JPS5938787Y2 - television receiver - Google Patents

Description

[Detailed Description of the Invention] This invention relates to an improvement of an automatic local oscillation frequency control circuit in a television receiver.

Figure 1 shows the conventional automatic local oscillation frequency control (hereinafter referred to as AFC).
This figure shows a television receiver including a circuit (called ``1''), and in FIG.

In a television receiver that includes such an AFC circuit,
The television signal received by the antenna 1 is amplified by the tuner 2, mixed with the local oscillation frequency and converted into a video intermediate frequency (hereinafter referred to as VIP') signal, and then amplified by the VIF' amplifier 3. A composite video signal is obtained by detection by a detector 4.

- A part of the VIP signal is extracted from the VIE' amplifier 3 and applied to the AFC circuit 5 to discriminate the frequency of the VIP' signal, and always maintains a predetermined frequency (58.75 in the case of Japan).
The oscillation frequency of the local oscillation circuit (not shown) of the tuner 20 is controlled so that the oscillation frequency becomes MH2).

However, in such a conventional AF'C circuit, in a transient state of operation of the local oscillation circuit, such as when selecting a reception channel, the local oscillation frequency is 3 to 4 times higher than the normal one.
Starting from a frequency higher than MHz, AF is performed using an audio intermediate frequency signal with a frequency higher than the VIP signal by 4.5 MHz.
So-called AF where the local oscillation frequency is locked by C characteristics
It had the disadvantage of causing malfunctions in 'C.

This invention was made in order to eliminate the above-mentioned drawbacks of the conventional technology, and it generates DC voltage only during the period when the resonant voltage waveform that resonates at the frequency of the horizontal synchronization signal exceeds the reference potential, and the DC voltage The purpose of this invention is to provide a television receiver that can perform automatic local oscillation frequency control to prevent malfunctions by stopping the AFC operation when the frequency cannot be obtained.

Next, an embodiment of this invention will be explained with reference to the drawings. Fig. 2 shows a block diagram from an antenna to a video detector of a television receiver including an AFC circuit according to an embodiment of this invention. The figure is shown below.

In FIG. 2, the same symbols as in FIG. 1 indicate directions or corresponding parts, and the malfunction prevention circuit 6 according to this invention
and the well-known synchronous separation circuit 1 are newly added to FIG. 1, and the other parts are similar to FIG. 1.

In FIG. 2, the malfunction prevention circuit 6 is applied with a synchronization signal extracted from the composite video signal by the synchronization separation circuit 1, and in a steady state where this synchronization signal exists, the malfunction prevention circuit 6 is 6 is not supposed to work.

For this reason, the operation of the AFC circuit 5 is as already explained with reference to FIG. becomes a noise component.

Therefore, since the output of the synchronization separation circuit γ is almost only a noise component, no synchronization signal is applied to the input of the malfunction prevention circuit 6.

In this state, the malfunction prevention circuit 6 operates and the AFC
The operation of circuit 5 is stopped and the normal AFC characteristic is 4°5.
To prevent AF'C characteristics from occurring due to audio intermediate frequency signals at frequencies higher than MHz.

That is, it detects that the frequency of the VIF' signal shifts to a higher frequency side than the AFC pull-in range based on the presence or absence of a synchronization signal, stops the AF'C operation while there is no synchronization signal, and performs AFC according to the audio intermediate frequency signal. When the channel selection operation is almost completed, the frequency of the VIP' signal enters the AFC pull-in range, and even a slight synchronization signal component appears in the output of the synchronization separation circuit 7, , the operation of the malfunction prevention circuit 6 is canceled and normal AFC operation is performed.

A specific example of the configuration of the AFC circuit will be described below with reference to FIGS. 3 and 4.

FIG. 3 is a diagram showing the circuit configuration thereof, and FIG. 4 shows signal waveforms at various parts in FIG.

First, the circuit shown in FIG. 3 will be explained. In FIG. 3, 8 is a frequency discriminator which is a basic part of the AFC circuit.

This frequency discriminator 8 has a first differential pair 9 and a 20th differential pair 1.
This is a well-known double-balanced connection quadrature frequency discriminator constructed of a 30th differential pair 11 which serves as a current source for each of the 30th differential pairs 11 and 0.

Above 1. Two common connection points connecting the 20th differential pair 9 and 100 pace terminals to each other are connected to the first input terminal pair 12at12b.

The pace terminal of the third differential pair 11 is connected to the second input terminal pair 13at13b.

The VIP signal is differentially introduced into the first pair of input terminals 12a and 12b, and the second pair of input terminals 13a and 13b is configured with a resonant circuit that resonates at the normal frequency of the VIP signal. A quadrature circuit 14 is connected thereto.

