KR100397112B1 - Television tuner - Google Patents

Abstract

An object of the present invention is to provide a low-cost tuner by controlling the tuning frequency of the tuning circuit and the trap frequency of the trap circuit by one signal from the band decoder so that the circuit of the band decoder can be simplified.

To this end, in the present invention, a binary band selection voltage from the band decoder 10 is input to the switching circuit 11 and the intermediate frequency tuning circuit 6 is tuned by the switching voltage output from the second terminal 23. At the same time as the frequency is changed, the trap frequency of the trap circuit 7 is changed by the switching voltage output from the third terminal 24.

Description

TV tuner {TELEVISION TUNER}

The present invention relates to a tuner for receiving a television signal, and more particularly to a tuner for receiving a television signal which can receive an FM broadcast signal.

The configuration of a conventional television tuner will be described with reference to Figs. As shown in Fig. 3, the television tuner includes a VHF circuit portion 52, a VHF mixer 53, a UHF circuit portion 54, a UHF mixer 55, an intermediate frequency tuning circuit 56, and a trap. A circuit 57, an intermediate frequency amplifier circuit 58, a SAW filter 59, and a band decoder 60 are formed. When receiving the television signal or the FM broadcast signal of the VHF band, the television signal or the FM broadcast signal is converted into an intermediate frequency signal by the mixer 53 for VHF. On the other hand, when receiving a television signal of the UHF band, the television signal is converted into an intermediate frequency signal by the mixer 55 for the UHF.

The VHF circuit part 52 is connected to the antenna 51, and consists of the VHF antenna circuit 61, the VHF high frequency amplification circuit 62, and the VHF tuning circuit 63 connected in series. Here, the VHF antenna circuit 61 outputs a signal of a band to be received among the input high frequency signals to the VHF high frequency amplifier circuit 62, and the VHF high frequency amplifier circuit 62 amplifies the input high frequency signal. The circuit and the VHF tuning circuit 63 are circuits for tuning an input signal.

The VHF mixer 53 receives the VHF band television signal of 54 MHz to 216 MHz or the FM broadcast signal of 89 MHz to 173 MHz from the VHF tuning circuit 63, and from the local oscillator 64 for the VHF. Local oscillation signal is input. The local oscillator 64 for the VHF changes its oscillation frequency by a tuning voltage (TU) applied from a band decoder 60 having a PLL circuit, and is 44 MHz higher than a received television signal when receiving a television signal of the VHF band. A local oscillation signal is output, and when receiving an FM broadcast signal, a local oscillation signal 41.25 MHz higher than the received FM broadcast signal is output. For this reason, the television signal of the VHF band is frequency-converted into a 44 MHz intermediate frequency signal, and the FM broadcast signal is frequency-converted into a 41.25 MHz audio intermediate frequency signal corresponding to the frequency of the audio intermediate frequency of the television signal.

The UHF circuit portion 54 is configured in parallel with the VHF circuit portion 52, to which the antenna 51 is connected, and the UHF antenna circuit 65, the high frequency amplification circuit 66 for the UHF, and the UHF connected in series. It consists of a tuning circuit 67. Since the operation of each circuit in the UHF circuit portion 54 is the same as that of each circuit of the corresponding VHF circuit portion 52, the description thereof is omitted.

The UHF mixer 55 receives a UHF television signal of 470 MHz to 890 MHz from the UHF circuit section 54 and a local oscillation signal from the UHF local oscillator 68. The local oscillator 68 for UHF outputs a local oscillation signal that is 44 MHz higher than the television signal received by the oscillation frequency being changed by the tuning voltage TU applied from the band decoder 60 incorporating the PLL circuit. For this reason, the UHF band television signal is frequency-converted into a 44 MHz intermediate frequency signal.

The television signal or FM broadcast signal frequency-converted by the VHF mixer 53 or the UHF mixer 55 is tuned by the intermediate frequency tuning circuit 56. As shown in Fig. 4, the intermediate frequency tuning circuit 56 has a first capacitor 70 having one end connected to the varactor diode 69 and the anode of the varactor diode 69 and the other end grounded. And one end of the first inductor 71a, 71b at which one end is grounded at a high frequency, and the first inductor 71a, 71b is parallel to the series connection circuit of the varactor diode 69 and the first capacitor 70. Are connected to form a parallel tuning circuit. The band select voltage S1 is applied to the connection points with the first inductors 71a and 71b, and the voltage is supplied to the cathode of the varactor diode 69 via the first inductor 71a. The anode of the varactor diode 69 is grounded via a resistor 72. The connection point between the cathode of the varactor diode 69 and the first inductor 71a is connected to the output terminal of the mixer 53 and to the trap circuit 57.

