Television tuner

Abstract: Described herein is a television tuner capable of reproducing images and voices of respective TV broadcasts of PAL (B/G) and SECAN (L) systems. The television tuner is provided with means for lowering respective tuning frequencies of a high-frequency double tuned circuit (4) and an intermediate frequency tuning circuit (7) as compared with the reception of other TV broadcasts when a TV broadcast of the SECAM (L) system lying in a VHF low-band is received. Added bias voltages are applied to turning variable capacitances diodes (4 1 , 4 2 ) as the means for reducing the tuning frequency of the high-frequency double tuned circuit. As first means for lowering the tuning frequency of the intermediate frequency tuning circuit, the intermediate frequency tuning circuit is provided with a variable capacitance diode (7 7 ) to which an added bias voltage is applied. As second means for lowering the tuning frequency of the intermediate frequency tuning circuit, an intermediate frequency tuning circuit is provided with an added capacitor connected in series with a diode. A switching voltage is applied to the added capacitor to turn on the diode.

Description: The present invention relates to a shared television tuner capable of receiving both of respective television broadcasts (hereinafter abbreviated as "TV broadcasts") of the PAL-B/G and SECAM-L systems, and particularly to a television tuner capable of preventing degradation in receiving performance, which takes place due to the differences in intermediate frequency between the respective systems and between broadcast frequency bands. A television tuner according to the preamble of claim 1 is known e.g. from EP-A-0 065 150. As TV broadcast systems which have been adopted in Europe, there are known the PAL-B/G system adopted in Germany, etc. and the SECAM-L system adopted in France, etc. A shared television tuner capable of receiving TV broadcasts of these two systems has already been developed. Fig. 3 is a block diagram partly including a circuit diagram, showing one example of a configuration of the already-known shared television tuner. As shown in Fig. 3, the already-known television tuner comprises an input terminal 61, an antenna (ANT) tuning circuit 62, a high-frequency amplifier circuit (RFAMP) 63, a high-frequency double tuned circuit 64, a mixer (MIX) 65, a local oscillator (OSC) 66, an intermediate frequency tuning circuit 67, a channel selection IC 69, and an output terminal 70. The high-frequency double tuned circuit 64 has an input terminal electrically connected to an output terminal of the high-frequency amplifier circuit 63 and an output terminal electrically connected to a first input terminal of the mixer 65. Further, the high-frequency double tuned circuit 64 comprises variable capacitance diodes 64 1 and 64 2 , inductors 64 3 through 64 7 , capacitors 64 8 through 64 12 , switching diodes 64 13 and 64 14 , and resistors 64 15 and 64 16 . Further, the local oscillator 66 has an output terminal electrically connected to a second input terminal of the mixer 65. The intermediate frequency tuning circuit 67 has an input terminal electrically connected to an output terminal of the mixer 65 and an output terminal electrically connected to the output terminal 70. Further, the intermediate frequency tuning circuit 67 comprises an inductor 67 1 and capacitors 67 2 through 67 4 . The IC 69 is provided with a band decoder in addition to a PLL circuit and has a tuning voltage terminal TU, and band switching terminals BS2 and BS3. These respective terminals are respectively electrically connected to the high-frequency double tuned circuit 64 and the local oscillator 66. Incidentally, the tuning voltage terminal outputs a tuning voltage therethrough, the band switching terminal BS2 outputs a positive voltage therethrough when a TV broadcast lying in a VHF high-band is selected, and the band switching terminal BS3 outputs a positive voltage therethrough when a TV broadcast lying in a VHF low-band is selected. The already-known shared television tuner constructed as described above is activated as follows: A received signal applied to the input terminal 61 is inputted to the antenna tuning circuit 62 where a received signal corresponding to a desired channel is selected and thereafter amplified by the high-frequency amplifier circuit 63. The amplified received signal is inputted to the high-frequency double tuned circuit 64 where a received signal corresponding to a desired channel is further selected. The mixer 65 mixes the selected received signal and a local oscillation signal supplied from the local oscillator 66 so as to be converted into an intermediate frequency signal. The intermediate frequency signal is inputted to the intermediate frequency tuning circuit 67 where an intermediate frequency signal having a predetermined frequency is selected and outputted from the output terminal 70. When a TV broadcast lying in a VHF low-band is now selected, a positive voltage is outputted from the band switching terminal BS3 of the IC 69. Thus, the high-frequency double tuned circuit 64 takes such a configuration that in response to the positive voltage, the switching diodes 64 13 and 64 14 are respectively turned off and hence the inductors 64 5 through 64 7 and the capacitor 64 10 are inserted into and connected to the variable capacitance diodes 64 1 and 64 2 , the inductors 64 3 and 64 4 , and the capacitors 64 8 and 64 9 . On the other hand, when a TV broadcast lying in a VHF high-band is selected, a positive voltage is outputted from the band switching terminal BS2 of the IC 69. Thus, the high-frequency double tuned circuit 64 takes such a configuration that in response to the positive voltage, the switching diodes 64 13 and 64 14 are respectively turned on so that a circuit portion comprised of the inductors 64 5 through 64 7 and the capacitor 64 10 is short-circuited and hence only the variable capacitance diodes 64 1 and 64 2 , the inductors 64 3 and 64 4 and the capacitors 64 8 and 64 9 are electrically connected to one another. Even when the