When the first input terminal N12a, 12bKVIF signal is applied, this VIP signal is applied to the first input terminal N12a, 12bKVIF signal. A part of this VIP signal is applied to the base terminals of the second differential pair 9,100 as a switching signal of the differential pair, and a part of this VIP signal is also applied to the base terminal of the 30th differential pair 11 via the capacitors 15 and 16. be done.

A seven-way Ohdrecher circuit 14 is connected to the base terminal of the third differential pair 11 through a second input terminal pair 13a and 13b, and the capacitors 15 and 16 are connected to the base terminal of the third differential pair 11.
At the same time, the VIP signal is shifted by 9C at the resonant frequency.

In such a configuration, the deviation of the frequency of the input VIP signal from the normal frequency is converted into a phase deviation centered on 9C, and is applied as the 30th differential pair 110 input.

And the first. The 20th differential pair 9, 10 is the 30th differential pair 1
1 output current is switched by the non-phase shifted VIF' signal to generate a rectangular wave signal having a tute proportional to the phase difference with the input signal to the 30th differential pair 11.

This rectangular wave signal is introduced into a differential amplifier 17, where the rectangular wave signal is smoothed and amplified by the differential amplifier 11, and a DC voltage proportional to the phase difference is generated between the differential output terminals. 18a, available from ISB.

The above is the operation of the well-known quadrature frequency discriminator with double balanced connection.Next, we will explain the details of the malfunction prevention circuit 6 used in the automatic local oscillation frequency control circuit of this embodiment. do.

The use of synchronization signals to prevent AF"C malfunction during channel selection has already been proposed, but these methods detect the presence or absence of synchronization signals by rectifying the synchronization signal to its single peak. There was a risk of mistaking a peak or audio intercarrier signal for a synchronization signal, but the present invention takes this into consideration and provides an AFC malfunction prevention circuit with good noise resistance, which will be described in detail below.

In FIG. 3, a positive synchronizing signal is applied via a capacitor 19, a first NPN transistor 210 is connected to a terminal 20, and a resistor 22 is connected between the terminal 20 and ground. There is.

The emitter of the transistor 21 is grounded through the resistor 23, and the collector is connected to the power supply (to which a voltage of 10 B is applied in the figure) through the resistor 24. Connected to the base of transistor 25.

A resistor 26 is connected between the emitter of the second transistor 25 and the power supply, and a parallel resonant circuit 2γ consisting of a coil and a capacitor and having a resonance point at approximately 15°75 kHz is connected between the collector and ground. ing.

Furthermore, this collector is connected via a resistor 28 to the base of a third transistor 290 of NPN type.

A bias resistor 30 is connected between the base of the third transistor 290 and the power supply, the emitter is connected to ground, and a load resistor 3 is connected between the collector and the power supply.
1 is connected.

Further, a holding circuit 33 consisting of a capacitor and a resistor is connected to this collector via a diode 32, thereby forming a positive peak rectifier circuit.

- the first pair of input terminals 12at of the frequency discriminator 8;
The collector and emitter of a PNP type (1) fourth transistor 34, which is an AFC operation stop circuit, are connected to 12b, respectively, and the base of this fourth transistor 34 is connected to the holding circuit 33.

In the malfunction prevention circuit having such a configuration, first, in a steady state, a positive polarity synchronization signal is applied to the terminal 20 via the capacitor 19, and the first transistor 21 and the second transistor 25 are connected during the period of the synchronization signal. It is on during some periods and off during other periods.

Here, if the collector load of the second transistor 25 is a resistance, the voltage waveform of the collector of the second transistor 25 becomes a pulse train similar to the input synchronization signal, and this is peak-rectified to maintain the above-mentioned state. If the peak acupressure is maintained in the circuit 33, it is possible to detect the presence or absence of a synchronization signal, and this is basically the same as what has been proposed in the past.

However, in such a method, when the input synchronization signal disappears and the noise signal increases, a pulse is generated at the collector of the second transistor 25 due to the amplification of the noise signal, which is peak-rectified and becomes the synchronization signal. It had the drawback of being misjudged.

However, in this invention, in order to eliminate such drawbacks, a parallel resonant circuit 27 having a resonance point approximately at the repetition frequency (15.75 KHz) of the synchronous signal is connected as a load to the collector of the second transistor 25. When the second transistor 25 is switched by the input synchronizing signal, the voltage waveform at the collector of the second transistor 25 is determined by the resonant frequency of the parallel resonant circuit 2T, rather than by a conventional pulse train. The waveform is close to a sine wave.

A characteristic of a resonant circuit is that the reference potential on the collector side (
In this case, a waveform that oscillates above the ground voltage (in this case, the ground voltage) is obtained, that is, a resonant voltage waveform that exceeds the reference potential.

