Abstract:
A television tuner includes an unbalanced tuning circuit and a balanced mixer provided at the next stage of the tuning circuit. An output port of the tuning circuit is grounded through a serial circuit composed of an inductive element and a capacitive means. The output port of the tuning circuit is connected to one input port of the mixer. A connection point provided between the inductive element and the capacitive means is connected to the other input port of the mixer. This permits phases of the signals input to two input ports of the mixer to be inverted to each other.

Description:
This application claims the benefit of priority to Japanese Patent Application No. 2004-000868 filed on Feb. 26, 2004, herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a television tuner and, more particularly, to a television tuner for balance-inputting a television signal to a balanced mixer. 
   2. Description of the Related Art 
     FIG. 5  shows a structure of a conventional television tuner. Balanced input ports  51  and  52  are connected to a tuning coil  53 , and a serial circuit composed of a variable-capacitance diode  54  and a capacitor  55  is connected in parallel to the tuning coil  53 . Further, a tuning DC-voltage input port  56  for controlling the capacitance of the variable-capacitance diode  54  is connected to a connection point between the variable-capacitance diode  54  and the capacitor  55  through a resistor  57 . The above-mentioned structure is a primary side structure. 
   As a secondary side structure, a tuning coil  61  is connected to a double balanced mixer  20  through capacitors  62  and  63 . In addition, a serial circuit composed of a variable-capacitance diode  64  and a capacitor  65  is connected in parallel to the coil  61 . Also, a tuning DC-voltage input port  66  for controlling the capacitance of the variable-capacitance diode  64  is connected to the connection point between the variable-capacitance diode  64  and the capacitor  65  through a resistor  67 . Further, one end of a resistor  68  is connected between the variable-capacitance diode  64  and the tuning coil  61  and the other end of the resistor  68  is grounded. 
   In addition, a predetermined frequency signal output from a local oscillator  14  is mixed to the output of the tuning circuit to make a predetermined intermediate frequency signal, and the intermediate frequency signal is supplied to intermediate frequency output ports  21 ,  22  (For example, see Patent Document 1). 
   [Patent Document 1] Japanese Unexamined Patent Application Publication No. 07-007452 (FIG. 1) 
   In the above-mentioned structure, as a means for balance-inputting the television signal to the double balanced mixer, a balanced tuning circuit is provided. However, as the tuning circuit used in a television tuner, an unbalanced tuning circuit including a single tuning circuit and a tuning circuit having a simple structure and a stable characteristic is mainly used. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a novel television tuner for balance-inputting a television signal to a balanced mixer by using an unbalanced tuning circuit. 
   As a means for solving the above-mentioned problem, a television tuner comprises an unbalanced tuning circuit, and a balanced mixer provided at the next stage of the tuning circuit, wherein an output port of the tuning circuit is grounded through a serial circuit composed of an inductive element and a capacitive means having a variable capacitance value, the output port of the tuning circuit is connected to one input port of the mixer, and a connection point between the inductive element and the capacitive means is connected to the other input port of the mixer. 
   In addition, the inductive element is provided at the output port side of the tuning circuit, the capacitive means is provided at the ground side, and a serial resonance frequency of the inductive element and the capacitive means is lower than a frequency of a signal input to the mixer from the tuning circuit. 
   Moreover, the capacitive means is provided at the output port side of the tuning circuit, the inductive element is provided at the ground side, and a serial resonance frequency of the inductive element and the capacitive means is higher than a frequency of a signal input to the mixer from the tuning circuit. 
   Furthermore, a capacitance value of the capacitive means decreases as a tuning frequency of the tuning circuit increases. 
   Also, the tuning circuit includes a first varactor diode applied with a tuning voltage, for changing the tuning frequency, the capacitive means includes at least a second varactor diode, and the second varactor diode is applied with the tuning voltage. 
   In addition, the capacitive means includes a capacitance element serially connected to the second varactor diode. 
   In one embodiment, a television tuner comprises an unbalanced tuning circuit, and a balanced mixer provided at the next stage of the tuning circuit, wherein an output port of the tuning circuit is grounded through a serial circuit composed of an inductive element and a capacitive means having a variable capacitance value, the output port of the tuning circuit is connected to one input port of the mixer, and a connection point provided between the inductive element and the capacitive means is connected to the other input port of the mixer. Accordingly, a signal having a frequency higher or lower than the resonance frequency of the serial circuit can be balance-input to the mixer. 
