JP2002064757A - Television tuner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive television tuner by controlling a tuning frequency of a tuning circuit and a trap frequency of a trap circuit with a signal from a band decoder, so as to simplify the circuitry of the band decoder. SOLUTION: A changeover circuit 11 receives a binary band selection voltage from the band decoder 10, a switching voltage outputted from a 2nd terminal 23 changes a tuning frequency of an intermediate frequency tuning circuit 6, a switching voltage outputted from a 3rd terminal 24 changes a trap frequency of a trap circuit 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tuner for receiving a television signal, and more particularly to a tuner for receiving a television signal capable of receiving an FM broadcast signal.

[0002]

2. Description of the Related Art The structure of a conventional television tuner will be described with reference to FIGS. As shown in FIG. 3, the television tuner includes a VHF circuit section 52, a VHF mixer 53, a UHF circuit section 54, a UHF mixer 55,
It comprises an intermediate frequency tuning circuit 56, a trap circuit 57, an intermediate frequency amplifier circuit 58, a SAW filter 59, and a band decoder 60. When receiving a television signal or an FM broadcast signal in the VHF band, the television signal or the FM broadcast signal is converted into an intermediate frequency signal by the VHF mixer 53. On the other hand, when receiving a television signal in the UHF band,
The signal is converted into an intermediate frequency signal by the F mixer 55.

The VHF circuit section 52 is connected to the antenna 51, and is connected to the VHF antenna circuit 6 connected in series.
1, a VHF high frequency amplifier circuit 62, and a VHF tuning circuit 63. Here, the VHF antenna circuit 61
Is a circuit that outputs a signal of a band to be received among the input high-frequency signals to the VHF high-frequency amplifier circuit 62, the VHF high-frequency amplifier circuit 62 amplifies the input high-frequency signal, and the VHF tuning circuit 63 includes: This is a circuit that tunes the input signal.

[0004] A VHF mixer 53 receives a VHF band television signal of 54 MHz to 216 MHz or an FM broadcast signal of 89 MHz to 173 MHz from a VHF tuning circuit 63 and a local oscillator 64 from a VHF local oscillator 64. A signal is input. Local oscillator 64 for VHF
The oscillator outputs a local oscillation signal that is higher by 44 MHz than the received television signal when the oscillation frequency is changed by the tuning voltage TU applied from the band decoder 60 having a built-in PLL circuit and the television signal in the VHF band is received. Then, when receiving an FM broadcast signal, it outputs a local oscillation signal 41.25 MHz higher than the received FM broadcast signal. For this reason, the television signal in the VHF band is frequency-converted to an intermediate frequency signal of 44 MHz, and the FM broadcast signal is frequency-converted to an audio intermediate frequency signal of 41.25 MHz corresponding to the frequency of the audio intermediate frequency of the television signal. You.

The UHF circuit section 54 is configured in parallel with the VHF circuit section 52, is connected to the antenna 51, and has a UHF antenna circuit 65 connected in series and a UHF antenna circuit 65.
It comprises an HF high-frequency amplifier circuit 66 and a UHF tuning circuit 67. Here, the operation of each circuit in the UHF circuit unit 54 is the same as that of each circuit in the corresponding VHF circuit unit 52, and a description thereof will be omitted.

[0006] The UHF mixer 55 includes a UHF circuit section 54.
470 MHz to 890 MHz in the UHF band, and a UHF local oscillator 68
Receives a local oscillation signal. Local oscillator for UHF 6
8, the oscillation frequency is changed by the tuning voltage TU applied from the band decoder 60 having a built-in PLL circuit, and the frequency is 44 MHz higher than the received television signal.
Outputs a high local oscillation signal. For this reason, the UHF band television signal is frequency-converted into a 44 MHz intermediate frequency signal.

