Abstract:
A television tuner comprises a VHF tuner section and a UHF tuner section and is configured in the two tuned modes of VHF high band and VHF low band between which the input tuning circuit of the VHF tuner section is switched by a diode in response to a source voltage. The television tuner further comprises a bias circuit for biasing said band-switching diode in reverse or forward direction by use of the UHF band source voltage during the receipt of UHF band.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a television tuner circuit having VHF and UHF tuner sections. 
     2. Description of the Prior Art 
     A certain tuner circuit for television uses a band switching diode for switching the low and high bands of the VHF tuner section. 
     For selecting the VHF low band, the band switching diode is turned off so that the input tuning circuit enters a VHF low band receiving mode. For selection of the VHF high band, on the other hand, the band switching diode is turned on so that the input tuning circuit is placed in the VHF high band receiving mode. Further, for selection of UHF, the band switching diode is subjected to zero bias mode. 
     This tuner circuit has the advantage that the band-switching diode is operated, i.e., the band is switched with a single-polarity power supply. 
     If the band switching diode is placed in the zero bias mode for selection of the UHF band in the VHF or UHF tuner circuit, however, the VHF component of a large signal higher than 1 V reaching the VHF antenna input tuner circuit is applied to the band switching diode, with the result that the band switching diode generates a high harmonic or snow noise attributable to the non-linear rise characteristics of the diode. This component is radiated on the UHF antenna input tuner circuit of the UHF tuner section, thereby deteriorating the S/N ratio of the UHF signal, resulting in the lower quality of the picture. 
     In order to solve this problem, the conventional tuner circuits use a power supply of two polarities, positive and negative, for controlling the band-switching diode. During the receipt of the VHF low band, the band-switching diode is turned off by the positive power supply; during the receipt of the VHF high band, the band-switching diode is turned on with the positive power supply; and during the receipt of the UHF band, the band-switching diode is impressed with a reverse bias from the negative power supply, thus preventing the diode from generating any high harmonic or snow noises. 
     The conventional methods are unsatisfactory both technically and in cost since the band switching operation requires a power supply of two polarities, positive and negative, and the number of required power terminals of the tuner is increased, thus complicating the circuit configuration. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a simple and low-cost television tuner circuit which is capable of band-switching operation with a single-polarity power supply for producing a UHF picture without any interference. 
     The feature of the circuit configuration according to the present invention lies in that during the UHF band receipt a single-polarity source voltage for driving the tuner section is used to apply a forward or reverse bias to the VHF low-high band switching diode of the VHF tuner section. 
     According to one aspect of the present invention, a proper bias voltage is applied to the band-switching diodes of the VHF tuner section during the receipt of the UHF band, and therefore high harmonics or snow noises are prevented from being generated from the band-switching diodes even when an electric wave of large amplitude arrives at the VHF tuner section, thus producing a clear UHF image without any interference. 
     According to another aspect of the present invention, each band is selected with a single-polarity power supply so that the tuner circuit according to the present invention is satisfactory both technically and in cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a wiring diagram showing a tuner circuit according to an embodiment of the present invention. 
     FIGS. 2A and 2B are diagrams showing equivalent circuits of the input tuner circuit included in FIG. 1. 
     FIGS. 3, 4 and 5 are wiring diagrams showing tuner circuits according to other embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the attached drawings. A wiring diagram of the tuner circuit according to an embodiment of the present invention is shown in FIG. 1. Reference numeral 1 shows a VHF antenna terminal, numeral 2 a high-pass filter, numeral 3 a coupling capacitor, and numeral 4 an input tuner circuit. The input tuner circuit comprises a tuning capacitor circuit including a variable capacitance diode 411 and a tracking correction capacitor 412; a tuning inductor circuit including a VHF high-band tuning coil 421 and a VHF low-band tuning coil 422; and a band-switching circuit including VHF high-low band switching diodes 431 and 432 for switching the coils 421 and 422 of the tuning inductor circuit. The anode of the band-switching diode 431 is connected via the capacitor 3 to the filter 2 on the one hand and to the radio-frequency amplifier 6 via the capacitor 5 on the other hand. The cathode of the diode 431 is grounded through the coil 421 and the capacitor 416 on the one hand and grounded through the resistor 443 on the other hand. The anode of the band-switching diode 432 is connected to the anode of the diode 431 through the capacitor 413 on the one hand and grounded through the coil 422, and the capacitors 415 and 416 on the other hand. Further, the anode of the diode 432 is grounded through the coil 422 and the resistor 444. The cathode of the diode 432 is connected to the radio-frequency amplifier 6 through the coil 423 and capacitor 414 in parallel with each other and through the capacitor 5. The anode of the variable capacitance diode 411 is directly grounded, while the cathode thereof is connected to the radio-frequency amplifier 6 through the capacitors 412 and 5 on the one hand and to the tuning voltage source V T  through the resistor 445 on the other hand. Numeral 7 shows a band pass filter, numeral 8 a local oscillator, and numeral 9 a mixer/intermediate frequency amplifier which operates as a mixer during the receipt of VHF band and as an intermediate frequency amplifier during the receipt of UHF band. Numeral 10 shows an intermediate frequency signal output terminal. These parts make up a generally-known VHF tuner section. 
