JPH0720811A - Horizontal afc circuit - Google Patents

Abstract

PURPOSE:To provide fixed pull-in sensitivity even when horizontal synchronizing signals with various frequencies are inputted by varying a pulse width of an output pulse of a waveform shaping circuit according to the frequency of the horizontal synchronizing signal. CONSTITUTION:This circuit is provided with the waveform shaping circuit generating a fixed width pulse according to the horizontal synchronizing signal, a phase comparator 1 comparing phase of the output pulse of the waveform shaping circuit with phase of a flyback pulse from a horizontal deflection coil and a smoothing circuit 2 smoothing the output signal of the phase comparator 1. Further, the circuit is constituted so as to be provided with a horizontal oscillation circuit 3 whose oscillation frequency is changed according to the output DC signal of the smoothing circuit 2 and impressing the oscillation output signal to the horizontal deflection coil and a control circuit 8 varying the pulse width of the output pulse of the waveform shaping circuit according to the frequency of the horizontal synchronizing signal. Thus, even when the horizontal synchronizing signals with various frequencies are inputted, fixed pull-in sensitivity is provided. Further, the sensitivity of a horizontal AFC circuit is fixed without increasing the number of elements and increasing the external components of an IC.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a horizontal AFC (automatic frequency control) circuit of a CRT display device used in a computer or the like, and more particularly to a horizontal AFC circuit capable of supporting horizontal synchronizing signals of various frequencies.

[0002]

2. Description of the Related Art A horizontal AFC circuit is known which locks on a horizontal synchronizing signal in a video signal and stably applies a horizontal output pulse to a horizontal deflection coil. In the TV receiver, the pulse width of the horizontal synchronizing signal applied to the horizontal AFC circuit is almost constant. However, in a CRT display device used in a computer or the like, video signals from various sources are applied, and the pulse width of the horizontal synchronizing signal changes variously depending on the sources. For example,
The pulse widths of horizontal sync signals having the same frequency of 30 KHz may differ. When the pulse width is different, there is a problem that the operation time of the phase comparator in the horizontal AFC circuit at the subsequent stage is different and the response sensitivity of the horizontal AFC circuit is changed. Therefore, as shown in FIG. 2, an MM (monostable multivibrator) (5) is provided in front of a horizontal AFC circuit (4) including a phase comparator (1), a smoothing circuit (2) and a horizontal oscillator circuit (3). It is conceivable that the pulse width of the horizontal synchronizing signal is once inserted and the waveform is once shaped into a constant value.

According to the apparatus of FIG. 2, the horizontal synchronizing signal can be locked with a constant sensitivity regardless of the pulse width of the horizontal synchronizing signal.

[0004]

In the device of FIG. 2, the frequency of the horizontal synchronizing signal also changes depending on the source. In that case, the center frequency of the horizontal oscillator circuit (3) is controlled by the control signal from the terminal (6) to control transistor (7).
The current is changed to switch continuously. that way,
Two signals of equal frequency are applied to the phase comparator (1) and the horizontal AFC circuit (4) can lock to the input signal. However, in the apparatus of FIG. 2, the pulse width of the output pulse of M · M (5) does not change even if the frequency of the horizontal synchronizing signal changes. The pulse width defines the phase comparison operation execution time in the phase comparator (1) of the horizontal AFC circuit (4), and that time will not change. On the other hand,
During the period when the phase comparator (1) is not operating, it changes according to the frequency of the horizontal synchronizing signal.
There is a problem that the pull-in sensitivity of the C circuit (4) changes according to the frequency. In order to make the pull-in sensitivity constant, the time constant of the smoothing circuit (2) may be adjusted, but the increase in the number of elements and the increase in external parts when integrated into an IC poses a problem. Further, it is difficult to continuously change the time constant.

[0005]

The present invention has been made in view of the above points, and a waveform shaping circuit for generating a pulse having a constant width in response to a horizontal synchronizing signal, and an output pulse of the waveform shaping circuit and a horizontal pulse A phase comparator that performs phase comparison with the flyback pulse from the deflection coil, a smoothing circuit that smoothes the output signal of the phase comparator, and an oscillation frequency that changes according to the DC signal output from the smoothing circuit A horizontal oscillation circuit that applies an output signal to the horizontal deflection coil, and a control circuit that changes the pulse width of the output pulse of the waveform shaping circuit according to the frequency of the horizontal synchronization signal.

[0006]

According to the present invention, the pulse width of the output pulse generated from the waveform shaping circuit is changed according to the frequency of the incoming horizontal synchronizing signal. When the frequency is low, the pulse width is lengthened, and when the frequency is high, the pulse width is shortened. Therefore, the ratio of the phase comparison period (pulse width) per one cycle of the horizontal synchronizing signal is always constant regardless of the frequency of the horizontal synchronizing signal, and the sensitivity of the horizontal AFC circuit can be constant.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an embodiment of the present invention, in which (8) shows M.multidot.M in response to a control signal from a terminal (6) for controlling the center frequency of a horizontal oscillation circuit (3). It is a control transistor that changes the pulse width of the output pulse of M (5). In FIG. 1, the same circuit blocks as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted.