The television signal or the FM broadcast signal frequency-converted by the VHF mixer 53 or the UHF mixer 55 is tuned by an intermediate frequency tuning circuit 56. As shown in FIG. 4, the intermediate frequency tuning circuit 56 includes a varactor diode 69 and a first capacitor 70 having one end connected to the anode of the varactor diode 69 and the other end grounded.
And a first inductor 71 whose one end is grounded in a high frequency manner.
a, 71b, and the first inductors 71a, 71b are connected in parallel to a series connection circuit of the varactor diode 69 and the first capacitor 70 to form a parallel tuning circuit. A band selection voltage S1 is applied to a connection point between the first inductors 71a and 71b, and this 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 the resistor 72.
The connection point between the cathode of the varactor diode 69 and the first inductor 71 a is connected to the output terminal of the mixer 53 and to the trap circuit 57.

When a television signal is received, a high-level band selection voltage S
1 is applied to the cathode of the varactor diode 69 via the first inductor 71a, and a reverse bias voltage is applied to the varactor diode 69. At this time, the tuning frequency of the intermediate frequency tuning circuit 56 is determined by the capacitance value of the varactor diode 69, the capacitance value of the first capacitor 70, and the inductance values of the first inductors 71a and 71b. Becomes on the other hand,
When receiving an FM broadcast signal, the band decoder 60
, The low-level band selection voltage S1 is applied to the cathode of the varactor diode via the first inductor 71a, and the potential between the anode and the cathode of the varactor diode 69 becomes substantially the same. Then, the varactor diode 69
Becomes extremely large, and the tuning frequency becomes low.
The tuning frequency at this time is 41.25 MHz.

The television signal or the FM broadcast signal tuned by the intermediate frequency tuning circuit 56 is input to a trap circuit 57. The trap circuit 57 includes a second inductor 73 and a second capacitor 74 connected in parallel with each other.
And a switch diode 75. The power supply voltage + B is applied to the anode of the switch diode 75, and the band switching voltage S2 is applied to the cathode. When receiving a television signal, the band selection voltage S2 applied from the band decoder 60 to the cathode of the switch diode 75 becomes low level, and the switch diode 7
5 is turned on, and no signal of any frequency is attenuated. On the other hand, when receiving the FM broadcast signal, the band selection voltage S2 applied to the cathode of the switch diode from the band decoder 60 becomes high level, and the frequency of the signal attenuated by the trap circuit 57 becomes almost the middle of the intermediate frequency band. 43 MHz.

The television signal or the FM broadcast signal tuned by the intermediate frequency tuning circuit 56 is supplied to an intermediate frequency amplifying circuit 58.
And amplified. The television signal or the 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 from 41 MHz to 47 MHz, and passes the intermediate frequency band of the television signal. Although the pass band of the SAW filter 59 is wider than the band of the FM broadcast signal frequency-converted to the audio intermediate frequency, when the FM broadcast signal is received, the intermediate frequency tuning circuit 56 and the trap circuit 5
7 removes the signal adjacent to the FM broadcast signal to be received, so that only signals in the band to be received are output from the tuner.

[0011]

Such a conventional television tuner outputs a band selection 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 high level voltage of each band selection voltage was different. For this reason, the circuit of the band decoder becomes complicated, which hinders cost reduction.

Therefore, an object of the present invention is to control 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, and to improve the band decoder circuit. It is intended to provide a low-priced tuner so that it can be simplified.

[0013]

In order to solve the above problems, a tuner of the present invention converts a frequency of a television signal into an intermediate frequency band of a television and converts an FM broadcast signal into an audio intermediate frequency in the intermediate frequency band. An intermediate frequency tuning circuit that has a mixer for converting the frequency into a frequency, and a switching diode, wherein the tuning frequency is switched to be approximately the middle of the intermediate frequency band when the switch diode is off, and to be the audio intermediate frequency when on. A trap circuit having a varactor diode, and attenuating a frequency higher than the intermediate frequency band or a substantially intermediate frequency of the intermediate frequency band by switching a reverse bias voltage applied to the varactor diode in two ways, large and small. , At least a band for selecting a low band or a high band of the VHF band A first terminal for outputting a binary band selection voltage for selecting the television signal or the FM broadcast signal, wherein the band selection voltage is input, and the switch A switch for outputting a binary switching voltage for switching a diode on or off from a second terminal, and outputting a binary switching voltage for switching a reverse bias voltage of the varactor diode from a third terminal. A circuit for turning off the switch diode by a switching voltage output to the second terminal when receiving the television signal, and applying a voltage to the varactor diode by a switching voltage output to the third terminal. When the FM broadcast signal is received, the second terminal is connected to the second terminal. While turning on the switching diode by the switching voltage output was reduced reverse bias voltage applied to the varactor diode by the switching voltage output from the third terminal.