TV broadcast lying in any band is selected, a tuning voltage is outputted from the tuning voltage terminal TU of the IC 69, so that the high-frequency double tuned circuit 64 is activated as follows. That is, since the tuning voltage is applied to the variable capacitance diode 64 1 through the resistor 64 15 and the inductor 64 3 and further applied to the variable capacitance diode 64 2 through the resistor 64 16 and the inductor 64 4 , the respective capacitance of the variable capacitance diodes 64 1 and 64 2 are adjusted so as to be tuned to the selected TV broadcast. In a manner similar to the high-frequency double tuned circuit 64, the local oscillator 66 is supplied with the positive voltage outputted from the band switching terminal BS3 or BS2 of the IC 69 and the tuning voltage outputted from the tuning voltage terminal TU thereof to thereby output a local oscillation signal having a frequency higher than that of the selected TV broadcast by a predetermined difference. Meanwhile, the frequencies of a video or picture carrier and a voice or speech carrier of a TV broadcast of the SECAM (L) system lying in a VHF low-band are arranged in a high/low inverse relationship as compared with a TV broadcast of the PAL (B/G) system. Further, a frequency bandwidth per channel is widened by 1MHz. Therefore, when the TV broadcast of the PAL (B/G) system is selected, a video or picture intermediate frequency, a chrominance intermediate frequency and a voice or speech intermediate frequency are respectively set to 38.9MHz, 34.47MHz and 33.4MHz which are standard. On the other hand, when the TV broadcast of the SECAM (L) system lying in the VHF low-band is selected, they are respectively set to 33.9MHz, 38.33MHz and 40.4MHz in consideration of the above-described difference and the characteristic of an intermediate frequency filter (not shown) formed by a surface acoustic wave device or element. Fig. 4 shows tuning characteristics of the high-frequency double tuned circuit 64 of the already-known shared television tuner at the time that the TV broadcast lying in the VHF low-band is selected. Symbols P PAL and S PAL in the drawing indicate a video or picture carrier and a voice or speech carrier of the TV broadcast of the PAL (B/G) system, respectively, whereas symbols P SEC and S SEC indicate a picture carrier and a speech carrier of the TV broadcast of the SECAM (L) system, respectively. However, numeric values in the drawing are those represented by replacing the frequencies of the respective carriers with intermediate frequencies for convenience. The relationship in magnitude between the frequencies of the respective carriers is contrary to an arrangement of the numeric values in the drawing, i.e., they are small on the left side in Fig. 4, whereas they are large on the right side in Fig. 4. As shown in Fig. 4, the tuning characteristic of the high-frequency double tuned circuit 64 is set so as to coincide with the TV broadcast of the PAL (B/G) system. In the tuning characteristic thereof, the tuning frequency of the high-frequency double tuned circuit 64 is made high by 1MHz with respect to each carrier of the TV broadcast of the SECAM (L) system lying in the VHF low-band, and the speech carrier S SEC lies outside the band of the tuning characteristic. Fig. 5 shows tuning characteristics of the intermediate frequency tuning circuit 67 of the already-known shared television tuner at the time that the TV broadcast lying in the VHF low-band is selected. Symbols PI PAL and CI PAL in the drawing indicate a video or picture intermediate frequency and a chrominance intermediate frequency of the TV broadcast of the PAL (B/G) system, respectively, whereas symbols PI SEC and CI SEC indicate a video or picture intermediate frequency and a chrominance intermediate frequency of the TV broadcast of the SECAM (L) system, respectively. As shown in Fig. 5, the tuning characteristic of the intermediate frequency tuning circuit 67 is a single peak response, which is tuned to about 36.7MHz lying between a chrominance intermediate frequency CI PAL of 34.47MHz and a picture intermediate frequency PI PAL of 38.9MHz of the PAL (B/G) system. Thus, both selectivity to the picture intermediate frequency and the chrominance intermediate frequency are set so as to be substantially equal to each other. However, the tuning frequency is biased to a higher frequency rather than an intermediate frequency between a chrominance intermediate frequency CI SEC of 38.33MHz of the SECAM (L) system lying in the VHF low-band and a picture intermediate frequency PI SEC of 33.9MHz thereof with respect to the chrominance intermediate frequency CI SEC of 38.33MHz and the picture intermediate frequency PI SEC of 33.9MHz. When the TV broadcast of the SECAM (L) system lying in the VHF low-band is selected as described above, the speech carrier S SEC lies outside the band of the tuning characteristic of the high-frequency double tuned circuit 64. Therefore, a first problem arises in that the speech carrier is attenuated in the high-frequency double tuned circuit 64 so that a reproduced voice is low in level as compared with the selection of the TV broadcast lying in another band, whereby the voice is disagreeable to hear. Further, the tuning frequency of the intermediate frequency tuning circuit 67 is biased to the chrominance intermediate frequency CI SEC . Therefore, a second problem arises in that the selectivity of the intermediate frequency tuning circuit 67 is high with respect to the chrominance intermediate frequency CI SEC rather than the picture intermediate frequency PI SEC , and a reproduced image becomes deep in color as compared with the selection of a TV broadcast lying in another band, whereby a satisfactory image or picture cannot be obtained. As a method of solving the first problem, a first solving method is considered in which when the TV broadcast of the SECAM (L) system lying in the VHF low-band is selected, the tuning voltage outputted from the IC 69 is slightly reduced and the tuning frequency of the high-frequency double tuned circuit 64 is lowered by 