GB2312345A

GB2312345A - Tuner and image trap circuit for TV with common frequency control

Info

Publication number: GB2312345A
Application number: GB9707390A
Authority: GB
Inventors: Tsutomu Kunishima
Original assignee: Alps Electric Co Ltd
Current assignee: Alps Alpine Co Ltd
Priority date: 1996-04-17
Filing date: 1997-04-11
Publication date: 1997-10-22
Also published as: DE19715956A1; KR970073038A; GB9707390D0; JPH09284157A; DE19715956C2; MX9702782A; CN1164795A; KR100243838B1

Description

2312345 INPUT CIRCUIT FOR A TELEVISION TUNER The present invention relates

to an input circuit for a television tuner, including an image-trap circuit for eliminating image frequencies which act as disturbance waves to a target resonant frequency.

In general, according to television tuners and so forth, by mixing a desired reception frequency with a local oscillating frequency which is higher by an intermediate frequency, the intermediate frequency can be obtained. When a reception frequency (image frequency) which is higher than the local oscillating frequency by an intermediate frequency is inputted, image disturbance occurs.

Fig. 3 shows a basic input circuit 1 for a conventional television tuner. Among signals received through an antenna 2, a VHF band (40 to 470 MHz) is extracted by a VHF-band selection filter (IF filter) 3, and is applied to a high frequency amplifier circuit 4 via a coupling capacitor Cl and the input circuit 1. In the input circuit 1, ends of variable inductors Ll and L2 are connected to the coupling capacitor Cl, and the other end of the inductor Ll is connected to ground via a capacitor C2. The other end of the inductor L2 is connected to ground via a varactor diode VD1, and is connected to the high frequency amplifier circuit 4 via a varactor diode VD2. Both ends of the connected variable inductor L2 and varactor diode VD2 are connected in parallel to a capacitor C3.

The ratio of minimum frequency to maximum frequency of the VHF band (40 to 470 MHz) is large. Thus, in accordance with whether a desired resonant frequency is in a high channel band or a low channel band, by controlling the inductances of the variable inductors Ll and L2 to be small when a high frequency is received and to be large when a low frequency is received, tuning is performed in all the bands.

Also, by controlling the capacitances of the varactor diodes VD1 and VD2 to be small when a high frequency is received and to be large when a low frequency is received, matching of impedance at a desired frequency is performed. As shown in Fig. 4, by connecting the capacitor C3 in parallel to the series circuit of the variable inductor L2 and the varactor diode VD2, an image-trap circuit which traps image frequencies acting as disturbance waves to a ddsired resonant frequency is formed.

However, according to the conventional input circuit 1, the capacitor C3 is connected in parallel to the series circuit of the inductor L2 and the varactor diode VD2 in order to form the image-trap circuit. However, this arrangement makes it extremely difficult to control the camacitance of the capacitor C3, and as a result, disadvantageous image disturbance occurs. In other words, 1) by increasing the capacitance of the capacitor C3, the variable range of the image-trap frequency can.be widened but the variable range cannot be extended to a high frequency. Because the variable inductors L1 and L2 are tuned to have small inductances when a high frequency is received, the capacitor C3 is connected in parallel to the variable inductor L2 having.small inductance, and the target image-trap frequency is higher than the reception resonant frequency by twice the intermediate frequency. Thus, under these conditions, in order to tune to the target image-trap frequency, the capacitance of the capacitor C3 must be reduced.

2) However, reducing the capacitance of the capacitor C3 narrows the variable range of the image-trap frequency, which makes it difficult to tune the image-tram frequency to the target image frequency.

Accordingly, it is an object of the present invention to provide an input circuit for a television tuner, in which the variable range of an imagetrap frequency can be extended and the image-trap frequency can readily be tuned to the target image frequency.

According to the present invention, the foregoing object is achieved through the provision of an input circuit for a television tuner, in which a series circuit including an image-trap capacitor and a second varactor diode is connected in parallel to a series circuit including a tuning inductor and a first varactor diode, and a common voltage for tuning is applied to the first and second varactor diodes so that their capacitances change, whereby the resonant frequency and the image-trap frequency change.

According to the input circuit for the television tuner, even when 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 range of the image-trap frequency can be extended, and the image-trap frequency can readily be tuned to the target image frequency.

