4 , C 5  and C 6  provided on the signal line. The transmitting filter  60  has a signal line extending from a transmitting connection terminal  61  to the antenna terminal  71 , and a plurality of capacitance elements C 4 ′, C 5 ′ and C 6 ′ provided on the signal line. Two coaxial dielectric resonators  20 ,  20  are connected to each of the signal lines. A switch SW is connected via a capacitance element C 0  to the point of connection between each resonator  20  and the signal line. Accordingly, the pass bands of the receiving filter  50  and the transmitting filter  60  can be altered for a changeover between two kinds of receiving/transmitting frequencies by operating these switches SW. 
     It has been demanded in recent years that mobile communications terminal devices, such as portable telephones, be made ever smaller in size, giving rise to the great problem of how to reduce the number of electric or electronic components and how to diminish the sizes of such components. However, the dielectric resonator device shown in FIG. 37 has the problem that the need to connect the external capacitor C 0  in the form of a chip to the coaxial dielectric resonator  20  increases the number of components of the device and makes the device large-sized. Further since chip capacitors are great in capacity tolerance, the capacitor requires an additional circuit (not shown) for finely adjusting the capacity, hence a further increase in the number of components. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a dielectric resonator device having a resonance frequency which is accurately variable without necessitating an external capacitor, and a dielectric filter and a dielectric duplexer which comprise the resonator device. 
     The present invention provides a dielectric resonator device comprising a coaxial dielectric resonator  2  which comprises a dielectric block  21  having a bore  22  extending therethrough, an outer conductor layer  24  formed on an outer peripheral surface of the dielectric block  21 , an inner conductor layer  23  formed on the dielectric block  21  over an inner peripheral surface thereof defining the bore  22 , a short-circuiting conductor layer  25  formed on the dielectric block  21  over an end face thereof where the bore  22  has an opening and providing a short circuit between the outer conductor layer  24  and the inner conductor layer  23 , and a separated conductor layer  3  formed on the outer peripheral surface of the dielectric block  21  and electrically separated from the outer conductor layer  24 . 
     The separated conductor layer  3  of the resonator  2  has connected thereto a switch SW by which the capacitance C′ provided between the separated conductor layer  3  and the inner conductor layer  23  is connected to or disconnected from the capacitance C provided between the outer conductor layer  24  and the inner conductor layer  23  upon switching to thereby vary the resonance frequency of the resonator  2 . 
     With the dielectric resonator device of the invention, the inner conductor layer  3  of the resonator  2  is connected, for example, to a signal input terminal S, and the outer conductor layer  24  is connected to the ground. 
     With the device described above, the separated conductor layer  3  on the outer peripheral surface of the dielectric block  21  of the resonator  2  is opposed to the inner conductor layer  23 , providing a capacitance C′ between the two layers. The capacitance C′ is connected to or disconnected from the capacitance C between the outer conductor layer  24  and the inner conductor layer  23  by operating the switch SW, thus performing the same function as a conventional external capacitor. 
     Stated more specifically, the separated conductor layer  3  of the resonator  2  is connected to the ground via the switch SW. Accordingly, when closed, the switch SW connects the separated conductor layer  3  to the ground, whereby the capacitance C′ between the separated conductor layer  3  and the inner conductor layer  23  is connected to the capacitance C between the outer conductor layer  24  and the inner conductor layer  23  to shift the resonance frequency of the resonator  2  toward the lower frequency side. Alternatively when opened, the switch SW cuts off the separated conductor layer  3  from the ground, with the result that the capacitance C′ between the separated conductor layer  3  and the inner conductor layer  23  becomes no longer involved in the resonance frequency of the resonator  2  to shift the resonance frequency toward the higher frequency side. 