In the case of receiving a television signal, a high level band select voltage S1 is applied from the band decoder 60 to the cathode of the varactor diode 69 via the first inductor 71a, and to the varactor diode 69. Reverse bias voltage is applied. At this time, the tuning frequency of the intermediate frequency tuning circuit 56 is determined by the capacitance of the varactor diode 69, the capacitance of the first capacitor 70, and the inductance of the first inductors 71a and 71b. The frequency is 44 MHz. On the other hand, when receiving the FM broadcast signal, the low-level band select voltage S1 is applied from the band decoder 60 to the cathode of the varactor diode via the first inductor 71a, so that the anode and cathode of the varactor diode 69 are received. It is almost coin above. By doing so, the capacitance of the varactor diode 69 becomes very large, and the tuning frequency is lowered. The tuning frequency at this time is 41.25 MHz.

The television signal or FM broadcast signal tuned by the intermediate frequency tuning circuit 56 is input to the trap circuit 57. The trap circuit 57 consists of the 2nd inductor 73, the 2nd capacitor | condenser 74, and the switch diode 75 connected in parallel with each other. A power supply voltage (+ B) is applied to the anode of the switch diode 75, and a band switching voltage S2 is applied to the cathode. When receiving a television signal, the band select voltage S2 applied from the band decoder 60 to the cathode of the switch diode 75 becomes low level and the switch diode 75 is turned on to attenuate signals of any frequency. It doesn't work. On the other hand, when receiving the FM broadcast signal, the band select voltage S2 applied from the band decoder 60 to the cathode of the switch diode becomes a high level so that the frequency of the signal attenuated by the trap circuit 57 is about halfway. It is supposed to be 43 MHz.

The television signal or FM broadcast signal tuned by the intermediate frequency tuning circuit 56 is input to the intermediate frequency amplifier circuit 58 and amplified. The television signal or FM broadcast signal amplified by the intermediate frequency amplifier circuit 58 is input to the SAW filter 59. The pass band of the SAW filter 59 is 41 MHz to 47 MHz and passes the intermediate frequency band of the television signal. The pass band of the SAW filter 59 is wider than the band of the FM broadcast signal frequency-converted to the voice intermediate frequency. However, when the FM broadcast signal is received, the SAW filter 59 is provided by the intermediate frequency tuning circuit 56 and the trap circuit 57. Since the signal adjacent to the received FM broadcast signal is removed, only the signal of the band to be received is output from the tuner.

Such a conventional TV tuner must output a band select voltage from a band decoder to an intermediate frequency tuning circuit and a trap circuit in order to receive a television signal and an FM broadcast signal, and the level of the high level voltage of each band select voltage is increased. It was different. For this reason, the circuit of the band decoder is complicated, which hinders the lower price.

Accordingly, an object of the present invention is to reduce the cost of the band decoder by simplifying the circuit of the band decoder by controlling the tuning frequency of the intermediate frequency tuning circuit and the trap frequency of the trap circuit by one band selection voltage output from the band decoder. To provide a tuner.

1 is a block diagram of a tuner of the present invention;

2 is a circuit diagram of an intermediate frequency tuning circuit, a trap circuit, and a switching circuit used in the tuner of the present invention;

3 is a block diagram of a conventional tuner,

4 is a circuit diagram of an intermediate frequency tuning circuit and a trap circuit of a conventional tuner.

