JPS6028164B2 - automatic frequency control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic frequency frequency control circuit that automatically controls the local oscillation frequency of a tuner of a television receiver, and an object of the present invention is to reduce the number of adjustment points.

A follow-ratio frequency discrimination circuit is mainly used as an automatic frequency control circuit, but it has the drawback of requiring two tuning circuits.

In addition, even when the automatic frequency control output is obtained by coupling and amplifying the video carrier wave used for video synchronous detection using a tuning circuit and capacitor to extract the video carrier wave, and performing synchronous detection at a phase of 9 pins with the original video intermediate frequency signal, the automatic control frequency Adjustments still exist for the control circuit. FIG. 1 shows the configuration of an automatic frequency control circuit according to the present invention. Reference numeral 1 denotes a video synchronous detection circuit, which detects a video intermediate frequency signal from a terminal 2 and outputs the video signal to a terminal 3. 4 is a differential amplifier circuit that amplifies part of the signal input to terminal 2;

Reference numeral 5 denotes a tuning circuit for extracting a video carrier wave component from the output of the differential amplifier circuit 4, and an inductance 6 and a capacitor 7 are selected so that the tuning frequency matches the video carrier wave frequency in the video intermediate frequency band, and both ends of the tuning circuit 5 are are connected to the differential output of the difference amplifier 4, and are configured so that video carrier waves having different polarities can be obtained at both ends.

Therefore, the inductance 6 and capacitor 7 of the tuned circuit
It is also desirable to connect both terminals in a balanced manner. A part of the video carrier wave component extracted by the tuning circuit 5 is combined in phase with the video carrier wave supplied from the terminal 2 to the video synchronous detection circuit 1, and is used for synchronous detection of the signal input from the terminal 2. used. Reference numeral 8 denotes a first amplitude detection circuit whose input is connected to one end of the tuned circuit 5 through a ceramic resonator 9; second
An amplitude detection circuit 12 is an adder circuit which adds the outputs of the first and second amplitude detection circuits 8 and 10 to obtain a local oscillation frequency automatic control voltage of the tuner of the television receiver at a terminal 13.

In this way, the ceramic resonator 9 is connected to one of the balanced terminals of the tuned circuit 5, and the capacitor 11 is connected to the other.
By connecting , it becomes easy to maintain balance in terms of the connection arrangement and load of the tuning circuit 5 except near the resonance frequency of the ceramic resonator 9 . Note that the addition circuit 12
is configured to operate differentially when the detection polarities of the first and second amplitude detection circuits 8 and 10 are the same, and to operate additively when the detection characteristics are different. Also, the ceramic resonator 9
The capacitance between the electrodes and the capacitance of the capacitor 11 are selected to be approximately the same, and the ceramic resonator 9 is selected so that the image carrier frequency is located approximately at an intermediate value between the resonant frequency and the anti-resonant frequency. Now, assuming that the Q of the tuning circuit 5 is sufficiently low, and considering the vicinity of the resonance frequency of the ceramic resonator 9, the result will be as shown by the solid line in FIG.

FIG. 2 shows the amplitude characteristics of the high frequency signal applied to the input of the amplitude detection circuit 8 via the ceramic resonator 9 with respect to frequency. f, is the ceramic resonator 9
The impedance of the ceramic resonator 9 becomes the lowest at the series resonance frequency of , and therefore the input of the amplitude detection circuit 8 becomes the largest. On the other hand, at the parallel resonance frequency of the ceramic resonator 9, the impedance of the ceramic resonator 9 becomes the highest and the output amplitude becomes the lowest. Furthermore, the characteristic indicated by the broken line is the amplitude of the high frequency signal applied from the tuning circuit 5 to the input of the amplitude detection circuit 10 via the capacitor 11. The ceramic resonator 9 and the capacitor 11 are connected to different points of the tuning circuit 5, and the phases of the high-frequency signals are opposite, but when considering the amplitude, the phase is irrelevant, and as a result, even the detection polarity of the amplitude detection circuits 8 and 10 is If it is the same, the third
The outputs shown by the solid and broken lines in the figure are obtained respectively. That is, assuming that the amplitude detection circuits 8 and 10 have the same detection magnetism, the addition circuit 12 is designed to operate differentially. As a result, the output of the adder circuit 12 is obtained by subtracting the broken line from the solid line in FIG. The output at this time is DC
If an offset is added and displayed, the result will be as shown in FIG. 3, which is a normal local oscillation frequency automatic control voltage for a tuner, which is obtained at the terminal 13. With this configuration,
The center frequency of the discrimination output, which is the automatic control voltage obtained at the terminal 13, is uniquely determined by the resonance frequency of the ceramic resonator 9, and there is no need to adjust the center frequency of the discrimination output. As explained above, according to the present invention, the following effects can be obtained.

(1) Since the center frequency of the automatic frequency control output can be uniquely determined by the ceramic resonator, no adjustment is necessary.

(b) Since the tuning circuit in which the inputs of the first and second amplitude detection circuits are connected via a ceramic resonator and a capacitor is also used to supply a video carrier wave to the video synchronous detection circuit, It requires less tuning circuitry than conventional automatic frequency control circuits.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the automatic frequency control circuit of the present invention, FIG. 2 is a frequency characteristic diagram of a ceramic resonator and a capacitor, and FIG. 3 is a frequency characteristic diagram of an automatic control voltage. 1... Video synchronous detection circuit, 5... Tuning circuit, 8...
- First amplitude detection circuit, 10... second amplitude detection circuit, 12... addition circuit. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 A video synchronous detection circuit that synchronously detects a video intermediate frequency signal of a television signal based on a reference video carrier wave and outputs a video signal; A differential amplifier circuit to which a tuned tuning circuit is connected and outputs a video carrier wave in phase with the video intermediate frequency signal inputted to the video synchronous detection circuit as the reference video carrier wave, and a ceramic resonator having a two-terminal configuration. A first amplitude detection circuit has an input connected to one end of both ends of the tuned circuit, and an input is connected to the other end of the tuned circuit via a capacitor having a capacitance approximately equal to the interelectrode capacitance of the ceramic resonator. a second amplitude detection circuit, and an adder circuit that adds both outputs of the first and second amplitude detection circuits, and the ceramic resonator is set at a frequency approximately midway between the resonant frequency and the anti-resonance frequency of the ceramic resonator. An automatic frequency control circuit whose value is selected to be located at the video carrier frequency and whose output voltage of the adder circuit is a local oscillation frequency control voltage of a tuner of a television receiver.