JP2006088646A - Video clock correction circuit for laser beam scanning optical system in tandem color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To finely control a video clock frequency for a laser beam scanning optical system in a tandem color image forming apparatus, using a PLL. <P>SOLUTION: In an image forming region, a video clock is corrected as an open loop control, and a ceramic oscillator 51 is used in a voltage control oscillator 5. Then, even in a state where frequency follow-up property to correction operation is improved in the image forming region, the oscillation frequency is narrowed in range by a voltage control oscillator 5, to enable the video clock frequency being finely controlled. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technique for correcting a video clock of a laser beam scanning optical system in a tandem color image forming apparatus.

  Conventionally, the length magnification adjustment in the laser beam scanning direction in the laser beam scanning optical system of the tandem color image forming apparatus is performed by using a PLL (Phase Locked Loop), and a video clock of the laser beam scanning optical system for each color. In recent years, a video clock for correcting a difference in image size in the scanning direction due to non-uniformity in the lens such as an fθ lens has been developed with the improvement in image quality. Correction has become necessary.

In the PLL, normally, closed-loop control is performed, and the frequency resolution is required to be very high. Therefore, the frequency band is designed to be low by a low-pass filter. For this reason, even if the video clock is corrected, the correction operation cannot be followed due to the low frequency band, and the correction response cannot be made. Therefore, as shown in Patent Document 1, the PLL is provided with a function of switching between closed loop control and open loop control, and when the video clock is corrected, it is switched to open loop control so that the frequency can follow the correction operation. A technique for correcting the video clock has been proposed.
JP-A-9-152561

  However, as described above, when the video clock is corrected using the PLL as an open loop control, the frequency can be made to follow the correction operation, but there is a problem that the fluctuation range of the frequency with respect to the correction operation becomes large. For this reason, if a voltage is applied to a VCO (voltage-controlled oscillator) when the video clock is changed, the frequency changes greatly even with a small voltage, so that the frequency cannot be controlled finely.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a video clock correction circuit capable of finely controlling the video clock frequency of a laser beam scanning optical system in a tandem color image forming apparatus. It is what.

The invention according to claim 1 of the present invention is a video clock correction circuit of a laser beam scanning optical system in a tandem color image forming apparatus,
A reference frequency generation circuit for generating a reference frequency;
An oscillation circuit for oscillating a frequency according to the control signal;
A phase difference detection circuit for detecting a phase difference between a reference frequency generated by the reference frequency generation circuit and a frequency to be compared;
A control signal generation circuit that generates a control signal to be input to the oscillation circuit based on the phase difference detected by the phase difference detection circuit;
Switching means for switching to open loop control in the non-image forming area and closed loop control in the image forming area,
Correction for generating a control signal for correction based on the control signal generated by the control signal generation circuit when the switching means is in open loop control in order to change the frequency oscillated by the oscillation circuit A control circuit generating circuit, and a PLL circuit comprising:
A ceramic oscillator is used for the oscillation circuit.

  In this configuration, when a ceramic oscillator is used for the oscillation circuit and the clock is corrected by open loop control, the ceramic oscillator oscillates the clock according to the control signal for correction, so open loop control in the image forming area is performed. Sometimes, even when the frequency followability to the correction operation is increased, the range of the oscillation frequency can be narrowed, and the frequency can be finely controlled.

  Also, while improving the response by correcting the clock with the correction control signal during open loop control in the image forming area, the frequency is locked to the frequency according to the reference frequency during closed loop control in the non-image forming area. Therefore, the length magnification adjustment in the laser beam scanning direction and the video clock correction can be made compatible in the tandem color image forming apparatus.

  The invention according to claim 2 is the video clock correction circuit of the laser beam scanning optical system according to claim 1, wherein the oscillation circuit has a varicap diode connected to the ceramic oscillator. The frequency according to the control signal or the correction control signal is oscillated by changing the frequency at which the ceramic oscillator oscillates in accordance with a change in voltage applied to the cap diode. .