※ Explanation of symbols for main parts of drawing

1: antenna 2: VHF circuit part

3: VHF mixer 4: UHF circuit section

5: Mixer for UHF 6: Intermediate Frequency Tuning Circuit

7: Trap Circuit 8: Intermediate Frequency Amplification Circuit

9: SAW filter 10: Band decoder

11: switching circuit 12: antenna circuit for VHF

13: VHF high frequency amplifier circuit 14: VHF tuning circuit

15: local oscillator for VHF 16: antenna circuit for UHF

17: high frequency amplification circuit for UHF 18: UHF tuning circuit

19: local oscillator for UHF 20: switch diode

21 varactor diode 22 first terminal

23: second terminal 24: third terminal

25: first capacitor 26a, 26b: first inductor

27: second capacitor 28: second inductor

29: third capacitor 30: third inductor

31: first switch transistor 32: second switch transistor

33: third switch transistor 34: first resistor

35: second resistance 36: third resistance

37: fourth resistor 38: fifth resistor

39: sixth resistor 40: seventh resistor

In order to solve the above problems, the tuner of the present invention is tuned with a mixer and a switch diode for frequency converting a television signal into an intermediate frequency of a television and simultaneously converting an FM broadcast signal into an audio intermediate frequency in the intermediate frequency band. The reverse bias voltage applied to the varactor diode with the intermediate frequency tuning circuit and the varactor diode being switched so that the frequency is approximately halfway between the intermediate frequency band in the off state of the switch diode and the voice intermediate frequency in the on state. A band circuit for selecting a low band or a high band of at least a VHF band, and a trap circuit for attenuating two to large and small, and attenuating a frequency higher than the intermediate frequency band or an approximately intermediate frequency of the intermediate frequency band. The TV signal again And a first terminal for outputting a binary band selection voltage for selecting the FM broadcast signal, and inputting the band selection voltage to the second terminal for switching the binary voltage for switching the switch diode on or off. And a switching circuit for outputting a binary switching voltage for switching the reverse bias voltage of the varactor diode from a third terminal, and outputting the second signal when the television signal is received. The switch diode is turned off by the voltage, and the bias voltage of the reverse direction applied to the varactor diode is increased by the switching voltage outputted to the third terminal so that the second terminal is received when the FM broadcast signal is received. The switch diode is turned on by the switching voltage outputted from the By the switching voltage to be output it was reduced bias voltage of reverse direction is applied to the varactor diode.

The television tuner of the present invention applies a first power supply voltage to the anode of the switch diode, connects a cathode to the second terminal, and applies the first power supply voltage to the cathode of the varactor diode. An anode is connected to the third terminal so that the high level voltage of the switching voltage output from the second terminal is higher than the level of the first power supply voltage, and the low level voltage is lower than the level of the first power supply voltage, The level of the high level voltage and the low level voltage of the switching voltage output from the third terminal is made lower than the level of the first power supply voltage.

The switching circuit of the television tuner of the present invention has a first transistor of NPN type, a second transistor of PNP type, and a third transistor of NPN type, and connects the second terminal to the collector of the first transistor. Applying a second power supply voltage to the base of the first transistor to apply the band selection voltage, ground the emitter of the first transistor, connect the third terminal to the collector of the second transistor, and Applying the first power supply voltage to an emitter of a two transistor, connecting a collector of the third transistor to a base of the second transistor, and applying the band select voltage to a base of the third transistor, The emitter of the transistor was grounded.

Moreover, the switching circuit of the television tuner of this invention was integrated.

The tuner of the present invention will be described with reference to FIGS. 1 and 2. Fig. 1 shows a block diagram of a tuner of the present invention, and Fig. 2 shows a circuit diagram of an intermediate frequency tuning circuit, a trap circuit and a switching circuit shown in Fig. 1.

First, in Fig. 1, the tuner includes a VHF circuit portion 2, a VHF mixer 3, a UHF circuit portion 4, a UHF mixer 5, an intermediate frequency tuning circuit 6, and a trap circuit. (7), an intermediate frequency amplifier circuit 8, a SAW filter 9, a band decoder 10, and a switching circuit 11. When receiving the television signal or the FM broadcast signal of the VHF band, the television signal or the FM broadcast signal is converted into an intermediate frequency signal by the mixer 3 for VHF. On the other hand, when receiving a television signal of the UHF band, the television signal is converted into an intermediate frequency signal by the mixer 5 for the UHF.

The VHF circuit portion 2 is connected to the antenna 1, and has a configuration in which the VHF antenna circuit 12, the VHF high frequency amplification circuit 13, and the VHF tuning circuit 14 are connected in series. . Here, the VHF antenna circuit 12 is a circuit for outputting a received signal to the VHF high frequency amplification circuit 13 of the input high frequency signal, the VHF high frequency amplification circuit 13 is a circuit for amplifying the input high frequency signal, The VHF tuning circuit 14 is a circuit for tuning an input signal.

When receiving the television signal of the VHF low band, the VHF low band select voltage VL is input from the band decoder 10 to the VHF antenna circuit 12 so that the VHF low band television signal of 54 to 88 MHz is used for the VHF. Tuned in the antenna circuit 12. When receiving a television signal or an FM broadcast signal of the VHF high band, the VHF high band selection voltage VH is input from the band decoder 10 to the VHF antenna circuit 12 so as to receive the VHF high band of 174 to 216 MHz. A television signal or an FM broadcast signal of 89 MHz to 173 MHz is tuned in the antenna circuit 12 for the VHF.

