JPH04119714A - Television receiver - Google Patents

Abstract

PURPOSE:To obtain an excellent reception state by utilizing an automatic amplifier gain control voltage (AGC voltage) and detecting a change in the AGC voltage generated with respect to a change in the strength of an signal to a tuner so as to control the operation error of an automatic frequency adjustment(AFT) circuit. CONSTITUTION:A voltage generated at a terminal TP15 is detected to turn on/off a switching transistor(TR) Q1. A diode D1 and a Zener diode D2 decide the operating voltage of the TR Q1, and when a voltage higher than the sum of a VBE of the Zener voltage of the diode D2 and the on-voltage of the diode D1 are applied to the anode of the diode D1, the TR Q1 is conductive. When the TR Q1 is conductive, currents flowing to resistors are shown in i1, i2 in figure. An output voltage at a terminal AFT is clipped by a voltage dropped across a resistor R2 and the diode D3 with respect to the current i1, then a operating error at the terminal AFT at a weak electric field is avoided.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention detects the strength of an input signal received by a receiving antenna, and detects an operation error of an automatic frequency adjustment circuit (hereinafter referred to as AFT) when the input signal is weak. Regarding a television receiver that has been designed to suppress the noise.

2. Description of the Related Art An AFT circuit used in a conventional television receiver is a circuit that maintains the oscillation frequency of a local oscillation circuit at a normal frequency, and is controlled by a control voltage generated within the receiver.

Problems to be Solved by the Invention However, especially when receiving in a weak electric field, the above-mentioned control voltage decreases, so sufficient control is not possible, and the local oscillation frequency does not transmit at the correct frequency, making it impossible to receive images in a weak electric field. There was a drawback.

Means for Solving the Problems In order to improve these conventional drawbacks, the television receiver of the present invention utilizes the tuner's automatic amplification gain control voltage (hereinafter referred to as AGC voltage) to control the input to the tuner. This is intended to control operational errors in the AFT circuit by detecting changes in the AGC voltage that occur in response to changes in signal strength.

Function: In the present invention, the newly provided clip circuit operates only when the input signal to the antenna is weak and the local oscillation frequency deviates from the normal value, and when the input signal is strong enough and good, the clip circuit is activated. The circuit did not operate, ensuring good reception.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 2 is a characteristic diagram of an automatic gain control circuit (hereinafter referred to as an AGC circuit).

In Fig. 1, the AGC circuit outputs multiple voltages to terminal TP15 according to the input electric field strength received by the receiving antenna, and when the input signal from the receiving antenna is weak, the highest voltage value is It is applied to the high frequency amplifier circuit of the tuner, and the tuner operates to obtain the maximum gain.

Furthermore, as the input signal becomes stronger, the output voltage from the AGC circuit decreases, working to lower the gain of the tuner.

In this way, the gain of the high frequency amplification circuit of the tuner is controlled by the AGC output voltage, and the input signal to the intermediate frequency amplification circuit is kept constant. The relationship between the input electric field strength from the antenna and the output voltage of terminal TP1s is shown in FIG. AFT
A circuit designed to control operational errors. 1st
In the figure, the AGC voltage is from TPls to current limiting resistor R1.
After passing through the transistor operation determining diode D1 and the transistor operation determining Zener diode D2, it is applied to the pace side of the switching transistor Q1. On the other hand, the external DC power supply terminal V is divided by clip voltage determining resistors R2 and R3C and applied to the tuner's AFTi voltage terminal through a clip voltage determining diode D3. Here, diode D3 also serves as a backflow prevention diode. The operation will be explained below using FIG. The voltage generated by TP16 is detected and the switching transistor Q1 is turned on and off. Diode D1
and the Zener diode D is used to determine the operating voltage of the transistor Q1, and the transistor's vBE1
Zener voltage of Zener diode D2, diode D
When a voltage higher than the sum of the ON voltages of 1 and 1 is applied to the 7 nodes of the diode D1, the transistor Q1 operates to become conductive. Also, when transistor Q1 becomes conductive, the current flow is il.

It becomes 12. Here, the output voltage of the AFT terminal is the current i1
As a result of this flow, the voltage drops at R2 and D3 and is clipped by the fc voltage, so any further operational errors in the weak electric field at the AFT terminal can be prevented.

Effects of the Invention As is clear from the above embodiments, according to the present invention, when receiving a weak electric field, the AGC voltage is not changed beyond a certain value, and therefore the local oscillation frequency is not significantly deviated from the correct frequency. No more malfunctions.

[Brief explanation of drawings]

FIG. 1 is a schematic circuit diagram showing an embodiment of the present invention, and FIG. 2 is a characteristic diagram of an automatic gain control circuit. R2, R3... Clip voltage determining resistance, Ql
...Switch transistor, D2...
・Transistor operation determining Zener diode, D3...
...Clip voltage determining diode.

Claims (2)

[Claims] (1) A television receiver that has a clip circuit to suppress operational errors in the automatic frequency adjustment circuit when receiving a weak electric field. (2) A television receiver equipped with a circuit that monitors the strength of the received electric field and switches other circuits.