  In this configuration, changing the voltage applied to the varicap diode connected to the ceramic oscillator changes the oscillation frequency, resulting in a large change in frequency despite the small amount of voltage change. The frequency can be finely controlled and a stable clock with little jitter can be generated.

  According to a third aspect of the present invention, there is provided the video clock correction circuit of the laser beam scanning optical system according to the first or second aspect, wherein the non-image forming region is a time interval between sheets. .

  According to this configuration, since the non-image forming area is the inter-paper time area, it is possible to apply video clock correction for each main scan of the laser beam scanning optical system in the image forming area. Since it can be locked to the reference frequency clock, the image quality can be stabilized, the correction frequency can be reduced, and the control burden can be reduced.

  According to the first aspect of the present invention, in order to correct the video clock of the laser beam scanning optical system, the open-loop control is performed in the image forming area, and even if the frequency tracking performance with respect to the correction operation is increased, The range can be narrowed and the frequency can be finely controlled. In addition, it is possible to achieve both the length magnification adjustment in the laser beam scanning direction and the video clock correction in the tandem color image forming apparatus.

  According to the second aspect of the present invention, when a voltage is applied to the voltage controlled oscillation circuit for changing the video clock of the laser beam scanning optical system, the frequency does not change greatly with a small voltage change. The frequency can be finely controlled, and a stable clock with little jitter can be generated.

  According to the invention described in claim 3, since the non-image forming area is the inter-paper time area, in the image forming area, it becomes possible to apply video clock correction for each main scanning of the laser beam scanning optical system, Since it becomes possible to lock to the reference frequency clock between the papers in the non-image forming area, the image quality can be stabilized, the correction frequency can be reduced, and the control burden is reduced. be able to.

  Hereinafter, a video clock correction circuit of a laser beam scanning optical system according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a basic configuration of a video clock correction circuit according to an embodiment of the present invention. A video clock correction circuit 1 according to an embodiment of the present invention corrects a video clock of a laser beam scanning optical system in a tandem color image forming apparatus (printer, copier, fax, etc.). For example, the video clock correction circuit 1 includes a reference frequency generation circuit 21, a frequency divider 22, a phase comparator 3, a low-pass filter 4, a voltage controlled oscillator (VCO) 5, and a second frequency divider 6. In this embodiment, the phase comparator 3, the low-pass filter 4, the voltage controlled oscillator 5, and the second frequency divider 6 generate a frequency N times the reference frequency.

  The reference frequency generation circuit 21 generates a reference frequency. The frequency divider 22 divides the reference frequency Fref output from the reference frequency generation circuit 21 into 1 / M, for example, to generate a reference signal. The phase comparator 3 detects the phase difference between the reference signal and the comparison signal by comparing the reference signal generated by the frequency divider 22 with a comparison signal generated by the second frequency divider 6 described later. Then, an error signal corresponding to the phase difference is output.

  The low-pass filter 4 smoothes the error signal from the phase comparator 3 and determines the control signal (voltage) of the voltage controlled oscillator 5. The voltage controlled oscillator (oscillation circuit) 5 oscillates the frequency according to the control signal (voltage) output from the low pass filter 4. The second frequency divider 6 divides the frequency output from the voltage controlled oscillator 5 by 1 / N and outputs the result to the phase comparator 3.

  The video clock correction circuit 1 is normally closed-loop controlled, and generates and outputs an N / M frequency of the reference frequency Fref. The switching between the closed loop control and the open loop control is performed, for example, by outputting a signal indicating a high impedance state from the phase comparator 3 by a control signal input from the CPU (not shown) to the phase comparator 3. It is like that. As another method, although not shown, the same function can be achieved even if the low-pass filter output is constituted by a sample hold circuit.

  The DAC (digital-analog converter) 7 generates a correction control signal to be input to the voltage controlled oscillator 5 by adding a voltage to the output from the low-pass filter 4 while being switched to the open loop control. When this correction control signal is input to the voltage controlled oscillator 5, the frequency oscillated from the voltage controlled oscillator 5 is modulated in real time and modulated to an arbitrary frequency. In order to avoid the influence of the oscillation frequency due to the direct current component of the DAC, it is preferable to cut and add the DC component due to the capacitor or the like.