   In addition, in a second embodiment, the inductive element is provided at the output port side of the tuning circuit, the capacitive means is provided at the ground side, and a serial resonance frequency of the inductive element and the capacitive means is lower than a frequency of a signal input to the mixer from the tuning circuit. Accordingly, a signal having a frequency higher than the resonance frequency of the serial circuit can be balance-input to the mixer. 
   Moreover, in a third embodiment, the capacitive means is provided at the output port side of the tuning circuit, the inductive element is provided at the ground side, and a serial resonance frequency of the inductive element and the capacitive means is higher than a frequency of a signal input to the mixer from the tuning circuit. Accordingly, a signal having a frequency lower than the resonance frequency of the serial circuit can be balance-input to the mixer. 
   Furthermore, in a fourth embodiment, a capacitance value of the capacitive means decreases as a tuning frequency of the tuning circuit increases. Accordingly, flatness of the frequency characteristic of the level of the signal input to the other input port of the mixer can be improved. 
   Also, in a fifth embodiment, the tuning circuit includes a first varactor diode applied with a tuning voltage, for changing the tuning frequency, the capacitive means includes at least a second varactor diode, and the second varactor diode is applied with the tuning voltage. Accordingly, it is possible that as the tuning frequency increases, the capacitance value of the capacitive means decreases. 
   In addition, in a sixth embodiment, the capacitive means includes a capacitance element serially connected to the second varactor diode. Accordingly, flatness of the frequency characteristic of the level of the signal input to the other input port of the mixer can be further improved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a structure of a first embodiment of a television tuner of the present invention; 
       FIG. 2  is a circuit diagram showing a structure of a second embodiment of a television tuner of the present invention; 
       FIG. 3  is a frequency characteristic diagram of the input voltage of a mixer in the first embodiment related to the television tuner of the present invention; 
       FIG. 4  is a frequency characteristic diagram of the input voltage of a mixer in the second embodiment related to the television tuner of the present invention; and 
       FIG. 5  is a circuit diagram showing a structure of a conventional television tuner. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a structure according to a first embodiment of the present invention. At a primary side of a double tuning circuit  1 , a tuning coil  2  connected between an input port  1   a  and a ground, and a serial circuit which is composed of a first varactor diode  3  and a DC cut capacitor  4  and which is connected in parallel to the tuning coil  2  are provided. The anode of the first varactor diode  3  is connected to the input port  1   a  and the cathode thereof is connected to a tuning voltage port  6  through a resistor  5 . The tuning voltage port  6  is applied with a tuning voltage for controlling the capacitance value of the first varactor diode  3 . 
   At the secondary side, a tuning coil  7  connected between an output port  1   b  and the ground, and a serial circuit which is composed of a first varactor diode  8  and a DC cut capacitor  9  and which is connected to the tuning coil  7  are provided. The anode of the first varactor diode  8  is connected to an output port  1   b  and the cathode thereof is connected to the tuning voltage port  6  through a resistor  10 . 
   A serial circuit  13  composed of an inductive element  11  and a capacitive means  12  is connected between the output port  1   b  of a double tuning circuit  1  and the ground. The capacitive means  12  is composed of a serial circuit including a capacitance element  12   b  and a second varactor diode  12   a . Further, one end of the inductive element  11  is connected to the output port  1   b  and the anode of the second varactor diode  12   a  is grounded. The capacitance element  12   b  is interposed between the cathode of the second varactor diode  12   a  and the other end of the inductive element  11 . The cathode of the second varactor diode  12   a  is applied with a tuning voltage through a resistor  14 . 
   In addition, the output port of the double tuning circuit  1  is connected to one input port of a balanced mixer (such as a double balanced mixer)  17  through a coupling capacitor  15 , and the connection point provided between the inductive element  11  and the capacitive means  12  is connected to the other input port of the balanced mixer  17  through a coupling capacitor  16 . Also, a local oscillating signal from an oscillator  18  is balance-input to the balanced mixer  17 . Thereby, an intermediate frequency signal is output from the balanced mixer  17  and is supplied to the output ports  17   a ,  17   b.    
   In the above-mentioned structure, if the voltage value of the television signal output to the output port  1   b  of the double tuning circuit  1  is e, the inductance value of the inductive element  11  is L, and the capacitance value of the capacitive means  12  is C, the voltage value e′ at the connection point provided between the inductive element  11  and the capacitive means  12  is expressed by equation (1).
 