When receiving the television signal or the FM broadcast signal of the VHF band, the VHF low band selection voltage VL or the VHF high band selection voltage VH is input from the band decoder 10 to the high frequency amplifier circuit 13 for VHF. The high frequency amplifier circuit 13 for the VHF operates. In this case, since the UHF band selection voltage U is not input from the band decoder 10 to the UHF high frequency amplifier circuit 17, the UHF high frequency amplifier circuit 17 does not operate.

When receiving the television signal of the VHF low band, the VHF low band selection voltage (VL) is input from the band decoder 10 to the VHF tuning circuit 14 so that the VHF low band television signal of 54 to 88 MHz is converted to VHF. It is tuned in the tuning circuit 14. When receiving a television signal or an FM broadcast signal of the VHF high band, the VHF high band selection voltage VH is input from the band decoder 10 to the VHF tuning circuit 14 so that the VHF high band of 174 to 216 MHz is received. The television signal or the FM broadcast signal of 89 MHz to 173 MHz is tuned in the VHF tuning circuit 14.

The VHF mixer 3 receives a VHF band television signal of 54 MHz to 216 MHz from the VHF tuning circuit 14, or an FM broadcast signal of 89 MHz to 173 MHz and is localized from the local oscillator 15 for VHF. The oscillation signal is input. The local oscillator 15 for VHF changes its oscillation frequency by a tuning voltage (TU) applied from a band decoder 10 having a PLL circuit so that the local oscillator 15 is 44 MHz higher than the received television signal when receiving a television signal of the VHF band. When the oscillation signal is output and the FM broadcast signal is received, a local oscillation signal 41.25 MHz higher than the received FM broadcast signal is output. For this reason, the television signal of the VHF band is frequency converted into a 44 MHz intermediate frequency signal, and the FM broadcast signal is frequency converted into a 41.25 MHz audio intermediate frequency signal corresponding to the frequency of the audio intermediate frequency of the television signal.

The UHF circuit section 4 is configured in parallel with the VHF circuit section 2, and the antenna 1 is connected to the UHF circuit section 4, the UHF antenna circuit 16, the UHF high frequency amplification circuit 17, and the UHF tuning circuit. It is comprised by connecting in series in the order of (18). Here, the UHF antenna circuit 16 is a circuit for outputting the required UHF band television signal among the input high frequency signals to the UHF high frequency amplifier circuit 17, and the UHF high frequency amplifier circuit 17 amplifies the input high frequency signal. The circuit and the UHF tuning circuit 18 are circuits for tuning an input signal.

When receiving the television signal of the UHF band, the television signal of the UHF band of 470 to 890 MHz is tuned in the UHF antenna circuit 16.

In addition, the UHF band selection voltage U is input from the band decoder 10 to the UHF high frequency amplifier circuit 17 so that the UHF high frequency amplifier circuit 17 operates. In this case, since the VHF low band select voltage VL and the VHF high band select voltage VH are not input from the band decoder 10 to the VHF high frequency amplifier circuit 13, the VHF high frequency amplifier circuit 13 it does not work.

The television signal of the UHF band is also tuned by the UHF tuning circuit 18.

In the UHF mixer 5, a UHF band television signal of 470 MHz to 890 MHz is input from the UHF tuning circuit 18, and a local oscillation signal is input from the UHF local oscillator 19. The UHF local oscillator 19 outputs a local oscillation signal that is 44 MHz higher than the received television signal by changing the oscillation frequency by the tuning voltage TU applied from the band decoder 10 having the PLL circuit. For this reason, the UHF band television signal is frequency-converted into a 44 MHz intermediate frequency signal.

The television signal or FM broadcast signal, which is frequency-converted in the VHF mixer 3 or the UHF mixer 5, is tuned in the intermediate frequency tuning circuit 6. The intermediate frequency tuning circuit 6 is a balanced circuit as shown in Fig. 2, and includes a first capacitor 25, a first inductor 26a, 26b, a switch diode 20, connected in parallel with each other, One end of each is composed of a second capacitor 27 and a second inductor 28 connected to the cathode of the switch diode 20 and connected in parallel with each other. The first power supply voltage B1 of 5V is applied between the first inductors 26a and 26b, and the first power supply voltage B1 is applied to the anode of the switch diode 20 via the first inductor 26a.

