INPUT CIRCUIT PfíRfl A TELEVISION TUNER
BACKGROUND OF THE INVENTION
1- FIELD OF THE INVENTION
The present invention relates to an input circuit for a television tuner, including? N
T ime image circuit for eliminating image frequencies that act as a disturbing wave or as a resonant object.
2. DESCRIPTION OF Lfl TECNTCO REÍ OCTONflDA
In general, according to the tuners or television, etc., by mixing a desired reception frequency with a local oscillation frequency which is higher with an intermediate frequency, the intermediate frequency can be obtained. When a reception frequency (image frequency) that is greater than the local oscillation frequency by the intermediate frequency is input, an image disturbance occurs. Figure 3 shows a basic input circuit 1 for a conventional television tuner. Among the sera received through an antenna 2, a VHF UO band at 470 MHz) is extracted by a VHF band selection filter (IF filter) 3, and is applied to a high frequency amplifier circuit 4 through a capacitor of acomplarment Cl and the input circuit 1. In the input circuit 1, the ends of the inductors Ll and L2 are connected to the coupling capacitor Cl, and the other end of the inductor I 1 is connected to earth through a capacitor C2. The other end of the inductor L2 is connected to earth through a varactor diode VD1, and is connected to a high frequency amplifier circuit 4 through a varactor diode VÜ2. The inductor I 2 connected in series and the varactor diode VO2 are connected in parallel with a capacitor C3. The ratio of minimum frequency to maximum frequency of the VHF band (40 to 470 MHz) is large. Therefore, according to the fact whether a desired resonant frequency is in a high channel band or a low channel band, controlling the inductances of the inductors Ll and L2 to be small when a high frequency is received and for When it is large when a frequency ba is received, the tuning is done in all the bands. Also, by controlling the capacitances of varactor diodes Vül and VD2 to be small when a high frequency is received and to be large when a low frequency is received, the pedanci equalization is performed at a desired frequency. As shown in Figure 4, by connecting the capacitor C3 in parallel to the series circuit of the driver 1.2 and the varactor diode VD2, an image trap circuit is formed which traps image frequencies which act as disturbing waves at a resonant frequency. . According to the conventional input circuit 1, the capacitor C13 is connected in parallel to the series circuit of the inductor L2 and the varactor diode VD2 to form the image trap circuit. However, this arrangement makes it extremely difficult to control the capacitance of capacitor C3 and as a result, disadvantageous image disturbance occurs. In other words, 1) by increasing the capacitance of the C-capacitor, the variable scale of the image trap frequency can be extended but the variable scale can not be extended to a high frequency. Since the inductors Ll and L2 are tuned to have small inductances when a high frequency is received, the capacitor C3 is connected in parallel to the inductor L2 which has small inductance, and the target image trap frequency is greater than the Reception resonant frequency twice the intermediate frequency. Therefore, under these conditions, in order to tune to the target image trap frequency, the capacitance of capacitor C3 must be reduced. 2) However, reducing the capacitance of capacitor C3 narrows the variable scale of the image trap frequency, which makes it difficult to tune the image trap frequency to the target image frequency.
BRIEF DESCRIPTION OF THE INVENTION
Accordingly, it is an object of the present invention to provide an input circuit for a television tuner, in which a variable scale of an image trap frequency can be extended and the image trap frequency can be easily tuned to. the target image frequency. According to the present invention, the above object is achieved by providing an input circuit for a television tuner, in which a series circuit q? E includes an image trap capacitor and a second varactor diode is connected in parallel to a series circuit that includes a tuning inductor and a varactor diode, and a common voltage for tuning is applied to the first and second varactor diodes so their capacitance changes, so the resonant frequency and frequency of image trap changes. According to the input circuit for the television tuner, even though the capacitance of the image trap capacitor is relatively increased, the combined capacitance of the capacitor and the varactor diode can be reduced when a high frequency is received. As a result, the variable scale of the image trap frequency can be extended, and the image trap frequency can be easily tuned to the target image frequency. As described above, in accordance with the present invention, by additionally connecting the varactor diode to the image trap capacitor, the capacitance of the varactor diode can be changed according to the reception frequency. Therefore, even though the capacitance of the capacitor is relatively increased, the combined capacitance of the capacitor and the varactor diode can be reduced when a high frequency is received, so that the variable scale of the image trap frequency can be extended, and the image trap frequency can be tuned to the target image frequency.