 e′=−e /(ω 2   LC− 1)   (1)
 
   As can be seen from this equation, when ω 2 LC&gt;1, the phase of the voltage e′ is inverted with respect to that of the voltage e. That is, in a frequency higher than the serial resonance frequency (angle frequency is ω 0 ) of the inductive element  11  and the capacitive means  12 , the phase of the voltage e′ is inverted with respect to that of the voltage e. 
   Accordingly, if the serial resonance frequency of the inductive element  11  and the capacitive means  12  is lower than the frequency of the television signal, the television signal can be balance-input to the balanced mixer by using the unbalanced double tuning circuit  1 . 
   Further, this voltage has a frequency characteristic shown in  FIG. 4  and, in a frequency range higher than the serial resonance frequency ω 0 , the absolute value thereof decreases as the frequency increases. Accordingly, as can be seen from equation (1), if the capacitance value of the capacitive means  12  decreases as the frequency increases, the reduction of the voltage value e′ is suppressed and thus the flatness of the television signal input to the balanced mixer can be improved. 
   Accordingly, as shown in  FIG. 1 , if the tuning voltage is applied to the second varactor diode  12   a , the capacitance value of the capacitive means  12  decreases as the frequency increases, thereby accomplishing the above-mentioned object. 
     FIG. 2  shows the structure according to a second embodiment of the present invention. In  FIG. 2 , the structure of the serial circuit  13  is different from that in  FIG. 1  in that a connection location between the inductive element  11  and the capacitive means  12  are swapped. The other structure thereof is similar to that in  FIG. 1 . In other words, the anode of the first varactor diode  12   a  is connected to the output port  1   b  of the double tuning circuit  1 . Also, one end of the inductive element  11  is grounded and the capacitance element  12   b  is connected between the cathode of the second varactor diode  12   a  and the other end of the inductive element  11 . 
   In the structure of  FIG. 2 , if the voltage value of the television signal output to the output port  1   b  of the double tuning circuit  1  is e, the inductance value of the inductive element  11  is L, and the capacitance value of the capacitive means  12  is C, the voltage value e′ at the connection point provided between the inductive element  11  and the capacitive means  12  is expressed by equation (2).
 
 e′=−eω   2   LC /(1−ω 2   LC )   (2)
 
   As can be seen from this equation, when 1&gt;ω 2 LC, the phase of the voltage e′ is inverted with respect to that of the voltage e. That is, at a frequency lower than the serial resonance frequency (angle frequency is ω 0 ) of the inductive element  11  and the capacitive means  12 , the phase of the voltage e′ is inverted with respect to that of the voltage e. 
   Accordingly, if the serial resonance frequency of the inductive element  11  and the capacitive means  12  is higher than the frequency of the television signal, the television signal can be balance-input to the balanced mixer by using the unbalanced double tuning circuit  1 . 
   If equation (2) is transformed, equation (3) is obtained.
 
 e′=−e /((1/ω 2   LC )−1)   (3)
 
   Accordingly, this voltage has a frequency characteristic shown in  FIG. 4  and, in a frequency range lower than the serial resonance frequency ω 0 , the absolute value thereof increases as the frequency increases. 
   Accordingly, as can be seen from equation (3), if the capacitance value of the capacitive means  12  decreases as the frequency increases, the reduction of the voltage value e′ is suppressed and thus the flatness of the television signal input to the balanced mixer can be improved. 
   In addition, the capacitance element  12   b  in the capacitive means  12 , which is serially connected to the second varactor diode  12   a , corrects the frequency characteristic of the entire capacitance value of the capacitive means  12 , thereby the frequency characteristic of the voltage e′ can be as flat as it can be.