     Further stated more specifically, the separated conductor layer  3  of the resonator  2  is provided by forming a groove  26  in the outer conductor layer  24  covering the outer peripheral surface of the dielectric block  21  and separating off a portion of the outer conductor layer  24 . The groove  26  can be formed, for example, by ultrasonic machining. The resonance frequency of the resonator  20  can be made to match the designed value with high accuracy by finely adjusting the area of the separated conductor layer  3  during machining of the groove  26 . 
     Further stated more specifically, the separated conductor layer  3  comprises a first separated conductor layer  31  and a second separated conductor layer  32  which are electrically separated from each other, the first separated conductor layer  31  being connected to an input signal terminal S, the second separated conductor layer  32  being connected to the ground via the switch SW, the outer conductor layer  24  being connected to the ground. With this specific construction, a capacitance C′ is provided between the second separated conductor layer  32  and the inner conductor layer  23 , and a capacitance C″ is provided between the first separated conductor layer  31  and the inner conductor layer  23 . Accordingly, when a high-frequency signal to be input to the inner conductor layer  23  is input to the first separated conductor layer  31 , the input signal is input to the inner conductor layer  23  through the capacitance C″. As a result, the wire for feeding the input signal to the inner conductor layer  23  can be dispensed with. 
     The present invention provides a dielectric filter comprising a first dielectric resonator device  11  and a second dielectric resonator device  12  which are connected to, and located respectively at two positions on, a signal line extending from an input terminal  42  to an output terminal  43 , at least one of the dielectric resonator devices comprising the coaxial dielectric resonator  2  of the invention described. The separated conductor layer  3  of the resonator  2  has connected thereto a switch SW by which the capacitance C′ provided between the separated conductor layer  3  and the inner conductor layer  23  is connected to or disconnected from the capacitance C provided between the outer conductor layer  24  and the inner conductor layer  23  upon switching to thereby give altered signal passage characteristics. 
     The present invention provides a dielectric duplexer comprising a receiving filter  5  and a transmitting filter  6  which are connected in parallel with an antenna terminal  71  for connecting an antenna  7  thereto, each of the receiving filter  5  and the transmitting filter  6  comprising the coaxial dielectric resonators of the invention described. The separated conductor layer  3  of the resonator  2  has connected thereto a switch SW by which the capacitance C′ provided between the separated conductor layer  3  and the inner conductor layer  23  is connected to or disconnected from the capacitance C provided between the outer conductor layer  24  and the inner conductor layer  23  upon switching to thereby alter the signal passage characteristics of the receiving filter  5  or the transmitting filter  6 . 
     The present invention provides another dielectric duplexer comprising a receiving filter  54  and a transmitting filter  64  which are connected in parallel with an antenna terminal  71  for connecting an antenna  7  thereto, the receiving filter  54  comprising a main filter circuit  82  having a pass band in the frequency band of the signal to be received and a trap circuit  83  connected in series with the main filter circuit  82  for attenuating the frequency band of the signal to be transmitted, the trap circuit  83  comprising the coaxial dielectric resonator  2  of the invention described. The separated conductor layer  3  of the resonator  2  has connected thereto a switch SW by which the capacitance C′ provided between the separated conductor layer  3  and the inner conductor layer  23  is connected to or disconnected from the capacitance C provided between the outer conductor layer  24  and the inner conductor layer  23  upon switching to thereby alter the signal passage characteristics of the receiving filter  54 . 