SHORT DESCRIPTION PE QS DRAWINGS
Figure 1 is a circuit diagram showing an input circuit for a television tuner, according to one embodiment of the present invention. Fig. 2 is a circuit diagram showing an equivalent image trap circuit included in the input circuit of Fig. 1. Fig. 3 is a circuit diagram showing a basic pattern of a conventional input circuit for a tuner of TV. Figure 4 is a circuit diagram showing the image trap circuit shown in Figure 3.
DESCRIPTION OF THE PREFERRED ROLLOVER
Referring to the accompanying drawings, an embodiment of the present invention is described below. Figure 1 shows a circuit diagram of an input circuit for a television tuner, according to an embodiment of the present invention. Figure 2 is a circuit diagram showing an image trap circuit included in the input circuit shown in Figure 1. Components identical to those of the conventional example are denoted by the same reference numerals. As shown in FIG. 1, between the signals received through an antenna 2, a VHF band (40 to 470 MHz) is extracted by a selection filter 3 of the VHF band, and is applied to a high frequency amplifier circuit. 4 through a 1P input circuit and a capacitor < 5. The input terminal 23 of the input circuit IA is connected to the output terminal 24 with the inductors 6, 7, 8, a capacitor 9, an inductor 10 and a varactor diode 23 provided therebetween, and connected to a. input terminal 24 with an image trap capacitor 11 and? n varactor diode 29 provided therebetween. The inductors 6 and 7 are connected at a point that is also connected to the ends of the resistors 20, 22 and a capacitor 21 through a diode 13 and an inductor 16. Fl another end of the resistor 22 is connected to a terminal ? 7 selection of low band VHF. The other ends of resistor 20 and capacitor 21 are connected to ground. The inductors 7 and 8 are connected to a point that is also connected to an extirm of an inductor-12 and the other end of the inductor 12 is connected to ground through a capacitor 18 and is also connected to a selection terminal 25 of high VHF band through a resistor 31. Also, the point at which the inductor 8 and the capacitor 9 are connected is also connected to the point between the diode 13 and the inductor IB through the diode 14. A reverse voltage supplied through a tuning voltage supply terminal 25 is applies to the varactor diodes 15 and 28 through a resistor 19 and is applied to the varactor diode 29 through resistor 30. At this time, a voltage is selectively applied to the low VHF band selection terminal 27 and to the VHF band selection terminal high 26. In other words, when a VHF band signal is received ba a, a voltage is selectively applied to the VHF band selection terminal ba to 27 so that diodes 13 and 14 are turned off . When a high VHF band signal is received, a voltage is selectively applied to the high VHF band selection terminal 26, so that the diodes 13 and 14 are turned on. Figure 2 is an equivalent circuit diagram showing an image ramp circuit that is extracted <; 1 the input circuit 10 shown in Figure 1. The capacitor 9 (shown in Figure 1), which has a large capacitance and is used to avoid direct current flow, is shorted at high frequencies and therefore both are shown in Figure 2. Further, a combined variable capacitor VD represents a serial combination of the varactor diode 29 and the capacitor 11, and the combined inductor L represents an inductance of the image trap circuit that is changed by the diodes 13 and 14. When the diodes 13 and 14 are turned off, the combined inductor L comprises the inductors 6, 7, 8 and 10 which are connected in series. When the diodes 13 and 14 are turned on, the combined inductor L comprises the inductors 7 and 8, which are shorted, and the inductors 6 and 10, which are connected in series. In other words, the inductance of the image trap circuit is changed to a large value when a VHF band signal ba a is received, and is changed to a small value when a high V HF band signal is received. In addition, the capacitance of the combined variable capacitor VD is changed by changing an applied reverse voltage to a tuning voltage supply terminal 25, along with the capacitances of the variable capacitance diodes 15 and 28. The capacitance of the combined variable capacitor Vül is changed to a small value when a high channel signal is received, and it is changed to a large value when a channel signal ba a is received. Consequently, when a VHF band signal is received, the inductor of the combined inductor L can be switched between two states together with the received band. Also, the capacitance of the combined variable capacitor VDl can be changed together with the selection of a channel of the received band. In this way, the image trap sequence is changed according to a change of reception channels. Therefore, the image trap frequency can be changed simultaneously in the scale between the minimum frequency and the maximum frequency according to the resonant frequency. Therefore, the variable scale of the image trap sequence can be extended and the image trap frequency can be easily tuned to the target image frequency.