1MHz. As a method of solving the second problem, a second solving method is considered wherein when the TV broadcast of the SECAM (L) system lying in the VHF low-band is selected, the tuning voltage outputted from the IC 69 is slightly increased to increase the oscillation frequency of the local oscillator 66, whereby the respective intermediate frequencies are increased by 0.6MHz. However, since the high-frequency double tuned circuit 64 and the local oscillator 66 are controlled by the same tuning voltage, the respective methods referred to above cannot solve the two problems together. With the foregoing problems in view, it is therefore an object of the present invention to provide a television tuner capable of reproducing both voices and an image in a satisfactory state even when either of TV broadcasts of the PAL (B/G) or SECAM (L) systems is received. The invention is defined as set out in appended claim 1. According to the present invention, for achieving the above object, there is provided a television tuner, comprising: a high-frequency double tuned circuit; an intermediate frequency tuning circuit; and means which is capable of receiving respective television broadcasts of PAL-B/L and SECAM L systems, and which sets respective tuning frequencies of the high-frequency double tuned circuit and the intermediate frequency tuning circuit so as to be adapted to the television broadcast of the PAL-B/G system, and lowers the respective tuning frequencies of the high-frequency double tuned circuit and the intermediate frequency tuning circuit when the television broadcast of the SECAM-L system lying in a VHF low-band is received. According to the means referred to above, the tuning frequencies of the respective tuning circuits become low when the TV broadcast of the SECAM-L system lying in the VHF low-band is received, so that they result in tuning frequencies corresponding to the TV broadcast of the SECAM-L system lying in the VHF low-band. When another TV broadcast is received, the tuning frequencies become high, thus resulting in tuning frequencies corresponding to the TV broadcast of the PAL-B/G system and a TV broadcast of the SECAM-L system lying in a band other than the VHF low-band. Thus, even when either of the TV broadcasts is received, both selectivity of the high-frequency double tuned circuit to a picture carrier and a speech carrier can be made substantially equal to each other and both selectivity of the intermediate frequency tuning circuit to a picture intermediate frequency and a chrominance intermediate frequency can be substantially equalized. In one of embodiments according to the present invention, means for lowering a tuning frequency of a high-frequency double tuned circuit is constructed such that the high-frequency double tuned circuit is provided with variable capacitance diodes and added bias voltages are respectively applied to the variable capacitance diodes together with a tuning voltage, thereby increasing the respective capacitance of the variable capacitance diodes as compared with the application of the tuning voltage alone. Means for lowering a tuning frequency of an intermediate frequency tuning circuit is constructed such that the intermediate frequency tuning circuit is provided with a variable capacitance diode and an added bias voltage is applied to the variable capacitance diode together with a fixed bias voltage, thereby increasing the capacitance of the variable capacitance diode as compared with the application of the fixed bias voltage alone. In one preferred example showing the embodiment according to the present invention, added bias voltages are fractional voltages obtained by dividing a band switching voltage outputted from a channel selecting IC provided with a band decoder by resistance circuits. A fixed bias voltage is a voltage higher than the added bias voltages. A tuning voltage is applied to the cathodes of each individual variable capacitance diodes of a high-frequency double tuned circuit and the added bias voltages are respectively applied to the anodes thereof, whereas the fixed bias voltage is applied to the cathode of a variable capacitance diode of an intermediate frequency tuning circuit and an added bias voltage is applied to the anode thereof. In another one of the embodiments according to the present invention, means for lowering a tuning frequency of a high-frequency double tuned circuit is constructed such that the high-frequency double tuned circuit is provided with variable capacitance diodes and added bias voltages are applied to the variable capacitance diodes together with a tuning voltage, thereby increasing the respective capacitance of the variable capacitance diodes as compared with the application of the tuning voltage alone. Further, means for lowering a tuning frequency of an intermediate frequency tuning circuit is constructed such that the intermediate frequency tuning circuit is provided with a fixed capacitor and an added capacitor series-connected to a switching diode, and a switching voltage is applied to the switching diode so as to bring the switching diode to an on state, thereby connecting the added capacitor in parallel with the fixed capacitor. In another preferred example showing the embodiment of the present invention, added bias voltages are fractional voltages obtained by dividing a band switching voltage outputted from a channel selecting IC provided with a band decoder by resistance circuits. Further, a switching voltage is the band switching voltage. A tuning voltage is applied to the cathodes of variable capacitance diodes and the added bias voltages are respectively applied to the anodes thereof, whereas a fixed bias voltage lower than the band switching voltage is applied to the cathode of a switching diode and the band switching voltage is applied to the anode thereof. Typical aspects of the present invention have been shown in brief. However, the various embodiments of the present invention and specific configurations thereof will be understood from the following description. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figs. 1 is a block diagram partly including a circuit diagram, showing a configuration of a first embodiment of a television tuner according to the present invention; Fig. 2 is a block diagram partly including a circuit diagram, illustrating a configuration of a second embodiment of a television tuner according to the present invention; Fig. 3 is a block diagram partly including a circuit diagram, showing a configuration of a conventional television tuner; Fig. 4 is a tuning characteristic diagram of a high-frequency double tuned circuit employed in the conventional television tuner; Fig. 5 is a tuning characteristic diagram of an intermediate frequency tuning circuit employed in the conventional television tuner; Fig. 6 is a tuning characteristic diagram of a high-frequency double tuned circuit employed in each of the television tuners according to the first and second embodiments; and Fig. 7 is a tuning characteristic diagram of an intermediate frequency tuning circuit employed in each of the television tuners according to the first and second embodiments. Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. Fig. 1 is a block diagram partly including a circuit diagram, showing a configuration of a first embodiment of a television tuner according to the present invention. As shown in Fig. 1, the television tuner according to the first embodiment is provided with an input terminal 1, an antenna (ANT) tuning circuit 2, a high-frequency amplifier circuit (RFAMP) 3, a high-frequency double tuned circuit 4, a mixer (MIX) 5, a local oscillator (OSC) 6, an intermediate frequency tuning circuit 7, a channel selecting IC 9, and an output terminal 10. The high-frequency double tuned circuit 4 has an input terminal electrically connected to an output terminal of the high-frequency amplifier circuit 3 and an output terminal electrically connected to a first input terminal of the mixer 5. Further, the high-frequency double tuned circuit 4 comprises variable capacitance diodes 4 1 and 4 2 , inductors 4 3 through 4 7 , capacitors 4 8 through 4 12 , switching diodes 4 13 and 4 14 , resistors 4 15 and 4 16 , and two resistive potential dividers 4' and 4". The variable capacitance diode 4 1 has a cathode electrically connected to one end of the inductor 4 3 and electrically connected to the input terminal of the high-frequency double tuned circuit 4 through the capacitor 4 11 and has an anode electrically connected to one resistive potential divider 4'. The variable capacitance diode 4 2 has a cathode electrically connected to one end of the inductor 4 4 and electrically connected to the output terminal of the high-frequency double tuned circuit 4 and has an anode electrically connected to the other resistive potential divider 4". The inductors 4 3 and 4 4 are electrically inductively-coupled to each other. The other end of the inductor 4 3 is electrically connected to one end of the inductor 4 5 through the capacitor 4 8 and electrically connected to a tuning voltage terminal TU of the IC 9 through the resistor 4 15 . The other end of the inductor 4 4 is electrically connected to one end of the inductor 4 6 through the capacitor 4 9 and electrically connected to the tuning voltage terminal TU of the IC 9 through the resistor 4 16 . The inductors 4 5 and 4 6 are electrically inductively-coupled to each other. The inductor 4 5 has one end electrically connected to a band switching terminal BS3 of the IC 9 and electrically connected to a band switching terminal BS2 through the switching diode 4 13 and has the other end electrically connected to the other end of the inductor 4 6 and one end of the inductor 4 7 . The inductor 4 6 has one end electrically connected to the band switching terminal BS3 of the IC 9 and electrically connected to the band switching terminal BS2 through the switching diode 4 14 . The inductor 4 7 has the other end electrically connected to the ground through the capacitor 4 10 . The resistive potential divider 4' comprises resistors 4 17 and 4 18 and a bypass capacitor 4 19 . The resistor 4 17 has one end electrically connected to a band switching terminal BS4 of the IC 9 and the other end electrically connected to the anode of the variable capacitance diode 4 1 , one end of the resistor 4 18 and one end of the bypass capacitor 4 19 respectively. The other end of the resistor 4 18 and the other end of the bypass capacitor 4 19 are respectively electrically connected to the ground. Further, the resistive potential divider 4" comprises resistors 4 20 and 4 21 and a bypass capacitor 4 22 . The resistor 4 20 has one end electrically connected to the band switching terminal BS4 of the IC 9 and the other end electrically connected to the anode of the variable capacitance diode 4 2 , one end of the resistor 4 21 and one end of the bypass capacitor 4 22 respectively. The other end of the resistor 4 21 and the other end of the bypass capacitor 4 22 are respectively electrically connected to the ground. Further, the local oscillator 6 has an output terminal electrically connected to a second input terminal of the mixer 5. The intermediate frequency tuning circuit 7 has an input terminal electrically connected to an output terminal of the mixer 5 and an output terminal electrically connected to the output terminal 10. Further, the intermediate frequency tuning circuit 7 comprises an inductor 7 1 , capacitors 7 2 through 7 6 , a variable capacitance diode 7 7 , and resistors 7 8 and 7 9 . The inductor 7 1 has one end electrically connected to an input terminal of the intermediate frequency tuning circuit 7 through the capacitor 7 3 and electrically connected to the output terminal 10 through the capacitor 7 4 . Further, the inductor 7 1 has the other end electrically connected to a power supply terminal B and electrically connected to the ground through the capacitor 7 5 . The capacitor 7 2 is electrically connected in shunt with the inductor 7 1 . The variable capacitance diode 7 7 has a cathode electrically connected to one end of the inductor 7 1 and an anode electrically connected to the other end of the inductor 