     With the dielectric resonator device, the dielectric filter and the dielectric duplexer according to the invention, the coaxial dielectric resonator  2  itself is provided with a capacitance for varying the resonance frequency as described above, so that the resonance frequency can be altered without necessitating an external capacitor. Further the coaxial dielectric resonator  2  can be set at a designed resonance frequency with high accuracy by finely adjusting the area of the separated conductor layer  3 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the construction of a dielectric resonator device according to the invention; 
     FIG. 2 is a perspective view of a coaxial dielectric resonator constituting the device; 
     FIG. 3 is a view in section of the resonator; 
     FIG. 4 is a diagram showing the construction of a dielectric filter of the invention comprising such resonator devices; 
     FIG. 5 is a perspective view showing the actual construction of the dielectric filter; 
     FIG. 6 is a perspective view of a circuit board for use in the dielectric filter; 
     FIG. 7 is a plan view of the dielectric filter; 
     FIG. 8 is a circuit diagram showing the specific construction of a switch; 
     FIG. 9 is a diagram showing the construction of another dielectric filter of the invention; 
     FIG. 10 is a plan view showing the specific construction of the dielectric filter; 
     FIG. 11 is a diagram showing the construction of a dielectric duplexer of the invention; 
     FIG. 12 is a graph showing the signal pass characteristics of the dielectric filter shown in FIG. 4; 
     FIG. 13 is a graph showing the signal pass characteristics of the dielectric filter shown in FIG. 9; 
     FIG. 14 is a graph showing the signal pass characteristics of the dielectric duplexer shown in FIG. 11; 
     FIG. 15 is a diagram showing the construction of another dielectric resonator device of the invention; 
     FIG. 16 is a perspective view of a coaxial dielectric resonator constituting the device; 
     FIG. 17 is a diagram showing the construction of a dielectric filter of the invention comprising such resonator devices; 
     FIG. 18 is a perspective view showing the actual construction of the dielectric filter; 
     FIG. 19 is a perspective view of a circuit board for use in the dielectric filter; 
     FIG. 20 is a plan view of the dielectric filter; 
     FIG. 21 is a diagram showing the construction of another dielectric filter of the invention; 
     FIG. 22 is a diagram showing the construction of another dielectric duplexer of the invention; 
     FIG. 23 is a diagram showing an equivalent circuit of the coaxial dielectric resonator shown in FIG. 2; 
     FIG. 24 is a diagram showing an equivalent circuit of the coaxial dielectric resonator shown in FIG. 16; 
     FIG. 25 is a diagram showing the construction of another dielectric duplexer of the invention; 
     FIG. 26 is a diagram showing the construction of a receiving filter constituting the dielectric duplexer; 
     FIG. 27 is a diagram showing the construction of another receiving filter constituting the dielectric duplexer; 
     FIG. 28 is a diagram showing the specific construction of the dielectric duplexer; 
     FIG. 29 is a diagram showing the specific construction of another dielectric duplexer; 
     FIG. 30 is a diagram showing the construction of another receiving filter; 
     FIG. 31 is a perspective view showing the actual construction of the receiving filter; 
     FIG. 32 is a graph showing the signal pass characteristics of the receiving filter when the switch is opened; 
     FIG. 33 is a graph showing the signal pass characteristics of the receiving filter when the switch is closed; 
     FIG. 34 is a diagram for illustrating the transmitting and receiving bands of CDMA1900 system; 
     FIG. 35 is a block diagram showing the construction of a mobile communications terminal device; 
     FIG. 36 is a diagram showing an arrangement of conventional receiving filter and transmitting filter; 
     FIG. 37 is a diagram showing the construction of a conventional dielectric resonator device; 
     FIG. 38 is a perspective view showing the construction of a conventional coaxial dielectric resonator; and 
     FIG. 39 is a diagram showing an equivalent circuit of the conventional resonator. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows the construction of a dielectric resonator device  1  according to the invention. The device  1  comprises a coaxial dielectric resonator  2 , and a switch SW for varying the resonance frequency thereof. 
     With reference to FIGS. 2 and 3, the resonator  2  comprises a rectangular parallelepipedal dielectric block  21  made from a ceramic material such as BaTiO 3  or the like. The dielectric block  21  has a bore  22  centrally extending therethrough. The block  21  is covered with an outer conductor layer  24  over the outer peripheral surface thereof and with an inner conductor layer  23  over the inner peripheral surface thereof defining the bore  22 . The block  21  is further covered with a short-circuiting conductor layer  25  over one end face thereof in which the bore  22  has an opening for providing a short circuit between the outer conductor layer  24  and the inner conductor layer  23 . A square groove  26  is formed as by ultrasonic machining in the outer conductor layer  24  covering the outer peripheral surface of the dielectric block  21  to provide inside the groove  26  a separated conductor layer  3  electrically separated from the outer conductor layer  24 . 
