JPS5923645B2 - automatic frequency control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic frequency control device for a separate carrier type television receiver, and its purpose is to prevent malfunctions in which no sound is output when the power is turned on, when changing channel selection, etc. shall be.

In television receivers that place emphasis on sound quality, such as TV audio multiplex broadcast receivers and TV audio multiplex tuners, the output section of the tuner is connected to a video signal (58.75 MH2) and an audio signal (58.75 MH2).
4.25MH2) is often used. This method requires frequency stability in both the video intermediate frequency stage and the audio intermediate frequency stage. Therefore, as shown in Fig. 1, AFC is applied to the local oscillator of the tuner 1 (referred to as the first local oscillator) and the second local oscillator 7 for audio intermediate frequency, and as shown in Fig. 2, the second local oscillator There are cases in which AFC is applied to the tuner 1 from the audio FM detector 9 via the AFC circuit 10 using a highly stable one such as crystal oscillation, but the former case will mainly be described. In Figures 1 and 2, 1 is a tuner, 2 is a video intermediate frequency amplifier, 3 is a video detector, and 4 is a video intermediate frequency amplifier.
5 is a first audio intermediate frequency amplifier, 6 is a mixer, 7 is a second local oscillator, 8 is a second audio intermediate frequency amplifier, 9 is an audio FM detector, and 10 is a second AFC circuit.

In FIG. 1, it is desirable that the AFC circuit of the second local oscillator 7 has a small AFC error in terms of various characteristics such as distortion, but the AFC loop gain is set high and a large amount of AFC error is required.
If the error is reduced by applying feedback, the pulling range will be widened unless complex shaping is applied to the S curve of the AFC control voltage.

In addition, in areas such as adjacent interference, ghosts, and weak electric fields, the first local oscillation frequency may be set to about 100 to 300 to improve the image.
It may be lowered to about KHz. In such a case, the second
The AFC of the local oscillator 7 is required to have a wide pull-in range and small errors. If the AFC pull-in range is wide, when the first local oscillation frequency moves from low to high when the power is turned on or when switching stations, the video signal component of the own station or the audio signal carrier will be at -920 KHz. The difference between the color subcarrier and the first local oscillation frequency is 54.2
In order to form a frequency of 5 MHz, these signals are input to an AFC with a second local oscillation frequency of 7. Therefore, even if the first local oscillation frequency reaches a predetermined value, the second local oscillator 7 remains absorbed by the video signal and color subcarrier, and even if the image is normal, the audio may not be received. Particularly in areas where it is necessary to shift the first local oscillation frequency to a lower level, the frequency difference between the audio signal and the video signal or color subcarrier becomes small, making the above-mentioned malfunction more likely to occur. In addition, in order to facilitate the tuning operation when manually tuning, such as when presetting an electronic tuning device, or when moving the first local oscillation frequency from a lower side to a higher side, AFC of the second local oscillator 7 is applied. If you leave it as is, you may be unable to receive audio even if you can receive images normally.

In view of the above-mentioned problems, the present invention aims to provide an automatic frequency control device that can always receive both images and sounds normally. Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 shows an embodiment of the present invention, and the same parts as in FIG. 1 are given the same numbers.

The feature of the circuit shown in FIG. 3 is that the output of the second AFC circuit 10 is applied to the second local oscillator 7 via the switching circuit 11, and the switching circuit 11 is activated by the detection output of the power-on detection circuit 12 or the station selection change detection circuit 13. At the time of switching, turning on the power, or switching the channel selection, a voltage lower than the AFC control voltage in the normal tuning state is applied from the constant voltage circuit 14 instead of the AFC control voltage. FIG. 4 shows a specific circuit of a part of FIG.
Detection voltages of the power-on detection circuit 12 and the channel selection switching detection circuit 13 are applied from b.

A Zener diode may be used as the fixed voltage.
Further, when slicing the AFC control voltage, the Zener diode 17 is connected as shown by the broken line in FIG. In the circuit shown in FIG. 3, the television signal receiving circuit is a separate carrier type circuit, and the audio circuit is a double superheterodyne type circuit.

In order to temporarily fix the local oscillator frequency to a frequency lower than the reference frequency (normally tuned frequency) when the power is turned on or when changing the channel selection, the +B voltage is divided by resistors 15 and 16 and the A voltage is set.
The value VL is set lower than the FC reference voltage (AFC control voltage in normal tuning state).

Turn on FETlla of the switching circuit 11 with pulses from the power-on detection circuit 12 and the tuning detection circuit 13, add the above VL to the variable capacitance diode of the local oscillator circuit, and increase the capacitance from the reference state (regular tuning state). The frequency FL is increased to be lower than the reference frequency. A of the second AFC circuit 10
The S curve of the FC control voltage is as shown in Figure 5 with respect to the change in the second intermediate frequency, and the change in the first intermediate frequency (
The change in the first local oscillation frequency is as shown in FIG.

