Abstract:
There is provided a frequency-variable-type filter, comprising: a voltage control terminal; at least one resonator; a switching device for being switched on/off by a control voltage supplied to the voltage control terminal; and the switching device being in the off-state, when a control circuit as an external circuit which is electrically connected to the voltage control terminal has high impedance at 0 V.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a frequency-variable-type filter, an antenna duplexer, and a communication apparatus, for example, which are used in the microwave band. 
     2. Description of the Related Art 
     In a conventionally-known frequency-variable-type filter, a resonator is connected to a switching device such as a PIN diode or a variable capacitance diode through a capacitor or the like to perform voltage control thereof so as to vary a resonance frequency (cf. Japanese Unexamined Patent Publication No. 7-321509). When a PIN diode is used, a frequency is switched by switching it on/off, so that two bands including a band in the on-period and a band in the off-period are provided. In the on-period, usually, a positive control voltage is supplied in order to switch the PIN diode on, whereas a negative voltage is supplied in the off-period. The reason why the negative voltage is necessary in the off-period is that when high-frequency signals of large electric power are input, a high-frequency voltage is applied to the PIN diode and it is switched on, which should be avoided. In other words, when a large amount of electric power is input, the PIN diode becomes unstable and thereby the frequency characteristic varies. Consequently, this needs to be avoided. 
     However, in the conventional frequency-variable-type filter, in order to switch a PIN diode off, a power-supply circuit for generating a negative voltage (approximately −3 through −10 V) is required, so that the circuitry is complicated, leading to an obstacle to miniaturization and cost reduction of a mobile phone or the like. 
     SUMMARY OF THE INVENTION 
     To overcome the above describe problems, preferred embodiments of the present invention provide a frequency-variable-type filter, an antenna duplexer, and a communication apparatus, which are small and low-cost. 
     One preferred embodiment of the present invention provides a frequency-variable-type filter, comprising: a voltage control terminal; at least one resonator; a switching device for being switched on/off by a control voltage supplied to the voltage control terminal; and the switching device being in the off-state, when a control circuit as an external circuit which is electrically connected to the voltage control terminal has high impedance at 0 V. 
     Another preferred embodiment of the present invention provides a communication apparatus, comprising: a frequency-variable-type filter comprising a voltage control terminal; at least one resonator; and a switching device for being switched on/off by a control voltage supplied to the voltage control terminal; and a control circuit for supplying a control voltage to the voltage control terminal to perform voltage control of the frequency-variable-type filter; and the switching device being in the off-state when the control circuit has high impedance at 0 V. 
     In the above described frequency-variable-type filter and communication apparatus, a PIN diode may be used as the switching device and a dielectric resonator may be used as the resonator. Additionally, a transistor, a field-effect transistor, or the like, may be used in the control circuit, in which the impedance of the control circuit is 100 KΩ or more when the control circuit is at 0 V. 
     Furthermore, the anode of the PIN diode may be electrically connected to the resonator through either an inductor or a capacitor, whereas the cathode of the PIN diode may be electrically connected to a ground; and the voltage control terminal may be electrically connected to the anode of the PIN diode through a chalk coil, whereas a noise-cutting bypass capacitor may be electrically connected between the voltage control terminal and a ground. In addition, the impedance of the chalk coil may be set to be 350 Ω or more, and the capacity of the noise-cutting bypass capacitor may be set to be in a range of 10 through 1000 pF. 
     According to the above described structure and arrangement, even if high-frequency signals of large electric power are input when the switching device is in the off-state, since the control circuit connected to the voltage control terminal has high impedance, a negative voltage is constantly imposed on the switching device. Thus, even if high-frequency signals of large electric power are input to the frequency-variable-type filter, a stable frequency characteristic is obtainable. In addition, since there is no need of a power-supply circuit for generating a negative voltage to form a simple circuit, miniaturization and cost reduction of the communication apparatus can be achieved. 
     Other features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention which refers to the accompanying drawings, where like reference numerals indicate like elements to avoid duplicative description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an electrical circuit diagram of an antenna duplexer and a control circuit for showing a first preferred embodiment of a communication apparatus according to the present invention. 