     With the dielectric resonator device  1  shown in FIG. 1, the separated conductor layer  3  of the resonator  2  is connected to the ground via a switch SW. For example, a diode is usable for the switch SW. A signal input terminal S is connected to the inner conductor layer  23  of the resonator  2 . The outer conductor layer  24  of the resonator device  1  is connected to the ground. 
     FIGS.  23 ( a ), ( b ) show the resonator device  1  and an equivalent circuit thereof. A capacitance C′ provided between the separated conductor layer  3  and the inner conductor  23  is connected in series with a circuit comprising an inductance element L and a capacitance element C connected in parallel with each other, by connecting a terminal T connected to the separated conductor layer  3  to the ground. 
     With the resonator device  1  described, the switch SW, when closed, connects the separated conductor layer  3  to the ground, whereby the capacitance C′ between the separated conductor layer  3  and the inner conductor layer  23  is connected to the capacitance C between the outer conductor layer  24  and the inner conductor layer  23  to increase the capacity of the resonator  2 . Alternatively when opened, the switch SW cuts off the separated conductor layer  3  from the ground, with the result that the capacitance C′ between the separated conductor layer  3  and the inner conductor layer  23  no longer functions to reduce the capacity of the resonator  2 . Thus, the capacity of the resonator  2  is altered by operating the switch SW to thereby alter the resonance frequency of the resonator  2 . The external capacitor conventionally used can therefore be dispensed with. 
     In fabricating the resonator device  1 , the separated conductor layer  3  is formed by forming the outer conductor layer  24  over the entire area of the outer peripheral surface of the dielectric block  21  and thereafter forming a groove  26  in the layer  24  as by ultrasonic machining, so that the area of the separated conductor layer  3  can be adjusted as desired with high accuracy when the groove  26  is machined. In this way, the resonance frequency of the resonator  2  can be made to match the specified designed value. 
     FIG. 4 shows the construction of an exemplary dielectric filter comprising a trap circuit provided by the coaxial dielectric resonator  2  described. As illustrated, a first inductance element L 1 , a second inductance element L 2  and a third inductance element L 3  are provided on a signal line extending from an input terminal  42  to an output terminal  43 . A first dielectric resonator device  11  is connected via a coupling first capacitance element C 1  to the point of connection between the first inductance element L 1  and the second inductance element L 2 . A second dielectric resonator device  12  is connected via a coupling second capacitance element C 2  to the point of connection between the second inductance element L 2  and the third inductance element L 3 . The first and second resonator devices  11 ,  12  each have the same construction as the device  1  shown in FIG.  1 . 
     As shown in FIG. 8, the switch SW constituting the first and second resonator devices  11 ,  12  comprises a diode D and a resistor R, and the switch SW can be opened or closed by changing the voltage to be applied to a control terminal  44 . 
     FIGS. 5 to  7  show the dielectric filter described, as actually built on a circuit board  4 . As shown in FIG. 6, the circuit board  4  is provided with a conductor pattern  40  including the input terminal  42 , output terminal  43 , control terminal  44  and grounding electrode pattern  45 . The area where the grounding electrode pattern  45  is formed is indicated in FIG. 7 by hatching. Arranged on the conductor pattern  40  as shown in FIG. 5 are coaxial dielectric resonators  2 ,  2 , first to third inductance elements L 1 , L 2 , L 3 , first and second capacitance elements C 1 , C 2 , diodes D, D and resistors R, R. The two resonators  2 ,  2  are each fixed to the circuit board  4 , with the separated conductor layer bearing sides thereof in contact with the surface of the board. Conductor patterns extending from the first and second capacitance elements C 1 , C 2  are connected to the respective inner conductor layers  23  of the resonators  2  by wires  41 . 
