JPH0217966B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an AFT voltage selection circuit for television receivers and the like. [Summary of the Invention] The present invention provides a television receiver that selectively receives a television signal in which the video carrier frequency is higher than the audio carrier frequency and a television signal in which the video carrier frequency is lower than the audio carrier frequency. It is equipped with an AFT circuit that takes out each AFT voltage having characteristics corresponding to the television signal, and a switch means for selecting one of the respective television signals in accordance with the switching operation of the frequency band of the reception channel, and the switch means is the above-mentioned one. By selecting an AFT voltage with characteristics corresponding to one of the above television signals in conjunction with a switching operation, it is possible to select whether the local oscillation frequency is higher or lower than the reception frequency (so-called upper local or lower local). Even in these cases, normal AFT operation can be performed. [Prior Art] In the tuner of a general television receiver, the high frequency signal received by the antenna has a frequency band of approximately 6 MHz for one channel as a whole, as shown in FIG.
The interval f' between the video carrier frequency f _P and the audio carrier frequency f _S is, for example, 4.5 MHz. A signal from a local oscillator is mixed with such a high frequency signal, converted to an intermediate frequency signal, amplified by an intermediate wave amplifier, and then sent to an audio detection circuit section, a video detection circuit section, etc. Incidentally, in recent years, the internal circuits of television receivers have become increasingly integrated into ICs, and for example, it is no longer possible to use standardized IC circuits for the above-mentioned intermediate frequency amplifiers, audio detection circuits, video detection circuits, etc. This is commonly done. Therefore, it is required that the video intermediate frequency f _VIF and the audio intermediate frequency f _SIF in the intermediate frequency signal always satisfy a predetermined relationship (for example, f _VIF < f _SIF ). Therefore, when f _P > f _S and when f _P > f _S , the local oscillation frequency f _LOC is changed to the frequency of the above high frequency signal (f _P , f _S , etc.)
It is necessary to switch to an upper local higher than f _LOC or, conversely, to a lower local lower than f _P or f _S. That is, when the local oscillation frequency f _LOC is higher than each frequency f _P and f _S of the high frequency signal above, f _LOC − f _P = f _VIF ...... f _LOC − f _S = f _SIF ...... Therefore, when f _P < f _S , −f _P > −f _S f _LOC −f _P > f _LOC −f _S ∴f _VIF > f _SIF ……, and when f _P > f _S , −f _P ＜−f _S f _LOC −f _P ＜f _LOC −f _S ∴f _VIF ＜f _SIF ……. Next, in the case of a lower local where the local oscillation frequency f _LOC is lower than the above f _P and f _S , f _P − f _LOC = f _VIF ...... f _S − f _LOC = f _SIF ......, so f When _P <f _S , f _P −f _LOC <f _S −f _LOC ∴f _VIF <f _SIF ... Also, when f _P > f _S , f _P −f _LOC >f _S −f _LOC ∴f _VIF >f _SIF ... Therefore, in order to always satisfy the relationship f _VIF < f _SIF , for example, it is necessary to switch to make the equation lower local when f _P < f _S and to make it upper local than the equation when f _P > f _S. That's fine. For example, Table 1 shows f _P and f _S for each channel of French television broadcasting, and for these f _P and f _S , the local oscillation frequency f _LOC is calculated as shown in Table 1. By selecting , constant f _VIF (32.70MHz) and f _SIF (43.8MHz) intermediate frequency outputs can be obtained.

[Table] As is clear from this first table, F5, F7,
In each channel of F9 and F11, f _P <
Since f _S , the above lower locals with frequencies lower than these f _P and f _S , F2, F4, F6, F8,
In each channel of F8A, F10, and F12, since f _P > f _S , the above-mentioned upper local is used.

【table】

[Problem that the invention seeks to solve]