7 1 through the capacitor 7 6 . Further, the anode of the variable capacitance diode 7 7 is electrically connected to the band switching terminal BS4 of the IC 9 through the resistor 7 8 and electrically connected to the ground through the resistor 7 9 . The IC 9 is TSA5520 produced by Philips Corp. as an example. The IC 9 is provided with a band decoder in addition to a PLL circuit and has the tuning voltage terminal TU and the band switching terminals BS2, BS3 and BS4. Of these, the respective terminals excluding the band switching terminal BS4 are electrically connected to the high-frequency double tuned circuit 4 and the local oscillator 6, and the band switching terminal BS4 is electrically connected to the high-frequency double tuned circuit 4 and the intermediate frequency tuning circuit 7. Incidentally, the tuning voltage terminal TU outputs a tuning voltage therethrough, the band switching terminal BS2 outputs a positive voltage therethrough when a TV broadcast lying in a VHF high-band is selected, the band switching terminal BS3 outputs a positive voltage therethrough when a TV broadcast lying in a VHF low-band is selected, and the band switching terminal BS4 outputs a positive voltage therethrough when a TV broadcast of the SECAM (L) system lying in the VHF low-band is selected. The television tuner of the first embodiment according to the above-described construction operates as follows: A received signal applied to the input terminal 1 is inputted to the antenna tuning circuit 2 where a received signal corresponding to a desired channel is selected and thereafter amplified by the high-frequency amplifier circuit 3. The amplified received signal is inputted to the high-frequency double tuned circuit 4 where a received signal corresponding to a desired channel is further selected. The mixer 5 mixes the selected received signal and a local oscillation signal supplied from the local oscillator 6 so as to be converted into an intermediate frequency signal. The intermediate frequency signal is inputted to the intermediate frequency tuning circuit 7 where an intermediate frequency signal having a predetermined frequency is selected and outputted from the output terminal 10. Thus, when a TV broadcast lying in a VHF low-band is selected, a positive voltage is outputted from the band switching terminal BS3 of the IC 9. Thus, the high-frequency double tuned circuit 4 takes such a configuration that in response to the positive voltage, the switching diodes 4 13 and 4 14 are respectively turned off and hence the inductors 4 5 through 4 7 and the capacitor 4 10 are inserted into and connected to the variable capacitance diodes 4 1 and 4 2 , the inductors 4 3 and 4 4 , and the capacitors 4 8 and 4 9 . On the other hand, when a TV broadcast lying in a VHF high-band is selected, a positive voltage is outputted from the band switching terminal BS2 of the IC 9. Thus, the high-frequency double tuned circuit 4 takes such a configuration that in response to the positive voltage, the switching diodes 4 13 and 4 14 are respectively turned on so that a circuit portion comprised of the inductors 4 5 through 4 7 and the capacitor 4 10 is short-circuited and hence only the variable capacitance diodes 4 1 and 4 2 , the inductors 4 3 and 4 4 and the capacitors 4 8 and 4 9 are electrically connected to one another. Even when the TV broadcast lying in any band is selected, a tuning voltage is outputted from the tuning voltage terminal TU of the IC 9 so that the high-frequency double tuned circuit 4 is activated as follows. That is, since the tuning voltage is applied to the variable capacitance diode 4 1 through the resistor 4 15 and the inductor 4 3 and further applied to the variable capacitance diode 4 2 through the resistor 4 16 and the inductor 4 4 , the respective capacitance of the variable capacitance diodes 4 1 and 4 2 are adjusted so as to be tuned to the selected TV broadcast. In a manner similar to the high-frequency double tuned circuit 4, the local oscillator 6 is supplied with the positive voltage outputted from the band switching terminal BS3 or BS2 of the IC 9 and the tuning voltage outputted from the tuning voltage terminal TU to thereby output a local oscillation signal having a frequency higher than that of the selected TV broadcast by a predetermined difference. Further, in the television tuner according to the first embodiment, the respective tuning frequencies of the high-frequency double tuned circuit 4 and the intermediate frequency tuning circuit 7 are set so as to be adapted to a TV broadcast of the PAL (B/G) system. When a TV broadcast of the SECAM (L) system lying in the VHF low-band is selected, the television tuner shifts the respective tuning frequencies to low frequencies. That is, when the television tuner according to the first embodiment selects the TV broadcast of the SECAM (L) system lying in the VHF low-band, the positive voltage is outputted from the band switching terminal BS4 of the IC 9 and applied to the high-frequency double tuned circuit 4 and the intermediate frequency tuning circuit 7. The positive voltage is divided into fractions by the resistors 4 17 , 4 18 and 4 20 , 4 21 in the high-frequency double tuned circuit 4, which in turn are applied to the anodes of the variable capacitance diodes 4 1 and 4 2 as added bias voltages respectively. At this time, the tuning voltage outputted from the tuning voltage terminal TU of the IC 9 is applied to the cathodes of the variable capacitance diodes 4 1 and 4 2 . Thus, the voltages corresponding to the differences between the tuning voltage and the added bias voltages are respectively applied between the anodes and cathodes of the variable capacitance diodes 4 1 and 4 2 . Further, the respective capacitance of the variable capacitance diodes 4 1 and 4 2 become greater than those thereof at the time that only the tuning voltage is applied to the cathodes thereof. Further, the positive voltage is adjusted so as to make a fractional voltage by the resistors 7 8 and 7 9 in the intermediate frequency tuning circuit 7, which in turn is applied to the anode