     FIG. 2 is a perspective view of the structure in which the antenna duplexer shown in FIG. 1 is mounted. 
     FIG. 3 is a sectional view of an example of a resonator used in the antenna duplexer shown in FIG.  1 . 
     FIG. 4 is a view for illustrating the operation of the antenna duplexer shown in FIG.  1 . 
     FIG. 5 is a view for illustrating the operation of the antenna duplexer shown in FIG.  1 . 
     FIG. 6 is an electric circuit diagram of an antenna duplexer and a control circuit for showing a second preferred embodiment of a communication apparatus according to the present invention. 
     FIG. 7 is a block diagram for showing a third preferred embodiment of the communication apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Preferred Embodiment, FIGS.  1  through  5 ] 
     FIG. 1 shows the structures of an antenna duplexer  1  and a control circuit  11  inside the communication apparatus; and FIG. 2 is a perspective view of the antenna duplexer  1  in which individual components are mounted ON a circuit substrate  40 . In the antenna duplexer  1 , a transmitting-side circuit  25  is electrically connected between a transmission terminal Tx and an antenna terminal ANT, whereas a receiving-side circuit  26  is electrically connected between a reception terminal Rx and the antenna terminal ANT. 
     The transmitting-side circuit  25  has a frequency-variable-type band block filter circuit  27  and a phase circuit  29 . The band block filter circuit  27  is formed by coupling two stages of resonance circuits, in which a resonator  2  is electrically connected to the transmission terminal Tx through a resonance capacitor C 1  and a resonator  3  is electrically connected to the phase circuit  29  through a resonance capacitor C 2 . The resonance capacitors C 1  and C 2  are capacitors for determining the magnitude of block-band attenuation. The series resonance circuit composed of the resonator  2  and the resonance capacitor C 1  is electrically connected to the series resonance circuit composed of the resonator  3  and the resonance capacitor C 2  through a coupling coil L 1 . In addition, capacitors C 5  and C 6  are electrically connected in parallel with respect to the two series resonance circuits. 
     A PIN diode D 2  as a switching device, whose cathode is grounded, is electrically connected to the intermediate junction between the resonator  2  and the resonance capacitor C 1  in parallel with respect to the resonator  2  thorough a band-varying capacitor C 3 . Meanwhile, a PIN diode D 3  is electrically connected to the intermediate junction of the resonator  3  and the resonance capacitor C 2  in parallel with respect to the resonator  3  through a band-varying capacitor C 4 . The anode of the PIN diode D 3  is electrically connected to the band-varying capacitor C 4 , and the cathode of the PIN diode D 3  is grounded. The band-varying capacitors C 3  and C 4  are capacitors for changing two attenuation-pole frequencies of the attenuation characteristic of the frequency-variable-type band block filter circuit  27 . 
     A voltage control terminal CONT 1  is electrically connected to the intermediate junction between the anode of the PIN diode D 2  and the band-varying capacitor C 3  through a control-voltage supplying resistor R 1 , a capacitor C 22 , and a chalk coil L 2 , and also it is connected to the intermediate junction between the anode of the PIN diode D 3  and the band-varying capacitor C 4  through the control-voltage supplying resistor R 1 , the capacitor C 22 , and a chalk coil L 3 . The capacitor  22 , which serves as a noise-cutting bypass capacitor, is electrically connected between the voltage control terminal CONT 1  and a ground. Preferably, the impedance of the chalk coils L 2  and L 3  is 350 Ω or more, and the capacity of the capacitor C 22  is in a range of 10 through 1000 pF. 
     The phase circuit  29  is a T-letter-type circuit composed of a coil L 20  electrically connected between the band block filter circuit  27  and an antenna terminal ANT, a capacitor C 15  electrically connected between a ground and the antenna terminal ANT, and a coil L 21  electrically connected between a band pass filter circuit  28  (which will be described below) of the receiving-side circuit  26  (described below) and the antenna terminal ANT. 