By the way, it is desirable that the local oscillation frequency f _LOC has high frequency stability, but in black-and-white television broadcasting, even if the stability of f _LOC is not high, there will not be much deterioration in image quality. ) was omitted. However, in 1979
From December, color television broadcasting will begin using the channel classifications shown in Table 2, so the need for the AFT circuit described above has increased. This AFT circuit has the drawback that the control operation works in opposite directions between the above-mentioned upper local and lower local, so the circuit configuration for correcting this becomes complicated. The present invention has been made in view of the above-mentioned circumstances, and the local oscillation frequency has been changed to the above-mentioned frequency because, for example, the height relationship of f _P and f _S changes as the channels of television broadcasting change. In tuners that require switching between lower local and upper local, not only can automatic fine adjustment be performed normally even when switching between lower local and upper local, but the AFT voltage can be easily realized with a simple circuit configuration. The purpose is to provide a selection circuit. [Means for solving the problems] In order to solve the above problems, the present invention
The AFT voltage selection circuit consists of a local oscillator that generates a local oscillation signal mixed with the received signal, an AFT circuit that extracts an AFT voltage with characteristics corresponding to the received television signal, and the AFT voltage selection circuit.
and a switch means to which the AFT voltage from the circuit is supplied and selects a signal in accordance with the switching operation of the frequency band of the reception channel, and the video carrier frequency is higher than the audio carrier frequency according to the signal from the switch means. AFT of a television receiver configured to selectively receive signals and a television signal lower than the video carrier frequency by switching the oscillation frequency of the local oscillator.
In the voltage selection circuit, the switching means includes a first filter to which the AFT voltage having a first characteristic is supplied, a second filter to which the AFT voltage having a second characteristic is supplied, and a second filter to which the AFT voltage having a second characteristic is supplied. It is characterized by comprising a field effect transistor element connected to a second filter to select the signal supplied to the local oscillator. [Operation] As the frequency band of the reception channel is switched, an AFT with characteristics corresponding to either a television signal in which the video carrier frequency is higher than the audio carrier frequency or a television signal in which the video carrier frequency is lower than the audio carrier frequency is applied. By selecting the voltage, AFT operation is performed normally. Further, by using a field effect transistor element to select the AFT voltage and supplying a control signal for the above switching operation to the control terminal of the field effect transistor element, the signal sent to the local oscillator is selected, and the signal sent to the local oscillator is oscillation frequency can be switched. [Examples] Hereinafter, preferred embodiments of the present invention will be described.
This will be explained with reference to FIGS. 1 to 4. FIG. 1 is a schematic block circuit diagram of a television receiver equipped with an AFT voltage selection circuit as an embodiment of the present invention, in which a high frequency signal received by an antenna 1 is mixed via a high frequency amplifier 2. It is sent to vessel 3. This mixer 3 is supplied with a local oscillation signal from a local oscillator 4, mixes it with the high frequency signal, converts it into an intermediate frequency signal, and sends this intermediate frequency signal to an intermediate frequency amplifier 5. An intermediate frequency signal is taken out from this intermediate frequency amplifier 5 and sent to an automatic fine tuning circuit (AFT circuit) 6. For example, it is sufficient for the AFT circuit 6 to have a configuration in which the intermediate frequency signal is amplitude-limited and amplified to a constant amplitude, and then sent to a Foster-Schili type FM detector. However, regarding the AFT output, A device that generates two outputs with opposite polarities as shown in FIGS. 2 and 3 is used. This is because a balanced type, a differential type, a double-balanced differential type, etc., are often used in general ICs, so it can be easily obtained as an IC for AFT. The output characteristics shown in Figs. 2 and 3 show that when the frequency of the intermediate frequency signal deviates from a predetermined center frequency, for example, the video intermediate frequency f _VIF , a positive or negative output voltage is generated. The positive and negative polarities of the output voltage with respect to the direction of deviation are opposite between FIG. 2 and FIG. 3.
The output of the AFT circuit 6 is sent to the local oscillator 4 via the AFT switching circuit 7. This AFT switching circuit 7 switches, for example, to the output characteristic shown in FIG. 2 when the lower local is set, and to the output characteristic shown in FIG. 3 when the upper local is set, and this switching operation is performed by the control terminal 32. This is done in response to a switching signal. This switching signal can be easily obtained from, for example, a channel changeover switch (not shown). For example, in the case of channel allocation as shown in Table 2 above, a band changeover switch (not shown) selects the first frequency band, which includes channels A, B, and C; 4,