of the variable capacitance diode 7 7 as an added bias voltage. At this time, a power supply voltage is applied to the cathode of the variable capacitance diode 7 7 from the power supply terminal B through the inductor 7 1 as a fixed bias voltage. Thus, the voltage corresponding to the difference between the fixed bias voltage and the added bias voltage is applied between the anode and cathode of the variable capacitance diode 7 7 , and the capacitance of the variable capacitance diode 7 7 becomes greater than that thereof at the time that only the fixed bias voltage is applied to the cathode thereof. Accordingly, the respective tuning frequencies of the high-frequency double tuned circuit 4 and the intermediate frequency tuning circuit 7 are shifted to low frequencies as compared with when the added bias voltage is not applied to the cathode thereof. On the other hand, when a TV broadcast of the SECAM (L) system and a TV broadcast of the PAL (B/G) system lying in a band other than the VHF low-band are selected, no positive voltage is outputted from the band switching terminal BS4 of the IC 9. Thus, only the tuning voltage is applied to the cathodes of the variable capacitance diodes 4 1 and 4 2 and the respective capacitances of the variable capacitance diodes 4 1 and 4 2 become smaller than those thereof at the time that the added bias voltages are applied to the cathodes thereof. Further, only the power supply voltage is applied to the cathode of the variable capacitance diode 7 7 and the capacitance of the variable capacitance diode 7 7 becomes smaller than that thereof at the time that the added bias voltage is applied to the cathode thereof. Thus, the respective tuning frequencies of the high-frequency double tuned circuit 4 and the intermediate frequency tuning circuit 7 are shifted to high frequencies as compared with the application of the added bias voltage to the cathode thereof. Next, Fig. 2 is a block diagram partly including a circuit diagram, showing a configuration of a second embodiment of a television tuner according to the present invention. The television tuner according to the second embodiment is provided with an intermediate frequency tuning circuit 8 as an alternative to the intermediate frequency tuning circuit 7 employed in the television tuner according to the first embodiment shown in Fig. 1. Since the second embodiment is identical to the first embodiment in configurations and operations other than the intermediate frequency tuning circuit 8, the same elements of structure as those in Fig. 1 are identified by the same reference numerals in Fig. 2. Only the configuration and operation of the intermediate frequency tuning circuit 8 will be explained below. Other configurations and operations will not be described. The intermediate frequency tuning circuit 8 has an input terminal electrically connected to an output terminal of a mixer 5 and an output terminal electrically connected to an output terminal 10. The intermediate frequency tuning circuit 8 comprises an inductor 8 1 , capacitors 8 2 through 8 6 , a switching diode 8 7 , and resistors 8 8 through 8 10 . The inductor 8 1 has one end electrically connected to the input terminal of the intermediate frequency tuning circuit 8 through the capacitor 8 3 and electrically connected to the output terminal thereof through the capacitor 8 4 . Further, the inductor 8 1 has the other end electrically connected to a power supply terminal B through the resistor 8 9 and electrically connected to the ground through the parallel-connected resistor 8 10 and capacitor 8 5 . The capacitor 8 2 is electrically connected in shunt with the inductor 8 1 . The capacitor 8 6 has one end electrically connected to the anode of the switching diode 8 7 and the other end electrically connected to the other end of the inductor 8 1 . Further, the switching diode 8 7 has a cathode electrically connected to one end of the inductor 8 1 and an anode electrically connected to a band switching terminal BS4 of an IC 9 through the resistor 8 8 . When a TV broadcast of the SECAM (L) system lying in a VHF low-band is now selected, a positive voltage is outputted from the band switching terminal BS4 of the IC 9 and applied to the anode of the switching diode 8 7 through the resistor 8 8 of the intermediate frequency tuning circuit 8 as a switching voltage. At this time, a power supply voltage applied to the power supply terminal B is adjusted so as to make a fractional voltage by the resistors 8 9 and 8 10 , which in turn is applied to the cathode of the switching diode 8 7 through the inductor 8 1 as a fixed bias voltage. Since, however, the switching voltage is higher than the fixed bias voltage, the switching diode 8 7 is brought to an on state. Further, the capacitor 8 6 is electrically parallel-connected to the capacitor 8 2 and the tuning capacitance of the intermediate frequency tuning circuit 8 becomes greater than that thereof at the time that the switching voltage is not applied to the cathode referred to above. Thus, the tuning frequency of the intermediate frequency tuning circuit 8 is shifted to a low frequency as compared with the non-application of the switching voltage thereto. On the other hand, when a TV broadcast of the SECAM (L) system and a TV broadcast of the PAL (B/G) system lying in a band other than the VHF low-band are selected, no positive voltage is outputted from the band switching terminal BS4 of the IC 9, so that the switching diode 8 7 is brought to an off state, whereby the tuning capacitance of the intermediate frequency tuning circuit 8 becomes smaller than that thereof at the application of the switching voltage. Thus, the tuning frequency of the intermediate frequency tuning circuit 8 is shifted to a high frequency as compared with the application of the switching voltage. Fig. 6 shows tuning characteristics of the high-frequency double tuned circuit 4 in the television tuner according to each embodiment at the time that the TV broadcast lying in the VHF low-band is selected. In Fig. 6, a solid line indicates a characteristic obtained when the