JPS628577Y2 - - Google Patents

Description

[Detailed description of the invention] This invention is a television receiver that has an automatic search function that automatically searches for the broadcasting position and tunes to that position by instantly operating the search start button.
The present invention relates to an AGC circuit, and particularly to a device that controls the response speed of an AGC control signal.

Such tuning devices usually have an automatic fine tuning (AFT) function for accurate tuning.
The station searching operation is completely stopped only when the local oscillation frequency of the tuner is within a precise tuning range that provides the desired intermediate frequency signal. This type of tuning device compares the horizontal synchronization signal of the video signal obtained when the broadcast signal is detected and the flyback pulse, and when the two match,
A voltage synthesizer type tuning device that operates an AFT circuit is known. In such a tuning device, when the AFT circuit operates and the AFT voltage is extracted, this AFT voltage is compared with a predetermined reference level, and the tuning operation is stopped when the tuner reaches the desired tuning point. It's summery. On the other hand, depending on the installation location of a television receiver, the electric field strength of broadcast waves may vary greatly between adjacent channels. In such a case, if you increase the station searching speed by increasing the sweep frequency of the tuning device, the AGC circuit will not be able to follow and respond to changes in the station searching speed, and the broadcast signal will not be detected properly. , the drawback was that good video signals could not be obtained on channels where broadcasting was available.

The present invention was proposed in view of this point, and it is possible to operate the search start button while the channel selection device is searching for a station (actually, when the AFT circuit is operating, it is considered to be in a tuned state). The response speed of the AGC circuit is changed depending on the period from when the station is detected until the AFT circuit starts operating) and after the station searching operation has stopped.

Below, the circuit diagram of the conventional time constant circuit 50' is shown in the first section.
FIG. 2 shows a circuit diagram of a time constant circuit 50 according to the present invention. Note that in FIGS. 1 and 2, 17 indicates an AGC circuit.

Now, in the configuration shown in FIG. 1, reference numeral 11 indicates an input terminal to which a video detection output is applied. AGC control signal _S6 from AGC circuit 17 is connected to one end of the parallel circuit (first CR circuit) of resistor _R1 and capacitor _C1 via resistor _R3 , and the other end is connected to +B volts. A DC voltage is applied, and one end of the parallel circuit (second CR circuit) of resistor R ₂ and capacitor C ₂ is connected to one end of the parallel circuit, and the other end is grounded. The AGC control output signal S ₆₀ is then taken out through the connection point of each of the parallel circuits. In other words, a time constant circuit 50' is formed from R ₁ , R ₂ , R ₃ and capacitors C ₁ and C ₂ .

The configuration shown in FIG. 2 is such that a time constant switching circuit 51 is added to the conventional time constant circuit 50' shown in FIG.

That is, a series circuit of a PNP transistor TR ₂ having a resistor R ₅ between the base and emitter and a resistor R ₄ connected to the collector of this transistor TR ₂ is connected in parallel to the capacitor C ₁ , so that the transistor A DC voltage of +B volts is applied to the emitter of the transistor TR ₂ , and the collector of the NPN transistor TR ₁ is connected to the base of the transistor TR ₂ , and the emitter of the transistor TR ₁ is grounded. A control pulse is sent to the base of this transistor TR ₁ from the tuning device 13, which always maintains a low level and becomes a high level only during station searching operation.
_S30 given. Note that many parts within this channel selection device 13 are formed as digital circuits.

Here, the operation of the time constant circuit 50 according to the present invention will be explained with reference to FIGS. 1 and 2.

First, consider a case where a constant broadcast signal is being received without switching the reception channel (or after the station search operation has stopped).

In this state, the control pulse _S30 is given at a low level, so the transistor _TR1 is off, so the base voltage of the transistor _TR2 is approximately +B volts, and the transistor _TR2 is off. Therefore,
Since the equivalent resistance value of the time constant switching circuit 51 connected in parallel to the capacitor C ₁ is close to infinity, the time constant circuit 50 has the same time constant as when the time constant switching circuit section 51 is not included.

Next, let us consider the period from when the search start button is operated and a certain broadcast signal is received until when another broadcast signal is received due to the search operation, that is, when the search operation is in progress.