Further, the television tuner of the present invention comprises:
A first power supply voltage is applied to an anode of the switch diode, a cathode is connected to the second terminal, and the first power supply voltage is applied to a cathode of the varactor diode, and the anode is connected to the second power supply. Connected to the third terminal, the high-level voltage of the switching voltage output from the second terminal is higher than the level of the first power supply voltage,
The low-level voltage is made lower than the level of the first power supply voltage, and the high-level voltage and the low-level voltage of the switching voltage output from the third terminal are changed to the level of the first power supply voltage. Lower than the level.

Further, the switching circuit of the television tuner of the present invention comprises an NPN type first transistor and a PNP transistor.
Having a second transistor of the type and a third transistor of the 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 the base of the first transistor, grounding the emitter of the first transistor, connecting the third terminal to the collector of the second transistor, Applying the first power supply voltage to the emitter, connecting the collector of the third transistor to the base of the second transistor, applying the band selection voltage to the base of the third transistor, Transistor was grounded.

Further, the switching circuit of the television tuner of the present invention is integrated.

[0017]

1 and 2 show a tuner according to the present invention.
It will be described according to. FIG. 1 shows a block diagram of the tuner of the present invention, and FIG. 2 shows a circuit diagram of the intermediate frequency tuning circuit, trap circuit and switching circuit shown in FIG.

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

The VHF circuit section 2 is connected to the antenna 1 and has a configuration in which a VHF antenna circuit 12, a VHF high-frequency amplifier circuit 13, and a VHF tuning circuit 14 are connected in series in this order. Here, the VHF antenna circuit 1
Reference numeral 2 denotes a received signal of VH
The circuit for outputting to the F high-frequency amplifier circuit 13, the VHF high-frequency amplifier circuit 13 is a circuit for amplifying the input high-frequency signal, and the VHF tuning circuit 14 is a circuit for tuning the input signal.

When a VHF low-band television signal is received, a VHF low-band selection voltage VL is input from the band decoder 10 to the VHF antenna circuit 12, and the VHF low-band television signal of 54 to 88 MHz is transmitted to the VHF antenna circuit. Synchronized at 12. When receiving a VHF high band television signal or FM broadcast signal, the band decoder 10
The VHF high band selection voltage V is applied to the HF antenna circuit 12.
H is input, and a VHF high-band television signal of 174 to 216 MHz or an FM broadcast signal of 89 to 173 MHz is tuned by the VHF antenna circuit 12.

VHF band television signal or FM
When receiving a broadcast signal, the VHF low-band selection voltage VL or the VHF high-band selection voltage VH is input from the band decoder 10 to the VHF high-frequency amplification circuit 13, and the VHF high-frequency amplification circuit 13 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 a VHF low-band television signal, the VHF low-band selection voltage VL is input from the band decoder 10 to the VHF tuning circuit 14, and the VHF low-band television signal of 54 to 88 MHz is converted to a VHF tuning circuit. Synchronized at 14. Also, V
When receiving the HF high band television signal or the FM broadcast signal, the VHF high band selection voltage VH is input from the band decoder 10 to the VHF tuning circuit 14, and the VHF high band television signal of 174 to 216 MHz or An FM broadcast signal of 89 MHz to 173 MHz is tuned by the VHF tuning circuit 14.