TV broadcast of the PAL (B/G) system is selected, whereas a broken line indicates a characteristic obtained when the TV broadcast of the SECAM (L) system is selected. Symbols P PAL and S PAL in the drawing indicate a video or picture carrier and a voice or speech carrier of the TV broadcast of the PAL (B/G) system respectively, whereas symbols P SEC and S SEC indicate a picture carrier and a speech carrier of the TV broadcast of the SECAM (L) system respectively. However, numeric values in the drawing are those represented by replacing the frequencies of the respective carriers with intermediate frequencies for convenience. The relationship in magnitude between the frequencies of the respective carriers is contrary to an arrangement of the numeric values in the drawing, i.e., they are small on the left side in the drawing, whereas they are large on the right side in the drawing. As shown in Fig. 6, the tuning characteristic of the high-frequency double tuned circuit 4 remains unchanged upon selection of the TV broadcast of the PAL (B/G) system as compared with the tuning characteristic of the high-frequency double tuned circuit 64 employed in the already-known television tuner shown in Fig. 4. When the TV broadcast of the SECAM (L) system is selected, the tuning characteristic thereof is shifted to the low frequency. Even when the TV broadcast of either system is selected, response ranges substantially coincide with each other and the picture carrier and speech carrier are substantially equal in response level to each other. Fig. 7 shows tuning characteristics of the intermediate frequency tuning circuits 7 and 8 in the television tuner according to each embodiment at the time that the TV broadcast lying in the VHF low-band is selected. In Fig. 7, a solid line indicates a characteristic obtained when a TV broadcast of the PAL (B/G) system is selected, whereas a broken line indicates a characteristic obtained when a TV broadcast of the SECAM (L) system is selected. Symbols PI PAL and CI PAL in the drawing indicate a video or picture intermediate frequency and a chrominance intermediate frequency of the TV broadcast of the PAL (B/G) system, respectively, whereas symbols PI SEC and CI SEC indicate a video or picture intermediate frequency and a chrominance intermediate frequency of the TV broadcast of the SECAM (L) system, respectively. As shown in Fig. 7, the tuning characteristics of the intermediate frequency tuning circuits 7 and 8 remain unchanged upon selection of the TV broadcast of the PAL (B/G) system as compared with the tuning characteristic of the intermediate frequency tuning circuit 67 employed in the already-known television tuner shown in Fig. 5. When the TV broadcast of the SECAM (L) system is selected, the tuning characteristics thereof are shifted to the low frequency. Even when the TV broadcast of either system is selected, the tuning frequencies are located substantially in the centers of the picture intermediate frequencies and chrominance intermediate frequencies respectively. While the respective embodiments have been described by the example in which the added bias voltages are applied to the two variable capacitance diodes 4 1 and 42 of the high-frequency double tuned circuit 4, it is unnecessary to apply the added bias voltages employed in the present invention to both the variable capacitance diodes. When the less shift of the tuning frequency of the high-frequency double tuned circuit 4 is allowed, the added bias voltage may simply be applied to one variable capacitance diode. While the respective embodiments have been described by the example in which the tuning voltage is applied to the cathodes of the respective variable capacitance diodes 4 1 and 4 2 and the added bias voltages are applied to the anodes thereof, the form of the supply of the tuning voltage and the added bias voltages employed in the present invention is not necessarily limited to this example. Both the tuning voltage and the added bias voltage are superimposed on each other and the superimposed voltage may be applied to the above-described cathodes. While the positive voltage obtained from the band switching terminal BS4 of the IC 9 is used as the added bias voltage or the switching voltage in each embodiment, the added bias voltage or switching voltage employed in the present invention is not limited to this positive voltage. Another voltage may be used if the voltage obtained only when the TV broadcast of the SECAM (L) system lying in the VHF low-band is selected, is taken. According to the television tuner of each embodiment, as has been described above, the tuning frequencies of the high-frequency double tuned circuit and the intermediate frequency tuning circuit are set so as to be adapted to the TV broadcast of the PAL (B/G) system. When the TV broadcast of the SECAM (L) system lying in the VHF low-band is selected, the respective tuning capacitance of the high-frequency double tuned circuit and the intermediate frequency tuning circuit are made great. That is, when the TV broadcast of the SECAM (L) system lying in the VHF low-band is received, the tuning frequencies of the high-frequency double tuned circuit and the intermediate frequency tuning circuit, which have been set so as to be adopted to the TV broadcast of the PAL (B/G) system, are shifted to the low frequency and thereby adapted to the TV broadcast of the SECAM (L) system lying in the VHF low-band. Thus, according to the television tuner of the present invention, an effect can be obtained in that even if the TV broadcast of either system is selected, both selectivity to the picture carrier and speech carrier of the high-frequency double tuned circuit is substantially equal to each other, and both selectivity to the picture intermediate frequency and chrominance intermediate frequency of the intermediate frequency tuning circuit is substantially equal to each other, whereby the picture and voices can be reproduced in a satisfactory state. While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention which is, however, still defined by the appended claims.