     With the dielectric filter described, the voltage to be applied to the control terminal  44  is changed to open or close the switches SW of the resonator devices  11 ,  12  at the same time, whereby the resonance frequency of the resonator devices  11 ,  12  can be altered. Indicated in a solid line in FIG. 12 are signal pass characteristics when the switches SW are opened, and in a broken line are those when the switches are closed. In this way, the signal pass characteristics of the dielectric filter can be shifted toward the lower frequency side or higher frequency side. 
     FIG. 9 shows the construction of another embodiment of dielectric filter comprising the coaxial dielectric resonator  2  described. As illustrated, a first capacitance element C 4 , a second capacitance element C 5  and a third capacitance element C 6  are provided on a signal line extending from an input terminal  42  to an output terminal  43 . A first dielectric resonator device  11  is connected to the point of connection between the first capacitance element C 4  and the second capacitance element C 5 . A second dielectric resonator device  12  is connected to the point of connection between the second capacitance element C 5  and the third capacitance element C 6 . The first and second resonator devices  11 ,  12  each have the same construction as the device  1  shown in FIG.  1 . 
     FIG. 10 shows the dielectric filter described, as actually built on a circuit board  4 . The circuit board  4  is provided with a conductor pattern  40  including the input terminal  42 , output terminal  43 , control terminal  44  and grounding electrode pattern  45 . Arranged on the conductor pattern  40  are coaxial dielectric resonators  2 ,  2 , first to third capacitance elements C 4 , C 5 , C 6 , diodes D, D and resistors R, R. The two resonators  2 ,  2  are fixed to the circuit board  4 , with the separated conductor layer bearing sides thereof in contact with the surface of the board. A conductor pattern extending from the point of connection between the first and second capacitance elements C 4 , C 5  is connected to the inner conductor layer  23  of one of the resonators  2  by a wire  41 . A conductor pattern extending from the point of connection between the second and third capacitance elements C 5 , C 6  is connected to the inner conductor layer  23  of the other resonator  2  by a wire  41 . 
     With the dielectric filter described, the voltage to be applied to the control terminal  44  is changed to open or close the switches SW of the resonator devices  11 ,  12  at the same time, whereby the resonance frequency of the resonator devices  11 ,  12  can be altered. Indicated in a solid line in FIG. 13 are signal pass characteristics when the switches SW are opened, and in a broken line are those when the switches are closed. In this way, the signal pass characteristics of the dielectric filter can be shifted toward the lower frequency side or higher frequency side. 
     FIG. 11 shows the construction of a dielectric duplexer comprising the dielectric filter described. As illustrated, a receiving filter  5  and a transmitting filter  6  are connected in parallel with an antenna terminal  71 . Each of the filters  5 ,  6  comprises the dielectric filter shown in FIG.  9 . The point of connection between the receiving filter  5  and the transmitting filter  6  is connected to the ground via a fourth inductance L 4 , which diminishes the unnecessary low-frequency components. 
     With the duplexer described, the switches SW of the filters  5 ,  6  are operated at the same time, whereby the signal pass characteristics of the two filters  5 ,  6  can be altered. Indicated in solid lines in FIG. 14 are the signal pass characteristics Rx-H, Tx-H of the receiving filter  5  and the transmitting filter  6  when the switches SW are opened, and in broken lines are the signal pass characteristics Rx-L, Tx-L of the filters  5 ,  6  when the switches SW are closed. 
     In this way, the frequency bands of the signal to be received and the signal to be transmitted can be shifted toward the higher frequency side or lower frequency side by operating the switches SW. This makes it possible to provide mobile communications terminal devices usable for two communications systems which are different in frequency band. 