The VHF mixer 3 comprises a VHF tuning circuit 1
A television signal in the VHF band of 4 to 54 MHz to 216 MHz or an FM broadcast signal of 89 MHz to 173 MHz is input, and a local oscillation signal is input from the local oscillator 15 for VHF. Local oscillator for VHF 15
The oscillation frequency changes according to the tuning voltage TU applied from the band decoder 10 having a built-in PLL circuit. When a television signal in the VHF band is received, a local oscillation signal higher by 44 MHz than the received television signal is output. Then, when receiving an FM broadcast signal, it outputs a local oscillation signal 41.25 MHz higher than the received FM broadcast signal. For this reason, the television signal in the VHF band is frequency-converted to an intermediate frequency signal of 44 MHz, and the FM broadcast signal is frequency-converted to an audio intermediate frequency signal of 41.25 MHz corresponding to the frequency of the audio intermediate frequency of the television signal. You.

The UHF circuit section 4 is provided with a VHF circuit section 2.
The antenna 1 is connected in parallel with the UHF antenna circuit 16, the UHF high frequency amplifier circuit 17, and the UHF tuning circuit 18 in that order. Here, the UHF antenna circuit 16 outputs a required UHF-band television signal to the UHF high-frequency amplifier circuit 17 among the input high-frequency signals, and the UHF high-frequency amplifier circuit 17 outputs the input high-frequency signal. The amplifying circuit, the UHF tuning circuit 18, is a circuit for tuning the input signal.

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

The UHF band selection voltage U is input from the band decoder 10 to the UHF high-frequency amplifier circuit 17, and the UHF high-frequency amplifier circuit 17 operates. In this case, since the VHF low band selection voltage VL and the VHF high band selection voltage VH are not input from the band decoder 10 to the VHF high frequency amplifier circuit 13, V
The HF high-frequency amplifier circuit 13 does not operate.

Further, the UHF band television signal is tuned by a UHF tuning circuit 18.

The UHF mixer 5 includes a UHF tuning circuit 1.
8 to 470 MHz to 890 MHz in the UHF band, and a UHF local oscillator 1
9, a local oscillation signal is input. The local oscillator 19 for UHF changes its oscillation frequency by the tuning voltage TU applied from the band decoder 10 having a built-in PLL circuit, and is 44 MHZ lower than the received television signal.
Outputs a local oscillation signal having a higher z value. For this reason, the UHF band television signal is frequency-converted into a 44 MHz intermediate frequency signal.

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

The intermediate frequency tuning circuit 6 includes a switching circuit 1
1 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 an integrated circuit, and its equivalent circuit includes a first switch transistor 31, a second switch transistor 32, a third switch transistor 33, a first resistor 34, A second resistor 35;
It includes a third resistor 36 and a fourth resistor 37. The first switch transistor 31 is an NPN-type transistor whose emitter is grounded and whose base is connected to the first terminal 2 of the band decoder 10 via the first resistor 34.
2 and the collector is grounded via a second resistor 35 and connected to the second terminal 23. The second switch transistor 32 is a PNP-type transistor. The collector of the second switch transistor 32 is grounded via a third resistor 36 and connected to the third terminal 24.
A first power supply voltage B1 of V is applied, and the base is connected to the collector of the third switch transistor 33. The third switch transistor 33 is an NPN-type transistor and has a collector connected to the second switch transistor 3.
1, the emitter is grounded, and the base is connected to the first terminal 22 of the band decoder 10 via the fourth resistor 37. Therefore, when the binary band selection voltage S is input from the band decoder 10 to the switching circuit 11, the binary switching voltage is output from the second terminal 23 and the third terminal 24 of the switching circuit 11, respectively. .

When a television signal is received, 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. Then, the first switch transistor 31 is turned off, the second power supply voltage B2 of 30V is divided by the second resistor 35 and the fifth resistor 38, and the voltage of the second terminal 23 becomes 10V. A switching voltage of 10 V is applied to the cathode of the switching diode 20. Therefore, since the reverse bias voltage is applied to the switch diode 20, the switch diode 20 is turned off, and the intermediate frequency tuning circuit 6 is tuned in parallel by the first capacitor 25 and the first inductors 26a and 26b. In this case, the tuning frequency is 44MHz and the band is 6M
Hz.