The third transistor 29 is biased to be off only during the period of the negative voltage exceeding this ground voltage,
Therefore, the collector of the third transistor 29 has a high potential only during this negative voltage period, and the pulse signal generated at this time turns on the diode 32, performs peak rectification, and holds the high potential in the holding circuit 33. .

Therefore, the base of the fourth transistor 340 connected to the holding circuit 33 is also at a high potential, this fourth transistor 34 is held in the off state, and according to the frequency discriminator 8 the fourth transistor 34 is connected to the junction It has no effect on anything other than capacity.

Next, the waveforms of each part in the above description will be explained with reference to FIG. 4. FIG. 4a shows the base voltage waveform of the first transistor 210, FIG. 4c shows the collector voltage waveform of the second transistor 25, and FIG. 4d shows the collector voltage waveform of the third transistor 29.

It is clear from FIG. 4 C1 and FIG. 4 d that only the negative voltage waveform of the resonance waveform due to the input synchronizing signal component generates the charging voltage to the holding circuit 33, If the input signal to the terminal 20 is only a noise signal component unrelated to the synchronization signal period, no resonant waveform is generated at the collector of the second transistor 25, and therefore, a negative voltage does not occur, and at most it is connected to ground. Only a pulse with a voltage of 1 is generated, and the noise signal cannot be mistaken for a synchronization signal.

That is, in this embodiment, the parallel resonant circuit 2γ performs frequency separation and the third transistor 29 performs amplitude separation, thereby ensuring separation of the synchronization signal component and the noise signal component.

Therefore, in a transient state of operation of the tuner 20 local oscillation circuit, such as during tuning operation, the frequency of the VIP signal shifts to the high frequency side beyond the AFC pull-in range, and the synchronization signal component of the composite video signal of the detection output disappears. At this time, no resonant voltage is generated at the collector of the second transistor 25 and the third transistor 29 is not turned off, so the collector of the third transistor 29 is at ground potential 11.

Therefore, the diode 32 is not turned on, so the holding circuit 33 is not charged and has a low potential, and the fourth transistor 34 is turned on.

Furthermore, even if a noise signal with a period close to that of the horizontal synchronizing signal is input, a negative voltage portion does not occur in the resonant voltage waveform as described above, so the transistor 34 is turned on in this case as well.

As a result, the first input terminal pair 12a of the frequency discriminator 8
, 12b are shorted by a fourth transistor 34;
The input VIP signal is bypassed and this frequency discriminator 8
Since no voltage is applied to the AFC operation, the AFC operation stops.

Next, when the channel selection operation is almost completed and the frequency of the VIP signal enters the AFC pull-in range, a slight synchronizing signal component appears again in the input signal to the terminal 20, so the parallel resonant circuit 21 extracts it. , generates a resonant waveform.

During the negative voltage period of this resonant waveform signal, the third transistor 29 is turned off to generate a charging voltage.

Therefore, the holding circuit 33 becomes high potential, the fourth transistor 34 is turned off again, the AFC operation is restored, and the V
The IP' signal is pulled to the normal frequency.

If you replace it, the AFC operation will be stopped during the overacid period of the channel selection operation, suppressing the occurrence of pseudo AFC characteristics due to the audio intermediate frequency signal, and AF will be activated when it enters the pull-in range due to the regular AFC characteristics.
By restoring C operation, malfunction of AFC can be prevented.

Note that the malfunction prevention circuit in the automatic local oscillation frequency control circuit of this invention is not limited to the circuit described above, but can be similarly constructed using variously modified circuits.

That is, depending on the polarity of the synchronization signal input to the terminal 20, the first transistor 21 may be omitted;
The transistor 34 is connected between the second input terminal pair 13a, 13b of the frequency discriminator 8 or between the first . 20th differential pair 9,1
It may be connected between 00 collectors.

Furthermore, the output terminal pair 18a of the differential amplifier 11.

It is also possible to provide another switching circuit for both or Kazuo 18b, and use this switching circuit to turn on and off using the voltage of the holding circuit 33.Also, it is not limited to AF'C using the autorature frequency discriminator as described above. , other methods,
For example, a switching circuit provided at the input terminal or output terminal of the AFC circuit may be turned on or off by the voltage of the holding circuit 33 using a comparative frequency discriminator.

Furthermore, instead of the fourth transistor 34, two diodes connected in series in opposite directions may be used, and the center thereof may be connected to the holding circuit 33. The same configuration can be achieved even if the conductivity type of the transistor, the polarity of the power supply, etc. are reversed.