     FIG. 15 shows the construction of another dielectric resonator device  1  according to the invention. The device  1  comprises a coaxial dielectric resonator  2 , and a switch SW for varying the resonance frequency thereof. With reference to FIG. 16, the resonator  2  comprises a rectangular parallelepipedal dielectric block  21  having a bore  22  centrally extending therethrough. The block  21  is covered with an outer conductor layer  24  over the outer peripheral surface thereof and with an inner conductor layer  23  over the inner peripheral surface thereof defining the bore  22 . The block  21  is further covered with a short-circuiting conductor layer  25  over one end face thereof in which the bore  22  has an opening for providing a short circuit between the outer conductor layer  24  and the inner conductor layer  23 . 
     A first rectangular groove  27  and a second rectangular groove  28  are formed as by ultrasonic machining in the outer conductor layer  24  covering the outer peripheral surface of the dielectric block  21  to provide inside the respective grooves  2 ,  28  a first separated conductor layer  31  and a second separated conductor layer  32  which are electrically separated from the outer conductor layer  24 . 
     With the dielectric resonator device  1  shown in FIG. 15, the second separated conductor layer  32  of the resonator  2  is connected to the ground via a switch SW. A signal input terminal S is connected to the first separated conductor layer  31  of the resonator  2 . The outer conductor layer  24  of the resonator device  1  is connected to the ground. 
     FIGS.  24 ( a ), ( b ) show the resonator device  1  and an equivalent circuit thereof. A capacitance C′ provided between the second separated conductor layer  32  and the inner conductor layer  23  and a capacitance C″ provided between the first separated conductor layer  31  and the inner conductor layer  23  are connected to a circuit comprising an inductance element L and a capacitance element C which are connected in parallel with each other, by connecting a terminal T connected to the second separated conductor layer  32  to the ground. 
     With the resonator device  1  described, the switch SW, when closed, connects the second separated conductor layer  32  to the ground, whereby the capacitance C′ between the second separated conductor layer  32  and the inner conductor layer  23  is connected to the capacitance C between the outer conductor layer  24  and the inner conductor layer  23  to increase the capacity of the resonator  2 . Alternatively when opened, the switch SW cuts off the second separated conductor layer  32  from the ground, with the result that the capacitance C′ between the second separated conductor layer  32  and the inner conductor layer  23  no longer functions to reduce the capacity of the resonator  2 . Thus, the capacity of the resonator  2  is altered by operating the switch SW to thereby alter the resonance frequency of the resonator  2 . The external capacitor conventionally used can therefore be dispensed with. 
     With the resonator device  1 , a capacitance C′ is provided between the second separated conductor layer  32  and the inner conductor layer  23 , and a capacitance C″ between the first separated conductor layer  31  and the inner conductor layer  23  as shown in FIGS.  24 ( a ) and ( b ), so that when the high-frequency signal to be input to the inner conductor  23  is input to the first separated conductor layer  31 , the input signal is input to the inner conductor layer  23  through the capacitor C″. This eliminates the need for a wire for feeding the input signal to the inner conductor layer  23 . 
     FIG. 17 shows the construction of an exemplary dielectric filter comprising the coaxial dielectric resonator  2  described. As illustrated, a first inductance element L 1 , a second inductance element L 2  and a third inductance element L 3  are provided on a signal line extending from an input terminal  42  to an output terminal  43 . A first dielectric resonator device  13  is connected to the point of connection between the first inductance element L 1  and the second inductance element L 2 . A second dielectric resonator device  14  is connected to the point of connection between the second inductance element L 2  and the third inductance element L 3 . The first and second resonator devices  13 ,  14  each have the same construction as the device  1  shown in FIG.  15 . 
     The switches SW constituting the first and second resonator devices  13 ,  14 , like the switch SW shown in FIG. 8, comprise a diode D and a resistor R, and the switches SW can be opened or closed by changing the voltage to be applied to a control terminal  44 . 