The intermediate frequency tuning circuit 6 is connected to the switching circuit 11 and the cathode of the switch diode 20 is connected to the second terminal 23 of the switching circuit 11. The switching circuit 11 is integrated circuit, but the equivalent circuit includes the first switch transistor 31, the second switch transistor 32, the third switch transistor 33, the first resistor 34 and the like. And a second resistor 35, a third resistor 36, and a fourth resistor 37. The first switch transistor 31 is an NPN type transistor, the emitter is grounded, the base is connected to the first terminal 22 of the band decoder 10 via the first resistor 34, and the collector is the second resistor. It is grounded via 35 and connected to the second terminal 23. The second switch transistor 35 is a transistor of the PNP type, and the collector is grounded via the third resistor 36 and connected to the third terminal 24. The first power supply voltage B1 of 5V is connected to the emitter. Is applied, and the base is connected to the collector of the third switch transistor 33. The third switch transistor 33 is a transistor of the NPN type, the collector of which is connected to the base of the second switch transistor 31, the emitter is grounded, and the base of the band decoder 10 via the fourth resistor 37. It is connected to the first terminal 22 of. Therefore, when the two-value band selection voltage S is input from the band decoder 10 to the switching circuit 11, the switching voltage of the binary values from the second terminal 23 and the third terminal 24 of the switching circuit 11 are respectively. Is output.

When receiving a television signal, a band selection voltage S of 0 V is output from the first terminal 22 of the band decoder 10 to the switching circuit 11. In this case, the first switch transistor 31 is turned off, and the second power supply voltage B2 of 30 V is divided by the second resistor 35 and the fifth resistor 38, and the voltage of the second terminal 23 is increased. The voltage becomes 10V and a switching voltage of 10V is applied to the cathode of the switch diode 20. Therefore, since the bias voltage in the reverse direction is applied to the switch diode 20, it is turned off and the intermediate frequency tuning circuit 6 is parallel-tuned by the first capacitor 25 and the first inductors 26a and 26b. In this case, the tuning frequency is 44 MHz and the band is 6 MHz.

On the other hand, when receiving the FM broadcast signal, the 5 V band selection voltage S is output from the first terminal 22 of the band decoder 10 to the switching circuit 11. In this case, the first switch transistor 31 is turned on, and the voltage of the second terminal 23 is 0V, and a switching voltage of 0V is applied to the cathode of the switch diode 20. Therefore, since the forward bias voltage is applied to the switch diode 20, the intermediate frequency tuning circuit 6 is turned on so that the first capacitor 25, the first inductors 26a and 26b, and the second capacitor ( 27 and the second inductor 28 are parallel-tuned. In this case, the tuning frequency is 41.25 MHz and the band is 1.5 MHz.

The television signal or the FM broadcast signal passing through the intermediate frequency tuning circuit 6 is input to the trap circuit 7. The trap circuit 7 consists of the varactor diodes 21, the 3rd capacitor | condenser 29, and the 3rd inductor 30 connected in parallel with each other, and is inserted in one line of a parallel line. A 5 V first power supply voltage B1 is applied to the cathode of the varactor diode 21 via the first inductor 26a.

In the case of receiving a television signal, a band selection voltage S of 0 V is output from the first terminal 22 of the band decoder 10 to the switching circuit 11, and the third switch transistor 33 is turned off. The two switch transistor 32 is turned off. As a result, the voltage of the third terminal 24 becomes 0V, and the bias voltage in the reverse direction applied to the varactor diode 21 becomes 5V, so that the capacitance value of the varactor diode 21 becomes relatively small. In this state, the frequency of the signal attenuated by the trap circuit 7 is higher than the intermediate frequency band so as to be 41.25 MHz, which is the frequency of the intermediate frequency band corresponding to the audio carrier of the adjacent television channel.

On the other hand, when receiving the FM broadcast signal, the 5 V band selection voltage S is output from the first terminal 22 of the band decoder 10 to the switching circuit 11. In this case, the third switch transistor 33 is turned on and the second switch transistor 32 is turned on. As a result, a voltage of 5V is output from the third terminal 24, divided by the sixth resistor 39 and the seventh resistor 40, and a switching voltage of 4V is applied to the anode of the varactor diode 21. The bias voltage in the reverse direction applied to the varactor diode 21 decreases to 1 V and the capacitance of the varactor diode 21 becomes relatively large. In this state, the frequency of the signal attenuated by the trap circuit 7 is set to 43 MHz, which is approximately halfway between the intermediate frequency bands.