At the start of the station searching operation, the control pulse _S30 becomes high level, so the transistor _TR1 is turned on and the base potential of the transistor _TR2 becomes approximately zero volts. Therefore, the time constant of the time constant circuit 50 becomes smaller by turning on the transistor TR ₂ and connecting the resistor R ₄ in parallel to the capacitor C ₁ . In particular, one feature of the time constant circuit 50 of the present invention is that a PNP transistor is used as the transistor TR ₂ so that the base current of the transistor TR ₂ flows to the ground terminal via the transistor TR ₁ . As a result, the base current of the transistor TR ₂ flows into the capacitor C ₂ via the resistor R ₄ and the high frequency amplifier circuit 1
2 and the intermediate frequency amplification circuit 15.
It is possible to prevent changes to the AGC voltage and simplify the circuit. If an NPN transistor is used, the base current will be drawn into the emitter circuit and flow into capacitor _C2 , and the base current will affect the AGC voltage supplied to the subsequent amplifier circuit.

Here, in order to explain the reason for switching the time constant of the time constant circuit 50, which is the object of the present application, the electric field strength of the initially set broadcast signal is low, and when the other broadcast channel is being switched by the search operation. Consider each reception state when the electric field strength is high.

The receiving state of a broadcast signal with a low electric field strength is determined by the gain of the high frequency amplification circuit ₁₂ and the intermediate frequency amplification circuit 15 by the AGC control loop of the AGC circuit 17 in an attempt to raise the level of the composite video signal S5 to a normal level. becomes large. In this state, when searching for a broadcast signal with high electric field strength and switching to the next receiving state, the AGC control loop by the AGC circuit 17 includes the time constant circuit 50, so the AGC control is performed as follows according to the time constant. The high frequency amplification circuit 12 and the intermediate frequency amplification circuit 1 cannot change at the moment of switching to the reception state, and even though radio waves with high field strength are being received, the high frequency amplification circuit 12 and the intermediate frequency amplification circuit 1
Since the gain of 5 is large, the intermediate frequency amplification circuit 15
The intermediate frequency signal amplified by the video detection circuit (not shown) exceeds the dynamic range.
The synchronizing pulse has a good waveform and no longer appears in the video signal detected by the synchronous pulse, making it impossible to compare the phase with the flyback pulse. Therefore, even though the device is in the receiving state, the so-called reception detection signal becomes, for example, a high level and does not indicate the presence of the broadcast signal, so it is determined that the receiving state is not present. Therefore, by switching the time constant of the time constant circuit 50 to a smaller value, the responsiveness of the AGC loop control can be increased in the state where the reception channel is switched, and the control of the AGC loop can be normally adjusted to correspond to the field strength level of the received radio waves. The AGC loop will be controlled.

The time constant of the time constant circuit 50 is increased in normal reception conditions in order to prevent the AGC loop from responding due to disturbances in received radio waves caused by airplanes, automobiles, etc.

As described above, by reducing the time constant of the time constant circuit 50 during the station searching operation, switching between channels having a large difference in field strength of broadcast waves can be performed without malfunction.

Next, an application example of the present invention is shown in Fig. 3.