     FIGS. 18 to  20  show the dielectric filter described, as actually built on a circuit board  4 . As shown in FIG. 19, the circuit board  4  is provided with a conductor pattern  46  including the input terminal  42 , output terminal  43 , control terminal  44  and grounding electrode pattern  45 . The area where the grounding electrode pattern  45  is formed is indicated in FIG. 20 by hatching. Arranged on the conductor pattern  46  as shown in FIG. 18 are coaxial dielectric resonators  2 ,  2 , first to third inductance elements L 1 , L 2 , L 3 , diodes D, D and resistors R, R. The two resonators  2 ,  2  are fixed to the circuit board  4 , with the separated conductor layer bearing sides thereof in contact with the surface of the board. 
     With the dielectric filter described, the voltage to be applied to the control terminal  44  is changed to open or close the switches SW of the first and second resonator devices  13 ,  14  at the same time, whereby the resonance frequency of the resonator devices  13 ,  14  can be altered to shift the signal pass characteristics of the dielectric filter toward the lower frequency side or higher frequency side. 
     The construction shown in FIG.  24 ( a ) is used for the dielectric resonator devices  13 ,  14  in the dielectric filter described, so that the input signal for the resonator  2  is fed to the first separated conductor layer  31 . This eliminates the need for the wire for feeding the input signal to the inner conductor layer  23  as shown in FIG.  18 . 
     Further as shown in FIGS.  24 ( a ), ( b ), the capacitance C″ is provided between the first separated conductor layer  31  of the resonator  2  and the inner conductor layer  23  thereof, and this capacitance C″ serves the function of a coupling capacitance. Accordingly, the dielectric filter shown in FIG. 17 need not be provided with the coupling capacitance elements C 1 , C 2  required for the dielectric filter shown in FIG.  4 . 
     FIG. 21 shows the construction of another embodiment of dielectric filter comprising the coaxial dielectric resonator  2  described. As illustrated, a first capacitance element C 4 , a second capacitance element C 5  and a third capacitance element C 6  are provided on a signal line extending from an input terminal  42  to an output terminal  43 . A first dielectric resonator device  13  is connected to the point of connection between the first capacitance element C 4  and the second capacitance element C 5 . A second dielectric resonator device  14  is connected to the point of connection between the second capacitance element C 5  and the third capacitance element C 6 . The first and second resonator devices  13 ,  14  each have the same construction as the device  1  shown in FIG.  15 . 
     FIG. 22 shows the construction of a dielectric duplexer comprising the dielectric filter described. As illustrated, a receiving filter  5  and a transmitting filter  6  are connected in parallel with an antenna terminal  71 . Each of the filters  5 ,  6  comprises the dielectric filter shown in FIG.  17 . 
     With the duplexer described, the switches SW of the filters  5 ,  6  are operated at the same time, whereby the signal pass characteristics of the two filters  5 ,  6  can be shifted toward the higher frequency side or lower frequency side. This makes it possible to provide mobile communications terminal devices usable for two communications systems which are different in frequency band. 
     FIG. 25 shows the construction of another duplexer  73  according to the invention. Connected to an antenna  7  as illustrated are a transmitting filter  64  having a pass band in the frequency band of the signal to be transmitted and an attenuation band in the frequency band of the signal to be received, and a receiving filter  54  having a pass band in the frequency band of the signal to be received and an attenuation band in the frequency band of the signal to be transmitted. 
     The transmitting filter  64  is provided by connecting a switch SW to the coaxial dielectric resonator  2  described. On the other hand, the receiving filter  54  comprises a main filter circuit  82  having a pass band in the frequency band of the signal to be received, a matching circuit  81 , and a trap circuit  83  for attenuating the frequency band of the signal to be transmitted, these circuits  82 ,  81 ,  83  being connected in series. The main filter circuit  82  comprises, for example, a known surface acoustic wave filter  8  comprising interdigital input electrode and output electrode which are provided on a substrate of LiTaO 3 . The trap circuit  83  comprises a dielectric filter of the invention provided by connecting a switch SW to a coaxial dielectric resonator  2 . Usable as the resonator  2  constituting the trap circuit  83  is one comprising a single separated conductor layer  3  as shown in FIG. 26, or one comprising a first separated conductor layer  31  and a second separated conductor layer  32  as shown in FIG.  27 . 