Therefore, when the power is turned on or when changing the channel selection, the first local oscillation frequency moves from low to high, and as shown in Fig. Even if the AFC control voltage is set to L when the power is turned on and when changing the channel selection as in this configuration, when the first local oscillation frequency moves between F and FO, it will deviate from the S curve centered at F. , the second AFC operates normally as it is drawn into the S-kerf centered on FO. Also, as shown by the broken line in Figure 5, the AFC control voltage is set to a value H higher than the reference voltage.
When slicing with is not higher than FH, it deviates from the S-curve centered on Fv and is drawn into the S-curve centered on FO, and when the first local oscillation frequency becomes FO, the image as well as the video can be received normally.

It is clear from the above explanation that this is effective even when the power is turned on or when changing channel selection. Examples of specific effects will be explained. Conventionally, when the first local oscillation frequency moves from low to high during both tuning switching and manual tuning, when receiving a crosshatch pattern, the input electric field 7
At 0 dB, the first local oscillation frequency is (reference frequency FO+
100KHz) could not be received properly.

However, by slicing the S curve of the AFC control voltage at VH = 6.7V, the second local oscillation frequency is FH = 65.2MHz.
If the first local oscillation frequency is moved from a low level to above (FO - 150 KHz), it will be drawn into the S curve centered on FO and can be received normally. Also, you will be able to receive signals within the above range even when switching channels. However, FO=64.95MHz,
．． (V0 = 5.7V) Furthermore, if the AFC control voltage is temporarily fixed at VL2 3.7V (FL: 64.4MHz) when switching the channel selection, the first local oscillation frequency will be set to FO-550KH.
Normal reception is possible even if the signal is shifted as low as z.

As described above, with this device, it is possible to prevent malfunctions when the first local oscillation frequency moves from low to high when changing tuning, turning on the power, or adjusting tuning.
Above, the second local oscillation frequency is 64.

Although we have explained the case where the second local oscillation frequency is set higher than 95MHz and the first audio intermediate frequency of 54.25MHz, when the second local oscillation frequency is set lower than the first audio frequency, the relationship between the second audio frequency and the AFC control voltage in Fig. 5 is Although the former is the opposite, FIGS. 6 and 7 are similar to the former, so the above explanation can be applied as is.

As described above, according to the present invention, when the power is turned on or when switching to a channel, a voltage lower than the AFC control voltage in a normal tuned state is applied from the constant voltage circuit to the second local oscillator instead of the AFC control voltage. , by temporarily fixing the second local oscillation frequency to a frequency lower than the reference tuning state frequency during the above period, the first local oscillation frequency can be set to a specific frequency in order to improve the image especially in ghosts, weak electric field areas, etc. Even if the channel is set to a low level, audio can always be received normally, images with excellent image quality can be obtained, and audio can always be obtained normally, which is extremely effective in practical terms. It is. In addition, in the above-mentioned control, a switching circuit is provided between the second AFC circuit and the second local oscillator, and a circuit that generates the above-mentioned constant voltage is also provided, so that the pulse that is always generated when the power is turned on or when changing the channel selection is provided. Since the switching circuit is simply controlled using the constant voltage to be applied to the second local oscillator instead of the AFC control voltage, the configuration can be simplified.

[Brief explanation of the drawing]

1 and 2 are block diagrams of a conventional automatic frequency control device, FIG. 3 is a block diagram of an automatic frequency control device according to an embodiment of the present invention, and FIG. 4 is a block diagram of a conventional automatic frequency control device. Part of the circuit diagram, FIG. 5, FIG. 6, and FIG. 7 are characteristic diagrams for explaining the device. 1... Tuner, 2... Video intermediate frequency amplifier, 5... First audio intermediate frequency amplifier, 6
...Mixer, 7...Second local oscillator,
8...Second audio intermediate frequency amplifier, 9...
・Audio FM detector, 10...Second AFC circuit,
11... Switching circuit, 12...
Power-on detection circuit, 13...Tuition selection switching detection circuit, 14...Constant voltage circuit.

Claims (1)

[Claims] 1 A tuner for selectively processing a television signal of a specific channel from input television signals, a means for demodulating a video intermediate frequency signal output from the tuner, and a means for controlling a first local oscillation signal of the tuner. a first AFC circuit that is intermittently inserted with the demodulation processing means;
A mixer that inputs the audio intermediate frequency signal output from the tuner, a means for amplifying and detecting the audio intermediate frequency output signal from the mixer, and controlling a second local oscillation signal supplied to the mixer. a second AFC circuit for applying AFC to the second local oscillator, and a constant voltage circuit for outputting a voltage having a value that controls the tuning frequency to be lower than the AFC control voltage in a normal tuning state output from the second AFC circuit. Then, during the period when the power-on detection circuit or the channel selection switching detection circuit and the detection output pulse are not output from the detection circuit, the AFC control voltage of the second AFC circuit is supplied to the second local oscillator, and the detection output pulse is output from the second local oscillator. A switching circuit is provided which supplies the output voltage of the constant voltage circuit to the second local oscillator instead of the AFC control voltage during that period when the AFC control voltage is output. An automatic frequency control device characterized in that a local oscillation frequency is fixed at a frequency lower than a reference tuning state frequency for a certain period of time.