On the other hand, when an FM broadcast signal is received, a band selection voltage S of 5 V is output from the first terminal 22 of the band decoder 10 to the switching circuit 11. Then, the first switch transistor 31 is turned on, the voltage of the second terminal 23 becomes 0 V, and a switching voltage of 0 V is applied to the cathode of the switch diode 20. Therefore, a forward bias voltage is applied to the switch diode 20, so that the switch diode 20 is turned on, and the intermediate frequency tuning circuit 6 includes the first capacitor 25 and the first inductors 26a and 26b.
And the second capacitor 27 and the second inductor 28 perform parallel tuning. In this case, if the tuning frequency is 41.
The band becomes 1.5 MHz at 25 MHz.

The television signal or the FM broadcast signal that has passed through the intermediate frequency tuning circuit 6 is input to a trap circuit 7. The trap circuit 7 includes a varactor diode 21, a third capacitor 29, and a third inductor 30 connected in parallel to each other, and is inserted on one of the parallel lines. A first power supply voltage B1 of 5 V is applied to the cathode of the varactor diode 21 via the first inductor 26a.

When a television signal is received, 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, the third switch transistor 33 is turned off, and the second switch transistor 33 is turned off. Switch transistor 32 is turned off. Then, the voltage of the third terminal 24 becomes 0 V, the reverse bias voltage applied to the varactor diode 21 increases to 5 V, and 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 becomes higher than the intermediate frequency band, and becomes 47.25 MHz, which is the frequency of the intermediate frequency band corresponding to the audio carrier of the adjacent television channel. Has become.

On the other hand, when an FM broadcast signal is received, a band selection voltage S of 5 V is output from the first terminal 22 of the band decoder 10 to the switching circuit 11. Then, the third switch transistor 33 is turned on, and the second switch transistor 32 is turned on. Then, a voltage of 5 V is output from the third terminal 24, divided by the sixth resistor 39 and the seventh resistor 40, and
Is applied to the anode of the varactor diode 21, the reverse bias voltage applied to the varactor diode 21 is 1
V, and the capacitance value of the varactor diode 21 becomes relatively large. In this state, the frequency of the signal to be attenuated by the trap circuit 7 is 43
MHz.

The television signal or the FM broadcast signal that has passed through the trap circuit 7 is input to the intermediate frequency amplifier circuit 8 and amplified. The television signal or the FM broadcast signal amplified by the intermediate frequency amplifier 8 is input to the SAW filter 9. The pass band of the SAW filter 9 is 41 MHz
From 47 MHz to the intermediate frequency band of the television signal. Although the pass band of the SAW filter 9 is wider than the band of the FM broadcast signal frequency-converted to the audio intermediate frequency, when the FM broadcast signal is received,
Since signals adjacent to the FM broadcast signal received by the intermediate frequency tuning circuit 6 and the trap circuit 7 have been removed, only signals in the band to be received are output from the tuner.

[0037]

As described above, the television tuner of the present invention can change 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. In the television tuner, each switching voltage for switching between the tuning frequency of the intermediate frequency tuning circuit and the trap frequency of the trap circuit is output from the switching circuit, so that one band selection voltage is output from the band decoder to the switching circuit. It is possible to switch between the tuning frequency of the intermediate frequency tuning circuit and the trap frequency of the trap circuit,
The number of output terminals of the band decoder can be reduced,
The circuit of the band decoder can be simplified, and the price can be reduced.

Further, 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 the first power supply voltage. Lower, the high-level voltage and the low-level voltage of the switching voltage output from the third terminal are lower than the first power supply voltage, and a switch diode that changes the tuning frequency of the intermediate frequency tuning circuit; A varactor diode that changes the trap frequency of the trap circuit can be controlled with a voltage suitable for each.

Further, the switching circuit of the television tuner of the present invention is composed of the first transistor of the NPN type, the second transistor of the PNP type, and the transistor of the NPN type. It can be downsized.

Further, the television tuner of the present invention comprises:
It is integrated and can be miniaturized.