Additionally, although the above explanation is based on the case where a synchronization signal is used as an input to the malfunction prevention circuit, the same effect can also be obtained by directly inputting the composite video signal and extracting the horizontal synchronization frequency component instead. It's a bastard.

Note that the same effect cannot be obtained even if the resonant frequency of the parallel resonant circuit 2γ is selected to be an integral multiple of the horizontal synchronization frequency (15°75KH2), and the part surrounded by the broken line in Fig. 3 is integrated. The effect of this embodiment will be further increased if it is made into a circuit, and if the connection point between the holding circuit 33 and the diode 32 is grounded with a switch, it can also be used as an AFC release switch. It is.

As detailed above, according to this invention, it is detected that the frequency of the VIP signal deviates to a higher frequency side than the AFC pull-in range based on the presence or absence of a synchronization signal, and the AFC operation is stopped while there is no synchronization signal. Prevent AFC characteristics from occurring in the audio intermediate frequency signal, and when the channel selection operation is almost completed and the frequency of the VIP signal enters the AFC pull-in range, the output of the synchronization separation circuit will again receive even the slightest synchronization signal component. appears, the malfunction prevention circuit is deactivated and normal AF occurs.
Since the 'C operation is performed and the presence or absence of the synchronization signal component is determined by both frequency separation and amplitude separation, the synchronization signal component can be very well separated from the noise signal.

Therefore, synchronization signal detection with good noise resistance is performed.
This has the effect of being able to reliably prevent AFC malfunction without making the 9-inclusive range sensitive.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a television receiver using a conventional AFC circuit, Figure 2 is a block diagram of a television receiver according to an embodiment of this invention, and Figure 3 is an automatic local oscillation frequency diagram of the above embodiment. The circuit diagrams of the control circuit, FIGS. 4a to 4d, are diagrams showing signal waveforms at various parts of the automatic local oscillation frequency control circuit of FIG. 3, respectively. 2... Tuner, 3... Movie iron intermediate frequency amplifier, 4...
...Video detector, 5...Automatic local oscillation frequency control circuit, 6...Malfunction prevention circuit, 1...Synchronization separation circuit, 8
~11...Differential Kari, 14...Quadrature circuit, 11...Differential amplifier, 21.25, 29...Diode, 30...Parallel resonant circuit, 33...Holding circuit . Note that the same reference numerals in the drawings indicate the same or equivalent parts.

Claims (6)

[Scope of utility model registration request] (1) An AFC circuit that detects the frequency of the video intermediate frequency signal and controls the local oscillation frequency of the tuner so that the signal becomes a predetermined frequency. a video detector that detects a frequency signal to obtain a composite video signal; a synchronization signal separation circuit that obtains a horizontal synchronization signal from the composite video signal; a parallel resonant circuit that generates a resonant voltage waveform, a peak rectifier circuit that generates a pulse when the resonant voltage waveform from the parallel resonant circuit exceeds the reference potential and peak-rectifies the pulse to generate a DC voltage; It consists of an AFC operation stop circuit that stops the operation of the A10 circuit when voltage cannot be obtained.
A television receiver comprising: a malfunction prevention circuit that prevents malfunction of the A10 circuit due to noise. (2) The frequency discriminator of the above A10 circuit is a quadrature circuit consisting of three differential transistor pairs, a first input terminal pair to which a video intermediate frequency signal is differentially input, a coil and a capacitor. 2. A television receiver as claimed in claim 1, characterized in that it is a double-balanced connected quadrature frequency discriminator having a second pair of input terminals and a second pair of input terminals connected to each other. (3) The AFC operation stop circuit is a transistor whose collector and emitter are connected between the first pair of input terminals of the quadrature frequency discriminator of the A10 circuit, and is a transistor whose collector and emitter are connected between the first input terminal pair of the quadrature frequency discriminator of the A10 circuit. Claim 2 of the utility model registration characterized in that the video intermediate frequency signal to the quadrature frequency discriminator of the A10 circuit is bypassed when it is turned on and off by a rectified voltage and there is no horizontal synchronizing signal frequency component. television receiver. (4) The AFC operation stop circuit is a transistor whose collector and emitter are connected between the second input terminal pair of the quadrature frequency discriminator of the A10 circuit. A television receiver according to claim 2. (5) A utility model characterized in that the AFC operation stop circuit comprises two diodes connected in series in opposite directions between the second input terminal row of the quadrature frequency discriminator of the A10 circuit. A television receiver according to claim 2. (6) The frequency discriminator of the A10 circuit is a comparison frequency discriminator, and the AFC operation stop circuit is connected to the input terminal or output terminal of the A10 circuit and is turned on and off by the peak rectification voltage of the peak rectification circuit. Claim 1 characterized in that the switching circuit is
Television receiver as described in section.