     Meanwhile, the receiving-side circuit  26  has the frequency-variable-type band pass filter circuit  28  and the phase circuit  29 . In the first preferred embodiment, both the receiving-side circuit  26  and the transmitting-side circuit  25  share the phase circuit  29  for common use, but it is conceivable that the receiving-side circuit  26  and the transmitting-side circuit  25  may respectively have an individual phase circuit. 
     The band pass filter circuit  28  is formed by coupling three stages of resonance circuits, in which a resonator  4  is electrically connected to the phase circuit  29  through a resonance inductor L 9 , a resonator  6  is electrically connected to a reception terminal Rx through a resonance inductor L 10 , and a resonator  5  is electrically connected to the intermediate junction between the resonators  4  and  6  through coupling capacitors C 11 , C 12 , C 13 , and C 14 . 
     A series circuit including a band-varying capacitor C 7  and a PIN diode D 4  is electrically connected to the intermediate junction between the resonator  4  and the resonance inductor L 9  in parallel with respect to the resonator  4 , with the cathode of the PIN diode D 4  being grounded. A series circuit including a band-varying capacitor C 8  and a PIN diode D 5  is electrically connected to the intermediate junction between the resonator  5  and the coupling capacitors C 12  and C 13  in parallel with respect to the resonator  5 , with the cathode of the PIN diode D 5  being grounded. A series circuit including a band-varying capacitor C 9  and a PIN diode D 6  is electrically connected to the intermediate junction between the resonator  6  and the resonance inductor L 10  in parallel with respect to the resonator  6 , with the cathode of the PIN diode D 6  being grounded. 
     The voltage control terminal CONT 2  is electrically connected to the intermediate junction between the anode of the PIN diode D 4  and the band-varying capacitor C 7  through a control-voltage supplying resistor R 2 , a capacitor  23 , and a chalk coil L 6 , and is electrically connected to the intermediate junction between the anode of the PIN diode D 5  and the band-varying capacitor C 8  through the control-voltage supplying resistor R 2 , the capacitor C 23 , and a chalk coil L 7 . Furthermore, it is electrically connected to the intermediate junction between the anode of the PIN diode D 6  and the band-varying capacitor C 9  through the control-voltage supplying resistor R 2 , the capacitor  23 , and a chalk coil L 8 . The capacitor C 23 , which serves as a noise-cutting bypass capacitor, is electrically connected between the voltage control terminal CONT 2  and a ground. Preferably, the impedance of the chalk coils L 6 , L 7 , and L 8  is 350 Ω or more and the capacity of the capacitor C 23  is in a range of 10 through 1000 pF. 
     As the resonators  2  through  6 , for example, as shown in FIG. 3, dielectric resonators are used. FIG. 3 shows the resonator  2  as a typical one. The dielectric resonators  2  through  6  are formed by a tube-shaped dielectric member  21  made of a high dielectric-constant material such as the TiO 2 -based ceramic, an outer conductor  22  disposed on the outer periphery of the tube-shaped dielectric member  21 , and an inner conductor  23  disposed on the inner periphery of the tube-shaped dielectric member  21 . The outer conductor  22  is electrically open with respect to (is separated from) the inner conductor  23  at an opening-end face  21   a  of the dielectric member  21  (hereinafter referred to as an open-side end face  21   a ), whereas it is electrically short-circuited (conducted) with respect to the inner conductor  23  at the other opening-end face  21   b  (hereinafter referred to as a short-circuited-side end face  21   b ). In the dielectric resonator  2 , the series circuit including the band-varying capacitor C 3  and the PIN diode D 2  is electrically connected to the open-side end face  21   a  in such a manner that an end of the band-varying capacitor C 3  is connected to the inner conductor  23 , whereas the cathode of the PIN diode D 2  is grounded to a ground, and the outer conductor  22  is grounded to a ground. 