     FIG. 28 shows the specific construction of the duplexer  73  according to the invention. As illustrated, the receiving filter  54  is provided by connecting a main filter circuit  82  comprising a SAW filter  8 , a matching circuit  81  and a trap circuit  83  of the invention comprising a coaxial dielectric resonator  2  and a switch SW, to a signal line extending from a receiving connection terminal  51  to an antenna  7 . On the other hand, the transmitting filter  64  comprises a signal line extending from a transmitting connection terminal  61  to the antenna  7 , and a dielectric resonator device of the invention composed of a coaxial dielectric resonator  2  and a switch SW and connected to the signal line at each of two positions thereon. 
     With the duplexer  73  described above, the switches SW of the receiving filter  54  and the transmitting filter  64  are operated to shift the signal pass characteristics of the filters  54 ,  64 . With the receiving filter  54 , the impedance of the main filter circuit  82  and that of the trap circuit  83  are made to match by the matching circuit  81 , so that the signal pass characteristics of the receiving filter  54  are the combination of the signal pass characteristics of the main filter circuit  82  and those of the trap circuit  83 . 
     Indicated in a solid line in FIG. 32 are the signal pass characteristics of the receiving filter  54  when the switches SW are open. Indicated in a solid line in FIG. 33 are the signal pass characteristics of the receiving filter  54  when the switches SW are closed. Indicated in chain lines in FIGS. 32 and 33 are the signal pass characteristics of the main filter circuit  82  alone of the receiving filter  54 . 
     The effectiveness of the duplexer  73  of the invention will now be described. With mobile communications systems, the transmitting pass band and the receiving pass band include many channels. FIG. 34 shows the transmitting and receiving bands of CDMA1900 system as such an example. With this system, when a high channel (High Ch) is used for receiving, a high channel is used for transmitting, and when a low channel (Lo Ch) is used for receiving, a low channel is used for transmitting. 
     When high channels are used for transmitting and receiving, the switches SW of the duplexer  73  are opened, whereby the suppression band of the trap circuit  83  is shifted toward the higher frequency side. As a result, the signal pass characteristics are available with the high-channel band fully suppressed in the transmitting band as shown in FIG.  32 . Accordingly there is no likelihood that signals transmitted on the high channel will leak to the receiving circuit. 
     When low channels are used for transmitting and receiving, on the other hand, the switches SW of the duplexer  73  are closed, whereby the suppression band of the trap circuit  83  is shifted toward the lower frequency side. As a result, the signal pass characteristics are available with the low-channel band fully suppressed in the transmitting band as shown in FIG.  33 . Accordingly there is no likelihood that signals transmitted on the low channel will leak to the receiving circuit. 
     FIG. 29 shows the construction of another duplexer  73  according to the invention. The receiving filter  54  of this duplexer is the same as the receiving filter  54  of the duplexer  73  of FIG.  28 . The transmitting filter  64 , on the other hand, comprises a dielectric resonator device serving as a main filter circuit  84  and composed of a coaxial dielectric resonator  2  of the invention and a switch SW, and a SAW filter  80  serving as a trap circuit  85 . The same effectiveness as described is also available with this duplexer  73 . 
     FIG. 30 shows the construction of another receiving filter  54 , which comprises a trap circuit  83  of the invention composed of a coaxial dielectric resonator  2  and a switch SW, and a main filter circuit  82  provided by a plurality of coaxial dielectric resonators  9 . The main filter circuit  82 , matching circuit  81  and trap circuit  83  are made into an integral unit using a common dielectric block  91 , which is arranged on a circuit board  90  along with a diode D and a resistor R. 
     The receiving filter  54  is thus provided in the form of a module. This feature reduces the number of assembling steps and achieves a cost reduction in providing mobile communications terminal devices.