As described above, according to the present invention, by additionally connecting the varactor diode to the imagetrap capacitor, the capacitance of the varactor diode can be changed in accordance with the reception frequency. Thus, even when 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 7487GB received, so that the variable range of the image-trap frequency can be extended, and the image-trap frequency can be tuned to the target image frequency.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a circuit diagram showing an input circuit for a television tuner, according to one embodiment of the present invention; Fig. 2 is an equivalent circuit diagram showing an image trap circuit extracted from the input circuit of Fig. 1; Fig. 3 is a circuit diagram showing a basic pattern of a conventional input circuit for a television tuner; and Fig. 4 is a circuit diagram showing the imagetrap circuit shown in Fig. 3.

By referring to the attached drawings, an embodiment of the present invention will be described below.

Fig. 1 shows a circuit diagram of an input circuit for a television tuner, according to one embodiment of the present invention. The components identical to those in the conventional example are denoted by the same reference numerals.

As shown in Fig. 1, among signals received through an antenna 2, a VHF band (40 to 470 MHz) is extracted by a VHF band selection filter 3, and is applied to a high frequency amplifier circuit 4 via an input circuit la and a coupling capacitor 5. The input terminal 23 of the input circuit la is connected to the output terminal 24 with inductors 6, 7, 8, a capacitor 9, an inductor 10, and a varactor diode 28 provided therebetween, and is connected to the output terminal 24 with an image- trap capacitor 11 and a varactor diode 29 provided therebetween.

The inductors 6 and 7 are connected at a point also is connected to ends of resistors 20, 22, and a capacitor 21 via a diode 13 and an inductor 16. The other end of the resistor 22 is connected to a VHF-low-channel-band selection-voltage-supply terminal 27. The other ends of the resistor 20 and the capacitor 21 are connected to ground. The inductors 7 and 8 are connected at a point also connected to one end of an inductor 12, the other end of the inductor- 12 is connected to ground via a capacitor 18 and is also connected to a VHF-high-channel-band selection-voltagesupply terminal 26 via a resistor 31.

In addition, 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 16 via the diode 14.

7487GB 7 A reverse voltage supplied via a tuning-voltage supply terminal 25 is applied to the varactor diodes 15 and 28 via a resistor 19, and is applied to the varactor diode 29 via a resistor 30.

At this time, a voltage is selectively applied to the VHF-low-band selection terminal 27 and the VHFhigh-band selection terminal 26. In other words, when a VHF-low-band signal is received, a voltage is selectively applied to the VHF-low-band selection terminal 27, so that the diodes 13 and 14 are switched off. When a VHF-highband signal is received, a voltage is selectively applied to the VHF-high-band selection terminal 26, so that the diodes 13 and 14 are switched on.

Fig. 2 is an equivalent circuit diagram showing an image-trap circuit which is only extracted from the input circuit la shown in Fig. 1. In Fig. 2, the capacitor 9 shown in Fig. 1 is short-circuited since it has a large capacitance and is used to prevent a direct current to flow. In addition, VD represents a combined variable capacitor formed by combining the varactor diode 29 and the capacitor 11 in series, and L is a combined inductor whose inductance can be changed by the diodes 13 and 14.

When the diodes 13 and 14 are switched 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 switched on, the combined inductor L comprises the inductors 7 and 8 which are short-circuited 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-low-band signal is received, and it is changed to a small value when a VHFhigh-band signal is received.

In addition, the capacitance of the combined variable capacitor VD is changed by changing a reverse 7487GB 8 voltage applied-to a tuning-voltage supply terminal 25, together with the capacitances of the variable capacitance diodes 15 and 28. The capacitance of the combined variable capacitor VD is changed to a small value when a high channel signal is received, and it is changed to a large value when a low channel signal is received.

Consequently, when a VHF band signal is received, the inductance of the combined inductor L can be changed to two conditions in conjunction with a switch of the bands. Also, the capacitance of the combined variable capacitor VD can be changed in conjunction with selection of the channels in the same band, thus, the image-trap frequency can be changed in accordance with a change of the reception channels.

Therefore, the image-trap frequency can simultaneously be changed in the range between the minimum frequency and the maximum frequency in accordance with the resonant frequency. Thus, the variable range of the image-trap frequency can be extended, and the image-trap frequency can readily be tuned to the target image frequency.

7487GB 9

Claims

1. An input circuit for a television tuner in which a series circuit including an image-trap capacitor and a second varactor diode is connected in parallel to a series circuit including a tuning inductor and a first varactor diode, and a common tuning voltage is applied to said first and second varactor diodes so that the capacitances thereof change, whereby the resonant frequency and the image-trap frequency change.

2. An input circuit for a television tuner substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

3. A television tuner, having an input circuit as claimed in Claim 1 or Claim 2.