     Meanwhile, the control circuit  11  connected to the voltage control terminal CONT 1  is formed by two transistors  12  and  13 , three resistors  14 ,  15 , and  16 , and a selector switch  18 . The transistor  12  is the PNP type, in which a bias voltage of +2.8 V is applied to the emitter, the collector is electrically connected to the voltage control terminal CONT 1 , and the base is electrically connected to the collector of the transistor  13  through the resistor  14 . Meanwhile, the transistor  13  is the NPN type, in which the emitter is grounded, and the resistor  15  is electrically connected between the base and the emitter. The selector switch  18  is electrically connected to the base of the transistor  13  through the resistor  16 . Either the voltage of 0 V or +3 V is applied to the base of the transistor  13  by switching the selector switch  18 . Furthermore, in FIG. 1, a control circuit having the same structure as that of the control circuit  11  is connected to the voltage control terminal CONT 2 , but this is not shown in the figure. 
     A description will be given of the operational advantages of the antenna duplexer  1  and the control circuit  11  having the aforementioned structure. In this antenna duplexer  1 , transmission signals input to a transmission terminal Tx from a transmission circuit system are output from an antenna terminal ANT through a transmitting-side circuit  25 , and reception signals input from the antenna terminal ANT are output to a reception circuit system from a reception terminal Rx through a receiving-side circuit  26 . 
     A trap frequency of the frequency-variable-type band block filter circuit  27  in the transmitting-side circuit  25  is determined by each resonance frequency of a resonance system formed by the band-varying capacitor C 3 , the resonance capacitor C 1 , and the resonator  2 , and a resonance system formed by the band-varying capacitor C 4 , the resonance capacitor C 2 , and the resonator  3 . When a voltage of +3 V is applied to the base of the transistor  13  by the selector switch  18  of the control circuit  11  connected to the voltage control terminal CONT 1 , as shown in FIG. 1, the transistors  12  and  13  are in the on-state and then a bias voltage of +2.8 V is applied to the voltage control terminal CONT 1 . In this arrangement, the positive voltage as a control voltage is applied to the voltage control terminal CONT 1 , so that the PIN diodes D 2  and D 3  are in the on-state. Therefore, the band-varying capacitors C 3  and C 4  are respectively grounded through the PIN diodes D 2  and D 3 , the two attenuation-pole frequencies are both lowered and the pass band of the transmitting-side circuit  25  is lowered. 
     In contrast, when a voltage of 0 V is applied to the base of the transistor  13  by the switch  18  of the control circuit  11 , the transistors  12  and  13  are in the off-state and then the control circuit  11  has high impedance of 100 kΩ or more (for instance, 100 through 200 MΩ), so that no voltage is applied to the voltage control terminal CONT 1 . Since no voltage is applied to the voltage control terminal CONT 1 , the PIN diodes D 2  and D 3  are in the off-state. This permits the band-varying capacitors C 3  and C 4  to be in the open-state, whereby the two attenuation-pole frequencies are both heightened and the pass band of the transmitting-side circuit  25  is heightened. In such a manner, the transmitting-side circuit  25  can have the two different pass-band characteristics by grounding or opening the band-varying capacitors C 3  and C 4  by performing voltage control. 
     In the antennas duplexer  1 , high-frequency signals of large electric power (about 0.5 through 3 W) are input from the transmission terminal Tx, and applied to the transmitting-side filter circuit  27  and the receiving-side filter circuit  28 . As shown in FIG.  4  and FIG. 5, the large-power high-frequency signals generate two kinds of current I 1  and I 2  in each of the resonance systems of the resonators  2 ,  3 , through  6 . Furthermore, these two kinds of current I 1  and I 2  flow in such a manner that a negative voltage is constantly applied to the anode of the PIN diodes D 2 , D 3 , through D 6 , by allowing the control circuit  11  connected to the voltage control terminal  1  to be in high impedance. Thus, even though the large-power high-frequency signals are input, a negative voltage is constantly applied to the PIN diodes D 2  and D 3 , so that the PIN diodes D 2  and D 3  are not in the on-state. 