The television signal or FM broadcast signal passing through the trap circuit 7 is input to the intermediate frequency amplifier circuit 8 and amplified. The television signal or FM broadcast signal amplified by the intermediate frequency amplifier circuit 8 is input to the SAW filter 9. The pass band of the SAW filter 9 is 41 MHz to 47 MHz and passes the intermediate frequency band of the television signal. The pass band of the SAW filter 9 is wider than the band of the FM broadcast signal frequency-converted to the voice intermediate frequency. However, when the FM broadcast signal is received, the pass band of the SAW filter 9 and Since the signal adjacent to the FM broadcast signal to be received is removed, only the signal of the band to be received is output from the tuner.

As described above, the television tuner of the present invention is a television tuner capable of changing the tuning frequency of the intermediate frequency circuit and the trap frequency of the trap circuit in order to receive the television signal and the FM broadcast signal. Since each switching voltage for switching the trap frequency is output from the switching circuit, one band selection voltage is output from the band decoder to the switching circuit, whereby the tuning frequency of the intermediate frequency tuning circuit and the trap frequency of the trap circuit can be switched. Since the number of output terminals of the band decoder can be reduced, the circuit of the band decoder can be simplified and the cost can be reduced.

In the switching circuit of the television tuner of the present invention, the high level voltage of the switching voltage output from the second terminal is higher than the first power supply voltage, and the low level voltage is lower than the first power supply voltage, and the switching output is output from the third terminal. The voltage between the high level voltage and the low level voltage of the voltage is lower than the first power supply voltage, so that the switch diode for changing the tuning frequency of the intermediate frequency tuning circuit and the varactor diode for changing the trap frequency of the trap circuit are appropriate voltages. Can be controlled.

In addition, since the switching circuit of the television tuner of the present invention is composed of an NPN-type first transistor, a PNP-type second transistor, and an NPN-type transistor, the switching circuit can be configured with fewer elements, thereby making it possible to miniaturize.

In addition, the television tuner of the present invention is integrated circuit, which enables miniaturization.

Claims (4)

A mixer for frequency converting a television signal into an intermediate frequency of a television and simultaneously converting an FM broadcast signal into an audio intermediate frequency in the intermediate frequency band; An intermediate frequency tuning circuit having a switch diode, the tuning frequency of which is switched to be about halfway between the intermediate frequency band in the off state of the switch diode and to the voice intermediate frequency in the on state; A trap circuit having a varactor diode, which converts the reverse bias voltage applied to the varactor diode into two types at large and attenuates a frequency higher than the intermediate frequency band or approximately a frequency in the intermediate frequency band; A band decoder for selecting at least VHF band low or high band, The band decoder is provided with a first terminal for outputting a binary band selection voltage for selecting the television signal or the FM broadcast signal. The band selection voltage is input to output a binary switching voltage for switching the switch diode on or off from a second terminal, and a binary switching voltage for switching the reverse bias voltage of the varactor diode is provided. Install a switching circuit to output from three terminals, When the television signal is received, the switch diode is turned off by the switching voltage output to the second terminal and the reverse bias voltage applied to the varactor diode by the switching voltage output to the third terminal. Increase the When receiving the FM broadcast signal, the switch diode is turned on by the switching voltage output from the second terminal and the reverse bias applied to the varactor diode by the switching voltage output from the third terminal. A TV tuner characterized by low voltage. The method of claim 1, A first power supply voltage is applied to an anode of the switch diode and a cathode is connected to the second terminal. While applying the first power supply voltage to the cathode of the varactor diode, an anode is connected to the third terminal, The voltage at the high level of the switching voltage output from the second terminal is higher than the level of the first power supply voltage, and the voltage at the low level is lower than the level of the first power supply voltage. And the level of the high level voltage and the low level voltage of the voltage voltage output from the third terminal is lower than the level of the first power supply voltage. The method of claim 2, The switching circuit has a first transistor of NPN type, a second transistor of PNP type and a third transistor of NPN type, Connecting the second terminal to the collector of the first transistor and applying a second power supply voltage; Applying the band selection voltage to a base of the first transistor, Ground the emitter of the first transistor, The third terminal is connected to the collector of the second transistor, Applying the first power supply voltage to an emitter of the second transistor, A collector of the third transistor is connected to a base of the second transistor, Applying the band selection voltage to a base of the third transistor, A television tuner, wherein the emitter of the third transistor is grounded. The method of claim 3, wherein And said switching circuit is integrated circuit.