In the figure, reference numeral 11 denotes a tuning circuit, which has a VCO, i.e., an oscillator (voltage controlled oscillator) whose oscillation frequency can be changed by applying a voltage from an external source, and outputs an output signal _S1 of radio waves having a receiving frequency corresponding to the frequency of the voltage controlled oscillator.
Reference numeral 12 denotes a high-frequency amplifier circuit, which receives the output signal _S1 , amplifies the signal, and outputs an output signal _S2 .
is a local oscillator circuit, which outputs a signal having a local oscillation frequency. 14 is a mixer circuit, which receives the output signal _S2 and the signal having the local oscillation frequency, and outputs an intermediate frequency signal _S3 corresponding to a desired receiving channel. 15 is an intermediate frequency amplifier circuit, which receives the intermediate frequency signal _S3 , amplifies it, and outputs an intermediate frequency signal _S4 . 16 is a detection circuit, which receives the intermediate frequency signal _S4 , detects it, and outputs a composite video signal _S5 . 17 is an AGC circuit, which receives the composite video signal _S5 , and outputs an AGC control signal _S6 having a signal level corresponding to the level of the composite video signal _S5 . The AGC control signal _S6 is a signal for controlling the gain of the high frequency amplifier circuit 12 and the intermediate frequency amplifier circuit 15 in order to prevent the level of the composite video signal _S5 from fluctuating due to differences in the field strength of the received radio waves. Reference numeral 18 denotes a sync separation circuit which receives the composite video signal _S5 , separates and outputs a sync pulse _S7 contained in the composite video signal _S5 , and 19 denotes a sync detection circuit which receives the sync pulse _S7 and horizontal pulse _S8 and outputs a reception detection signal _S9 for determining the reception state.
A prescaler 20 receives a frequency signal _CO indicating the reception frequency from the voltage-controlled oscillator in the tuning circuit 11, divides the frequency by a fixed ratio (for example, 1/k), and outputs a frequency-converted frequency signal _1. A programmable frequency divider 31 divides the frequency signal ₁ and the station search signal S
_AS is input, and the frequency signal ₂ is divided by a predetermined frequency division ratio (for example, 1/N ₁ , 1/N ₂ , . . . 1/N _o ,
...1/N _M ) and outputs a frequency-converted frequency signal _2. That is, the frequency division ratio when receiving normal reception, for example, channel 3, is 1/N ₃ , and when the station search operation is performed, the frequency division ratio becomes 1/N _N corresponding to the channel on which the next broadcast exists by receiving the station search signal S _AS .
Reference numeral 2 denotes a reference oscillator, which outputs a reference frequency signal _0. Reference numeral 33 denotes a φ-DET, i.e., a phase detector, which detects the frequency signal ₂ and the reference frequency signal 0.
₀ , detects the difference between these frequencies, and outputs an error signal E _0. 34 is a control pulse generating circuit, which receives the error signal E ₀ and the reception detection signal
It receives the programmable frequency divider 3 and the station search signal _SAS , and outputs a control pulse _S30 . That is, in a normal receiving state, the control pulse _S30 is at a low level _, and when the station search signal _SAS rises during station search, the control pulse _S30 goes to a high level, and when the error signal _E0 becomes zero, the reception detection signal _S9 goes to a high level, i.e., the control pulse S30 goes to a low level in a broadcast receiving state. ₃₀ is a PLL frequency synthesizer, and the programmable frequency divider 3
1, the reference oscillator 32, the φ-DET 33, and the control pulse generating circuit 34. 40 is an LPF (low pulse filter).
The error signal _E0 is input, and a DC output signal _VD , which is a DC voltage corresponding to the level of the error signal _E0 , is output, and is input to the voltage controlled oscillator in the tuning circuit 11. 50 is a time constant circuit, and the AGC control signal _S6 and the control pulse
The AGC control output signal _S60 is outputted by delaying _the AGC control _{signal S6} by a time constant corresponding to the level of _the control pulse _S30 .
The signal is input to the high frequency amplifier circuit 12 and the intermediate frequency amplifier circuit 15 .

As explained above, the present invention increases the time constant of the time constant circuit during normal reception conditions, and switches the time constant to a lower value during station searching operations, thereby facilitating switching between channels with large differences in field strength of broadcast radio waves. Not only can it be done without malfunctions, but
By using a conductivity type transistor that does not affect the AGC voltage supplied to the high frequency amplifier circuit and intermediate frequency amplifier circuit in the subsequent stage due to the base current of the time constant switching transistor, the operation of the circuit becomes stable and the circuit operation becomes stable. This has the effect of simplifying the process.

[Brief explanation of drawings]

FIG. 1 is a conventional circuit diagram, FIG. 2 is a circuit diagram showing one embodiment of the present invention, and FIG. 3 is a circuit diagram showing an application example of the present invention. 50... Time constant circuit, 51... Time constant switching circuit, 30... PLL frequency synthesizer, 34...
Control pulse generation circuit, S _AS search signal, S ₉ ... Reception detection signal, S ₃₀ ... Control pulse, S ₆ ... AGC control signal, S ₆₀ ... AGC control output signal, TR ₁ , ₂ ...
Transistor, R ₁ to R ₅ ... Resistor, C ₁ , C ₂ ...
capacitor.

Claims (1)

[Claims for Utility Model Registration] An automatic station search that starts a station search operation by a predetermined channel selection operation, stops the station search operation when the presence of a broadcast signal is detected, and extracts an intermediate frequency signal of a desired broadcast signal. In a television receiver having the function,
a DC signal generating means for generating a DC signal that is maintained at a first DC level during normal reception and switched to a second DC level different from the first DC level during station searching operation; a DC power supply; a first CR having a first resistor and a first capacitor connected in parallel at first and second connection points, the first connection point being connected to a first electrode of the DC power supply; a second resistor and a second capacitor connected in parallel at first and second connection points; the second connection point is connected to a second electrode of the DC power source; The connection point is connected to the second CR circuit which is connected to the second connection point of the first CR circuit, one end is connected to the output of the AGC circuit, and the other end is connected to the connection point of the first and second CR circuits. A third resistor is connected, the base is connected to the output of the DC signal generating means, the emitter is connected to the second electrode of the DC power source, and the collector is connected to the first electrode of the DC signal generator through the fourth resistor. a first conductivity type transistor connected to an electrode of the first conductivity type transistor, a base connected to a collector of the first conductivity type transistor, an emitter connected to a first electrode of the DC power source, and a collector connected to a fifth resistor. A response speed control device for an AGC control signal, comprising a time constant circuit including a second conductivity type transistor connected to a connection point of the first and second CR circuits via a time constant circuit.