Band switching signals that are mutually inverted in the second frequency band including channels 5 and 6 may be supplied to the control terminal 32 as switching signals. The automatic fine adjustment device 20 consisting of these AFT circuits 6 and AFT switching circuits 7 sends a voltage output to the local oscillator 4 according to the fluctuation of the intermediate frequency, and changes the capacitance value of, for example, a variable capacitance diode in the local oscillator 4. By changing the oscillation frequency, control is performed such that the intermediate frequency is restored to a predetermined center frequency (for example, f _VIF ) in both lower local and upper local cases. In FIG. 1, the output from the intermediate frequency amplifier 5 is sent to a CRT (cathode ray tube) 10 via a video detector 8, a video amplifier 9, etc., and is also sent to a CRT (cathode ray tube) 10 via an audio detector 11 and an audio amplifier 12. and is sent to the speaker 13. Next, FIG. 4 is a circuit diagram showing a specific example of the AFT switching circuit 7 and its surroundings.
Outputs from transistors 14 and 15 in the output stage of the FM detector of the AFT circuit 7 are sent to output terminals 16 and 17, respectively. These transistors 1
4 and 15 perform operations opposite to each other (or differential operations). For example, when the output voltage of the first output terminal 16 is as shown in FIG. 2, the output voltage of the second output terminal 17 is as shown in FIG. It becomes like this. The first output terminal 16 is connected to a first smoothing circuit composed of a capacitor 21, a resistor 22, and a capacitor 23 in the AFT switching circuit 7, and the second output terminal 17 is connected to a first smoothing circuit composed of a capacitor 21, a resistor 22, and a capacitor 23. , and a second smoothing circuit consisting of a capacitor 26. The output terminals of these smoothing circuits are connected via a capacitor 27. 1st above
The output terminals of the smoothing circuit are connected to resistor 28 and resistor 2.
9 to the base of the output transistor 30. Further, the output terminal of the second smoothing circuit is connected to the connection point between the resistors 28 and 29 via a switching element such as a FET (field effect transistor) 31. A control terminal 32 of this FET 31 is supplied with a band switching signal as a switching control signal, which performs an inverted operation depending on the above-mentioned upper local and lower local. Further, the emitter output terminal of the output transistor 30 is connected to a variable capacitance diode (varicap diode) 33 of the local oscillator 4. In the AFT switching circuit 7 having the above configuration, when the FET 31 is in the OFF state, the AFT
Since the output from the first output terminal 16 of the circuit 6 is sent to the output transistor 30 via the smoothing circuit and the resistors 28 and 29, the output characteristics shown in FIG. 2 are obtained. Next, when the input signal of the control terminal 32 is switched and the FET 31 is turned on, this
Since the resistance value of the FET 31 is much smaller than that of the resistor 28, the output from the second output terminal 17 is mainly applied to the base of the output transistor 30, and the output characteristics of the output transistor 30 are as shown in FIG. become. As is clear from the above description, the AFT voltage selection circuit according to the embodiment of the present invention includes an AFT circuit that generates two outputs with different polarities, one output of the two outputs is connected through a resistor, and the other output is consists of an AFT switching circuit that sends signals to the local oscillator via a switching element, and the switching element is activated by a switch signal that is inverted in accordance with band switching operation or channel switching operation when the local oscillation frequency is higher or lower than the receiving frequency. It has a configuration that controls. Therefore, with an extremely simple configuration and the addition of a few parts, normal AFT operation can be performed in both lower local and upper local cases. Furthermore, the field effect transistor element, which is a switching element, has a low ON resistance and a high OFF resistance, so that good switching can be performed. [Effects of the Invention] According to the AFT voltage selection circuit of the present invention, when selectively receiving a television signal whose video carrier frequency is higher than the audio carrier frequency and a television signal whose video carrier frequency is lower than the audio carrier frequency, In addition, the switch is changed according to band switching operation or channel switching operation, and normal AFT operation can be realized with a simple circuit configuration. Furthermore, a field effect transistor element is used in the circuit configuration for switching the switch, and good switching can be performed by utilizing the characteristics of the field effect transistor element.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing a schematic configuration of a television receiver to which an embodiment of the present invention is applied;
Fig. 2 is a graph showing the first output characteristic of the AFT circuit, Fig. 3 is a graph showing the second output characteristic of the AFT circuit, Fig. 4 is a circuit diagram showing a specific example of the AFT switching circuit, Fig. 5 is a graph showing the frequency spectrum of a television broadcast reception signal. 3...Mixer, 4...Local oscillator, 5...Intermediate frequency amplifier, 6...AFT circuit, 7...AFT switching circuit, 20...Automatic fine adjustment device.

Claims (1)

[Claims] 1. A local oscillator that generates a local oscillation signal to be mixed with a received signal, an AFT circuit that extracts an AFT voltage having characteristics corresponding to the received television signal, and a AFT above
a television signal whose video carrier frequency is higher than the audio carrier frequency according to the signal from the switch means, and a switch means to which a voltage is supplied and selects a signal in accordance with the switching operation of the frequency band of the receiving channel; In the AFT voltage selection circuit of a television receiver, the AFT voltage selection circuit is adapted to selectively receive a lower television signal by switching the oscillation frequency of the local oscillator.
a first filter to which a voltage is supplied; a second filter to which the AFT voltage of a second characteristic is supplied; and a signal connected to the first and second filters to supply a signal to the local oscillator. An AFT voltage selection circuit comprising a selected field effect transistor element.