     Numeral 20 shows a UHF tuner section and numeral 21 a UHF antenna terminal which are connected as shown. Numeral 30 shows a circuit means which leads the source voltage for driving the UHF tuner section 20 to the band-switching diodes 431 and 432 of the input tuner circuit 4 of the VHF tuner section. This circuit means is comprised of a diode 301, a resistor 302 and a capacitor 303. Numeral 11 shows a power supply device including a power change-over switch 111. The VHF low-band power supply terminal BL of the switch 111 is connected in DC fashion to the cathode of the band-switching diode 431 through the diode 13 and the coil 421 of the VHF input tuner circuit 4 on the one hand and to the cathode of the band-switching diode 432 through the diode 13, the coil 421, the resistor 441 and the coil 424 on the other hand. The terminal BL is also connected to the radio-frequency amplifier 6 and the band pass filter 7 through the diode 22 and, in addition, to the mixer/intermediate frequency amplifier 9 through the diodes 22 and 16. The terminal BL is furthermore connected to the local oscillator 8 through the diode 22 and resistor 19. The VHF high-band power supply terminal BH is connected in DC fashion to the anode of the band-switching diode 432 through the coil 422 on the one hand and to the anode of the band-switching diode 431 through the coil 422, the resistor 442 and the coil 424 on the other hand. The terminal BH is also connected through the diode 17 to the high-frequency amplifier 6 and the band pass filter 7 and in addition, to the local oscillator 8 through the diode 18. The terminal BH is further connected through the diode 18, resistor 19 and diode 16 to the mixer/intermediate frequency amplifier 9. The UHF band power supply terminal BU of the switch 111 is connected to the UHF tuner section 20 on the one hand and in DC fashion to the cathodes of the band-switching diodes 431 and 432 of the input tuner circuit 4 through the resistor 302 and the diode 301 of the circuit means 30 on the other hand. The terminal BU is furthermore connected to the mixer/intermediate frequency amplifier 9 via the diode 15. The movable terminal of the switch 111 is connected to the power supply +B (12 V). 
     In this circuit configuration, assume that the power +B is selectively applied to the VHF low-band power supply terminal BL. The source voltage is applied through the diode 13 to the cathodes of the band-switching diodes 431 and 432 of the input tuner circuit 4, so that the diodes 431 and 432 are turned off by being reversely biased by the source voltage. Upon the turning off of the band-switching diodes 431 and 432, the input tuner circuit 4 makes up a low-band tuning circuit as shown in FIG. 2A and is ready to receive the VHF low band frequency. 
     When the power +B is applied selectively to the VHF high-band power supply terminal BH, on the other hand, the source voltage is applied to the anode of the band-switching diodes 431 and 432 of the input tuner circuit 4, so that the diodes 431 and 432 are biased in forward direction by the source voltage and turned on. Upon the turning on of the band-switching diodes 431 and 432, the coil 422 of the input tuner circuit 4 is shorted by the diode 431, with the result that a high-band tuning circuit as shown in FIG. 2B is configured, thus making the circuit ready to receive the VHF high band. 