Claims: A television tuner, comprising: a high-frequency double tuned circuit (4); an intermediate frequency tuning circuit (7,8); and means capable of receiving respective television broadcasts of PAL-B/G and SECAM-L-systems, said means setting respective tuning frequencies of said high-frequency double tuned circuit and said intermediate frequency tuning circuit so as to be adapted to the television broadcast of the PAL-B/G-system characterized in that said means is further adapted for lowering the respective tuning frequencies of said high-frequency double tuned circuit and said intermediate frequency tuning circuit when the television broadcast of the SECAM-L-system lying in a VHF low-band is received. The television tuner according to claim 1, wherein said high-frequency double tuned circuit and said intermediate frequency tuning circuit (7) are respectively provided with variable capacitance diodes (4 1 ,4 2 ,7 7 ), and when the television broadcast of the SECAM-L-system lying in the VHF low-band is received, an added bias voltage for increasing the capacitance of the variable capacitance diodes (4 1 ,4 2 ) of said high-frequency double tuned circuit as compared with the application of only a tuning voltage is applied to the variable capacitance diodes thereof together with the tuning voltage and an added bias voltage for increasing the capacitance of the variable capacitance diode (7 7 ) of said intermediate frequency tuning circuit as compared with the application of only a fixed bias voltage is applied to the variable capacitance diode thereof together with the fixed bias voltage, and when another television broadcast is received, only said tuning voltage is applied to the variable capacitance diodes of said high-frequency double tuned circuit and only said fixed bias voltage is applied to the variable capacitance diode of said intermediate frequency tuning circuit. The television tuner according to claim 2, wherein said added bias voltages are fractional voltages obtained by dividing a band switching voltage outputted from a channel selecting IC (9) provided with a band decoder by resistance circuits (4',4") and said fixed bias voltage is a voltage higher than said each added bias voltage, and the tuning voltage is applied to the cathodes of the variable capacitance diodes of said high-frequency double tuned circuit and said added bias voltage is applied to the anodes thereof, whereas the fixed bias voltage is applied to the cathode of the variable capacitance diode of said intermediate frequency tuning circuit and the added bias voltage is applied to the anode thereof. The television tuner according to claim 1, wherein said high-frequency double tuned circuit is provided with variable capacitance diodes (4 1 , 4 2 ) and said intermediate frequency tuning circuit (8) is provided with a fixed capacitor (8 2 ) and an added capacitor (8 6 ) series-connected to a switching diode (8 7 ), and when a television broadcast of the SECAM-L-system lying in a VHF low-band is received, an added bias voltage for increasing the capacitance of the variable capacitance diodes of said high-frequency double tuned circuit as compared with the application of only a tuning voltage is applied to the variable capacitance diodes thereof together with the tuning voltage, and a switching voltage for bringing the switching diode to an on state is applied to said switching diode so as to turn on said switching diode, thereby connecting said added capacitor in parallel with said fixed capacitor, and when another television broadcast is received, only the tuning voltage is applied to said variable capacitance diodes and a switching voltage for bringing the switching diode to an off state is applied to said switching diode so as to turn off said switching diode, thereby disconnecting said added capacitor from said fixed capacitor. The television tuner according to claim 4, wherein said added bias voltage is a fractional voltage obtained by dividing a band switching voltage outputted from a channel selecting IC (9) provided with a band decoder by a resistance circuit and said switching voltage is said band switching voltage, and said tuning voltage is applied to the cathodes of said variable capacitance diodes and said added bias voltage is applied to the anodes thereof, whereas a fixed bias voltage lower than said band switching voltage is applied to the cathode of said switching diode and said band switching voltage is applied to the anode thereof.

Inventors: SASAKI MICHINORI, YAMAMOTO MASAKI, SASAKI, MICHINORI, YAMAMOTO, MASAKI
Applicants: ALPS ELECTRIC CO LTD, ALPS ELECTRIC CO., LTD.
Application year: 1999
Main classification: H03J5
IPC codes: H04N5, H03J5