     The passing frequency of the frequency-variable-type band pass filter circuit  28  in the receiving-side circuit  26  is determined by each resonance frequency of a resonance system formed by the band-varying capacitor C 7 , the resonance inductor L 9 , and the resonator  4 , a resonance system formed by the band-varying capacitor C 8  and the resonator  5 , and a resonance system formed by the band-varying capacitor C 9 , the resonance inductor L 10 , and the resonator  6 . When a positive voltage as a control voltage is applied to the voltage control terminal CONT 2  from the control circuit connected to the voltage control terminal CONT 2  by the same operation as the above-described one, the PIN diodes D 4 , D 5 , and D 6  are in the on-state. Thus, the band-varying capacitors C 7 , C 8 , and C 9  are grounded through the PIN diodes D 4 , D 5 , and D 6 , whereby the passing frequencies is lowered. 
     In contrast, when no control voltage is applied to the voltage control terminal CONT 2 , the PIN diodes D 4 , D 5 , and D 6  are in the OFF-state. This permits the band-varying capacitors C 7 , C 8 , and C 9  to be in the open-state, whereby the passing frequency is heightened. In such a manner, the receiving-side circuit  26  can have the two different pass-band characteristics by grounding or opening the band-varying capacitors C 7  through C 9  by performing voltage control. 
     In the frequency-variable-type band pass filter circuit  28 , voltage control is performed in such a manner that it allows the band pass frequency to be lower when a low-frequency pass band is selected as a transmitting band, whereas it allows the band pass frequency to be higher when a high-frequency pass band is selected as a transmitting band, corresponding to the switching of the two high/low pass bands of the transmitting-side circuit  25 . In this arrangement, the phase combination with the transmitting-side circuit  25  is ideally performed. 
     In this manner, when the PIN diodes D 2  through D 6  are in the off-state, even if large-power high-frequency signals are input from the transmission terminal Tx or the like, since a negative voltage is applied constantly to the anode of the PIN diodes D 2  through D 6 , the antennas duplexer  1  can have a stable frequency characteristic. In addition, the control circuit  11  does not need a power-supply circuit for generating a negative voltage, whereby it is a simple circuit so that miniaturization and cost reduction of the communication apparatus can be achieved. 
     [Second Preferred Embodiment, FIG.  6 ] 
     FIG. 6 shows a structure of an antenna duplexer  41  and a control circuit  42  inside a communication apparatus of a second preferred embodiment. The antenna duplexer  41  is the same as the antenna duplexer  1  incorporating an inductor L 42  as an alternative to the band-varying capacitor C 4 . The control circuit  42  is the same as the control circuit  11  of the first embodiment incorporating field-effect transistors (FET)  43  and  44  as alternatives to the transistors  12  and  13 . The antenna duplexer  41  and the control circuit  42  having the structures above can achieve the same operational advantages as those in the antenna duplexer  1  and the control circuit  11  used in the first preferred embodiment. 
     [Third Preferred Embodiment, FIG.  7 ] 
     A third embodiment of the communication apparatus according to the present invention will be described referring to an example of a mobile phone. 
     FIG. 7 is an electric circuit block diagram of the RF part of a mobile phone  120 . In FIG. 7,  122  is an antenna device,  123  is a duplexer,  131  is a transmitting-side isolator,  132  is a transmitting-side amplifier,  133  is a transmitting-side inter-stage band-pass filter,  134  is a transmitting-side mixer,  135  is a receiving-side amplifier,  136  is a receiving-side inter-stage band-pass filter,  137  is a receiving-side mixer,  138  is a voltage-controlled oscillator (VCO), and  139  is a local band pass filter. 
     In this case, it is possible to use a duplexer, in which the antenna duplexers  1  and  41  and the control circuits  11  and  42  of the first and second preferred embodiments are combined as the duplexer  123 . With the antenna duplexers  1  and  41  and the control circuits  11  and  42  mounted, a compact mobile phone having a stable frequency characteristic can be obtained. Moreover, as the transmitting-side and receiving-side inter-stage band-pass filters  133  and  136  and the local band-pass filter  139 , for example, a filter, in which the frequency-variable-type filter circuit  28  and the control circuit  11  shown in FIG. 1 are combined, can be used. 
     Furthermore, the frequency-variable-type filter, the antenna duplexer, and the communication apparatus according to the present invention should not be limited to the embodiments above described, and various modifications can be applied within the range of the scope and spirits of the invention. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the forgoing and other changes in form and details may be made therein without departing from the spirit of the invention.