     When the power supply +B is supplied to the UHF band power supply terminal BU, the UHF band source voltage is applied through the resistor 302 and the diode 301 of the circuit means 30 to the cathodes of the band-switching diodes 431 and 432 of the input tuner circuit 4 of the VHF tuner section, thus turning off the diodes 431 and 432 by being reversely biased by the source voltage. By selecting appropriate values of the resistors 431, 432, 433 and 434 of the input tuning circuit 4, the reverse bias voltage to the band-switching diodes 431 and 432 is adjusted as desired. 
     In this way, an appropriate bias is applied to the band-switching diodes 431 and 432 of the input tuning circuit 4 of the VHF tuner section by use of the power supply +B supplied to the UHF band power supply terminal BU. Therefore, even if a VHF component of a large amplitude reaches the band-switching diodes 431 and 432 of the VHF tuner section at the time of UHF band receipt, an interference signal interfering with the UHF band receipt is prevented from being generated. 
     In the above-mentioned embodiment, the voltage applied to the UHF band power supply terminal BU of the power supply device 11 is introduced to the band-switching diodes 431 and 432 of the input tuner circuit 4, so that the diodes 431 and 432 are reversely biased by the same voltage. As an alternative shown in FIG. 3, the input of the circuit means 30 may be connected to the mixer/intermediate frequency amplifier 9 so that the band-switching diodes 431 and 432 may be biased in reverse direction by utilizing the voltage of the mixer/intermediate frequency amplifier 9. 
     In the above-mentioned embodiment, a reverse bias voltage is applied to the band-switching diodes 431 and 432 of the input tuning circuit 4 of the VHF tuner section. This may be replaced with equal effect by the circuit configuration shown in FIG. 4. In other words, the VHF high-band power supply terminal BH of the power supply device is connected through the diode 17 to the high-frequency amplifier 6 and the band pass filter 7 on the one hand and to the local oscillator 8 through the diode 18 on the other hand. The terminal BH is also connected to the mixer/intermediate frequency amplifier 9 through the diode 18, resistor 19 and diode 16. The UHF band power supply terminal BU is connected through the diode 15 to the mixer/intermediate frequency amplifier 9 on the one hand and through the diode 15 and the resistor 302 of the circuit means 30 to the junction point of the resistor 444 and the coil 422 of the input tuning circuit 4 on the other hand. The other component parts are connected in the same manner as in FIG. 1. In this way, during the receipt of the UHF band, a forward bias may be applied to the band-switching diodes 431 and 432 by the UHF band source voltage thereby to dampen the high harmonics and snow noise which otherwise might be generated by the zero biasing of the band switching diodes. 
     A circuit diagram of a further embodiment of the present invention is shown in FIG. 5. In this drawing, component elements identical to those in FIG. 1 are designated by the same reference numerals and will not be described in detail. In FIG. 5, the VHF low-band power supply terminal BL of the power supply device 11 is connected through the diodes 22 and 16 to the mixer 9 on the one hand and through the diode 22 and resistor 19 to the local oscillator 8 on the other hand. The VHF high-band power supply terminal BH is connected through the coil 422 of the input tuning circuit 4 to the band-switching diode 432 on the one hand and through the coil 422, resistor 442 and coil 424 to the band-switching diode 432 on the other hand. The terminal BH is also connected to the high-frequency amplifier 6 and the band filter 7 via the diode 17, to the local oscillator 8 through the diode 18, and further to the mixer/intermediate frequency amplifier 9 through the diode 18, the resistor 19 and the diode 16. The UHF band power supply terminal BU is connected via the diode 15 to the mixer/intermediate frequency amplifier 9 on the one hand and in DC fashion to the cathodes of the band-switching diodes 431 and 432 of the input tuning circuit 4 through the diode 15 and the resistor 302 of the circuit means 300 on the other hand. 
     In the circuit configuration according to the embodiment under consideration, the UHF band power supply terminal BU is connected to the cathodes of the band-switching diodes 431 and 432 of the input tuning circuit 4 through the resistor 302 of the circuit means 30 and the diode 15, so that the diodes are biased in reverse direction. As noted from the drawing, this circuit configuration is simpler than that of FIG. 1. 
     It will be seen from the foregoing description that according to the present invention, by use of the source voltage of single polarity (positive) of the tuner section during UHF band receipt, the band-switching diodes of the input tuning circuit 4 of the VHF tuner section are impressed with a bias voltage, thereby preventing the high harmonics or snow noise from being generated from the diodes, resulting in a clear UHF image at low cost.