A direct current voltage corresponding to a desired horizontal oscillation frequency is applied to the terminal (6) of FIG.
A direct current flows through the collectors of (8) and (8), respectively, and a signal of period T ₁ is generated from the horizontal oscillation circuit (3).
From (5), an output pulse having a pulse width T ₃ corresponding to the period T ₁ is generated. A horizontal FBP (flyback pulse) generated from the horizontal deflection coil (9) and having a period T ₁ as shown in FIG. 3A is smoothed by the smoothing circuit (10) in the line A of FIG. Such a sawtooth wave is converted, the direct current component is removed by the direct current blocking capacitor (11), and the result is applied to the phase comparator (1).

On the other hand, an output pulse having a pulse width of T ₃ as shown in FIG. 3C is generated from MM (5) and applied to the phase comparator (1). From this state, if the frequency of the horizontal synchronizing signal applied to the input terminal (12) is reduced to 1/2, the frequency of the horizontal oscillation circuit (3) needs to be reduced by that amount, so that a lower voltage is applied to the terminal (6), for example. Is applied. Then, a pulse having a period T ₂ of FIG. 3D is generated on the line A of FIG. 1, and the smoothing circuit (10) produces a pulse as shown in FIG. 3E, which is applied to the phase comparator (1).

On the other hand, MM (5) also doubles the width of its output pulse according to the control signal from the terminal (6),
The pulse shown in FIG. 3 (f) (pulse width T ₄ = 2T ₃ ) is generated. Here, since the ratio of the period T ₁ : T ₃ and the period T ₂ : T ₄ in FIG. 3 is constant, the response sensitivity of the horizontal AFC circuit (4) remains unchanged. Therefore, according to the apparatus of FIG. 1, the horizontal AFC circuit operates with a constant response sensitivity even when horizontal synchronizing signals of different frequencies are applied.

FIG. 4 shows an example of a concrete circuit of MM (5) of FIG. 1, which is RS-FF (RS type flip-flop).
(13), first and second transistors (14) and (15), current mirror circuit (16), charge and discharge capacitor (17), and first and second comparators (18) and (19) Consists of. In the initial state, RS-F
Since F (13) is reset, the first transistor (14) is off, and the second transistor (15) is on, the current mirror circuit (16) operates and the charge / discharge capacitor (17) discharges. It is in a state. Therefore, the voltage at point C is at the power supply voltage (+ V _CC ) level,
The outputs of the first and second comparators (18) and (19) are both at "L" level.

Now, when the horizontal synchronizing signal of FIG. 5 (a) is applied to the RS-FF (13) from the input terminal (12), its Q output becomes as shown in FIG. 5 (b), and the first transistor ( 1
4) is turned on, the second transistor (15) is turned off, and the constant current of the control transistor (8) charges the charge / discharge capacitor (17). Then, the voltage at point C in FIG.
When the voltage drops linearly as shown in FIG. 5 (c) and becomes lower than the reference voltage V _{1 of} the first comparator (18), the output of the first comparator (18) becomes “H” level and RS−
Reset the FF (13) and charge / discharge capacitor (17)
Is rapidly discharged, the voltage rapidly rises toward the power supply voltage. As a result, the output of the first comparator (18) is
As shown in FIG. 5 (d), the output V _out of the second comparator (19) becomes as shown in FIG. 5 (e). The rising timing of the output pulse of FIG. 5 (e) is determined by the charging curve of FIG. 5 (c) when the reference voltage V ₂ is constant, so that it depends on the DC voltage applied to the terminal (6) of the control transistor (8). it is possible to determine the pulse width of the output pulse V _{ou t.}

[0013]

As described above, according to the present invention, it is possible to provide a horizontal AFC circuit having a constant pull-in sensitivity even when horizontal synchronizing signals of various frequencies arrive. In particular, according to the present invention, since the pulse width of the output pulse is changed by the waveform shaping circuit according to the frequency of the horizontal synchronizing signal, the horizontal AFC circuit is not brought about without increasing the number of elements and the external parts of the IC. The sensitivity of can be made constant.

[Brief description of drawings]

FIG. 1 is a block diagram showing a horizontal AFC circuit of the present invention.

FIG. 2 is a block diagram showing a conventional horizontal AFC circuit.

FIG. 3 is a waveform chart for explaining FIG.

4 is a specific circuit diagram of MM (5) of FIG.

FIG. 5 is a waveform diagram for explaining FIG.

[Explanation of symbols]

 (1) Phase comparator (2) Smoothing circuit (3) Horizontal oscillation circuit (5) MM (8) Control transistor

Claims (3)

[Claims] 1. A horizontal AFC circuit that locks to a horizontal synchronizing signal of a plurality of frequencies, the waveform shaping circuit generating a pulse of a constant width according to the horizontal synchronizing signal, and an output pulse of the waveform shaping circuit and a horizontal pulse. A phase comparator that performs phase comparison with the flyback pulse from the deflection coil, a smoothing circuit that smoothes the output signal of the phase comparator, and an oscillation frequency that changes according to the DC signal output from the smoothing circuit A horizontal AFC circuit comprising: a horizontal oscillation circuit that applies an output signal to the horizontal deflection coil; and a control circuit that changes the pulse width of the output pulse of the waveform shaping circuit according to the frequency of the horizontal synchronization signal. . 2. The horizontal AFC circuit according to claim 1, wherein the control circuit changes an oscillation frequency of the horizontal oscillation circuit. 3. The horizontal A according to claim 1, wherein the waveform shaping circuit is a monostable multivibrator.
FC circuit.