[Brief description of the drawings]

FIG. 1 is a block diagram of a tuner of the present invention.

FIG. 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.

FIG. 3 is a block diagram of a conventional tuner.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Antenna 2 VHF circuit part 3 VHF mixer 4 UHF circuit part 5 UHF mixer 6 Intermediate frequency tuning circuit 7 Trap circuit 8 Intermediate frequency amplifier circuit 9 SAW filter 10 Band decoder 11 Switching circuit 12 VHF antenna circuit 13 For VHF High-frequency amplifier circuit 14 VHF tuning circuit 15 VHF local oscillator 16 UHF antenna circuit 17 UHF high-frequency amplifier circuit 18 UHF tuning circuit 19 UHF local oscillator 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 Motor 33 third switching transistor 34 first resistor 35 second resistor 36 third resistor 37 a fourth resistor 38 fifth resistor 39 the sixth resistor 40 seventh resistor

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 5/60 H04N 5/60 B 5/62 5/62 CF term (Reference) 5C025 AA13 AA25 AA27 CA19 5C026 DA22 5J103 AA02 BA02 BA03 BA06 CA07 CA08 CB04 CB05 DA02 DA03 DA05 DA06 DA16 DA41 EA01 EA03 EA05 EA08 EA10 EA12 GB03 KA00 5K062 AA05 AA06 AB11 AC02 AD04 AD05 BA03 BB01 BB03 BB09 BB15 BC05

Claims (4)

[Claims] 1. A mixer for frequency-converting a television signal into a television intermediate frequency band and frequency-converting an FM broadcast signal into an audio intermediate frequency in the intermediate frequency band, and a switch diode. The switch diode has an intermediate frequency tuning circuit that is substantially in the middle of the intermediate frequency band when the switch diode is off, and has an varactor diode that is switched to the audio intermediate frequency when the switch diode is on, and the reverse direction applied to the varactor diode A trap circuit for attenuating a frequency higher than the intermediate frequency band or substantially the intermediate frequency of the intermediate frequency band by switching a bias voltage between two levels, and a band decoder for selecting at least a low band or a high band of a VHF band; And the band decoder includes the television. A first terminal for outputting a binary band selection voltage for selecting a signal or the FM broadcast signal; a binary switch for receiving the band selection voltage and for turning on or off the switch diode; A switching circuit that outputs a voltage from a second terminal and outputs a binary switching voltage for switching a reverse bias voltage of the varactor diode from a third terminal;
Upon receiving the television signal, the switching diode is turned off by the switching voltage output to the second terminal, and the reverse voltage applied to the varactor diode by the switching voltage output to the third terminal. When receiving the FM broadcast signal, the switching diode is turned on by the switching voltage output from the second terminal, and the switching voltage output from the third terminal is turned on by the switching voltage output from the third terminal. A television tuner wherein a reverse bias voltage applied to a varactor diode is reduced. 2. A first power supply voltage is applied to an anode of the switch diode, a cathode is connected to the second terminal, and the first power supply voltage is applied to a cathode of the varactor diode. At the same time, an anode is connected to the third terminal, a high-level switching voltage output from the second terminal is higher than the level of the first power supply voltage, and a low-level voltage is the first voltage. The switching voltage output from the third terminal may be lower than the level of the power supply voltage, and the high-level voltage and the low-level voltage of the switching voltage may be lower than the level of the first power supply voltage. The television tuner according to claim 1. 3. The switching circuit has an NPN-type first transistor, a PNP-type second transistor, and an NPN-type third transistor, and has a collector of the first transistor connected to the second transistor. Connect the terminals and apply a second power supply voltage, apply the band selection voltage to the base of the first transistor, ground the emitter of the first transistor, and connect the collector of the second transistor to the collector. Connecting a third terminal, applying the first power supply voltage to the emitter of the second transistor, connecting the collector of the third transistor to the base of the second transistor, 3. The television tuner according to claim 2, wherein the band selection voltage is applied to a base of the television set, and an emitter of the third transistor is grounded. 4. The television tuner according to claim 3, wherein said switching circuit is integrated.