JPH0543574Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Technical Field> The present invention relates to an AGC circuit for a television receiver, particularly a small-sized television receiver.

<Prior art> Television receivers, especially small television receivers such as LCD televisions, are equipped with an internal antenna (rod antenna) for outdoor use, and this short rod antenna can provide sufficient image reception sensitivity. In order to achieve this, the sensitivity of the tuner has been increased compared to ordinary television receivers. Many of these small television receivers are equipped with an external antenna terminal as well as a rod antenna, and when switching from a rod antenna to an external antenna, if a strong electric field is input from the external antenna, the tuner may be damaged as described above. Because the sensitivity of
It is difficult to obtain stable images because interference and cross modulation are likely to occur.

<Purpose of the invention> The invention has been made in view of the above points, and regardless of whether the antenna is an internal antenna or an external antenna,
The purpose is to always obtain stable images.

<Structure of the invention> In order to achieve the above-mentioned object, the invention provides a television receiver that is equipped with an internal antenna and an external antenna and receives television waves by switching between the two.
In the AGC circuit, RF and
An operation start level variable means for varying the AGC operation start level of the AGC circuit is provided, and the AGC operation is started earlier when receiving by an external antenna than when receiving by an internal antenna.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the drawings. The drawing is a circuit diagram of an embodiment of the present invention. In the figure, 1 is an internal antenna (rod antenna), 2 is a plug for connecting an external antenna,
3 is a connection jack, 4 is a tuner, and this tuner 4 has an antenna terminal 4a and an AGC terminal 4.
b, a power supply terminal 4c, and an intermediate frequency signal output terminal 4d.

5 is an IC that constitutes a video intermediate frequency amplification circuit and an AGC circuit, and this IC5 is an intermediate frequency (IF)
It has an amplifier circuit 6, a video detection circuit 7, and a video amplification circuit 8, as well as an AGC circuit 12 consisting of an AGC detection circuit 9, an AGC amplifier circuit 10, and an RF/AGC circuit 11. AGC of AGC amplifier circuit 10
The gain of the intermediate frequency amplification circuit 6 and
RF/AGC voltage is controlled. This IC 5 has an input terminal 5a to which the intermediate frequency signal from the tuner 4 is input, an output terminal 5b to output the video signal from the video amplification circuit 8 to the next stage video amplification circuit, and an RF/
It includes an input terminal 5c to which the operation start voltage of the AGC circuit 11 is applied, and an output terminal 5d of the AGC voltage to the tuner 4. The operation start voltage of the RF/AGC circuit 11 can be adjusted by a variable resistor VR.
The AGC operation starts earlier as the operation start voltage from the input terminal 5c becomes higher.

The above configuration is basically the same as the conventional example.

The AGC circuit of this embodiment is provided with an operation start level variable means 13 that varies the operation start level of the RF/AGC circuit 11 in conjunction with insertion/removal of the external antenna connection plug 2 into the connection jack 3. There is. This operation start level variable means 13 basically includes a switch 14 that is linked to the insertion and removal of the connection plug 2, and first and second transistors Q1 and Q2.
and resistors R1, R2, R3, and R4.

In the state where the connection plug 2 of the external antenna is not inserted into the connection jack 3, that is, in the illustrated state, the movable contact 14a of the switch 14 is located on the side of the grounded first individual contact 14b, and the connection plug 2 is inserted into the connection jack, the power
It moves to the second individual contact 14c side connected to Vcc. A common contact 14d of this switch 14 is connected to the base of the first transistor Q1 via a resistor R1.

The collector of this first transistor Q1 is connected to the base of the second transistor Q2 via a resistor R3. The emitter and collector of the second transistor Q2 are connected to both ends of a resistor R6 that defines the operation start voltage of the RF/AGC circuit 11.

Next, the operation of the AGC circuit having the above configuration will be explained. First, in the illustrated state, that is, during reception by the internal antenna 1, both the first and second transistors Q1 and Q2 are off, and the RF
The operation start voltage of the AGC circuit 11 is determined by the resistors R5 and R
6, R7 and variable resistor VR.

Next, when the connection plug 2 of the external antenna is inserted into the connection jack 3 and the reception is switched to the external antenna, the movable contact 14a of the switch 14 moves to the second individual contact 14c side. Therefore, the first and second transistors Q1 and Q2
is turned on, the resistor R6 is shot, and the operation start voltage applied to the RF/AGC circuit 11 becomes higher, so that the AGC operation starts earlier than when receiving by the internal antenna 1. Therefore, when switching from internal antenna 1 to external antenna, interference,
It becomes possible to control the occurrence of cross modulation.

In addition, in the figure, resistors R8 and R9 are AGC
This resistor is used to divide the power supply voltage Vcc to obtain the AGC voltage of the tuner 4 before starting operation.

<Effects of the invention> As described above, according to the invention, the AGC operation start level of the RF/AGC circuit is varied in conjunction with switching between the internal antenna and the external antenna, and when reception is performed using the external antenna, reception is performed using the internal antenna. Since the start of the AGC operation is made earlier than when the internal antenna is switched from the internal antenna to the external antenna, it is possible to suppress the occurrence of interference and cross modulation and obtain a stable image.

[Brief explanation of the drawing]

The drawing is a circuit diagram of an embodiment of the present invention. 1... Internal antenna, 2... External antenna connection plug, 3... Connection jack, 11...
RF/AGC circuit, 13...Operation start level variable means.

Claims (1)

[Scope of claim for utility model registration] In the AGC circuit of a television receiver that is equipped with an internal antenna and an external antenna and receives TV radio waves by switching between the two, the AGC operation start level of the RF/AGC circuit in conjunction with the switching. The television receiver is provided with an operation start level variable means for varying the AGC operation, and starts the AGC operation earlier when receiving by an external antenna than when receiving by an internal antenna.
AGC circuit.