  The switching between the closed loop control and the open loop control is switched depending on whether the video clock output by the video clock correction circuit 1 is performed on an image forming area or a non-image forming area by the laser beam scanning optical system. Preferably, the switching is controlled so that the video clock is closed-loop controlled in the non-image forming area, and the video clock is corrected by the open-loop control in the image forming area.

  The resolution is determined by increasing the value of 1 / M of the frequency divider 22 and the second frequency divider 6, particularly the frequency divider 22. If the value of M is increased, the frequency band of the low-pass filter 4 must be set to the low frequency side in order to ensure the stability of the feedback system, and quick response becomes difficult. Since the correction in the main scanning direction is higher than the band of the low-pass filter 4, it cannot be corrected in the closed loop state. For this reason, in the image forming area, as the open loop control, the voltage from the magnification correction circuit is directly applied to the voltage controlled oscillator 5 to correct the frequency by clock modulation.

  In the non-image forming area, as a closed loop control, the PLL is operated to generate and output a magnification adjustment frequency of N / M of a preset reference frequency Fref. When the image area is set to open loop control, the output of the voltage controlled oscillator 5 becomes a fixed output. Therefore, the original clock output slightly deviates from the reference frequency due to leakage current, temperature, etc. As a closed-loop control again in the region, the frequency difference is restored to prevent the occurrence of a large difference.

  This improves the responsiveness by correcting the video clock during open loop control in the image forming area, while the video clock of the laser beam scanning optical system follows the magnification adjustment value during closed loop control in the non-image forming area. Since the clock is locked, the length magnification adjustment in the laser beam scanning direction and the video clock correction can both be achieved.

  Further, it is preferable that the non-image forming area is a time interval between sheets. In this case, the video clock can be corrected for each main scan, the image quality can be stabilized, and the magnification is adjusted between papers, so that the correction frequency can be reduced. The burden of control can be reduced.

  FIG. 2 is a diagram showing a schematic configuration of the voltage controlled oscillator 5. The voltage controlled oscillator 5 is provided with a ceramic oscillator 51. One end of a capacitor 52 is connected to one end of the ceramic oscillator 51, and one end of a capacitor 53 is connected to the other end of the ceramic oscillator 51. The other end of the capacitor 53 is connected to the cathode of a varicap diode 54. A resistor 55 and an inverter 56 are connected to the ceramic oscillator 51 in parallel. The other end side of the capacitor 52 and the anode side of the varicap diode 54 are grounded.

The ceramic oscillator 51 oscillates with electric power supplied from an unillustrated power source via an inverter 56. The resonance frequency of the ceramic oscillator 51 is determined according to a time constant based on the combined capacitance of the capacitor 53 and the varicap diode 54, the capacitance of the capacitor 52, and the resistance value of the resistor 55. In a state where the ceramic oscillator 51 is oscillating, the voltage Vctl is applied to the varicap diode 54, the capacitance C _D of the varicap diode 54 becomes a value corresponding to the voltage Vctl. As a result, an oscillation frequency corresponding to the voltage Vctl applied to the varicap diode 54 is output from the output terminal of the inverter 56. That is, the frequency generated by the ceramic oscillator 51 can be controlled by changing the voltage Vctl applied to the varicap diode 54.

  A resistor 57 is connected to the capacitor 53 and the varicap diode 54. The other end of the resistor 57 is connected to the low pass filter 4 and the DAC 7. A capacitor 58 is connected between the resistor 57 and the low-pass filter 4 and the DAC 7.

  As described above, by using the ceramic oscillator 51 as the oscillator of the voltage controlled oscillator 5, even when the video clock correction is performed as an open loop control, the frequency followability to the correction operation is improved. The frequency range oscillated by is narrow. Therefore, the frequency oscillated from the voltage controlled oscillator 5 does not change greatly even though the amount of change in the voltage input to the voltage controlled oscillator 5 (voltage Vctl applied to the varicap diode 54) is small. Therefore, by changing and controlling the voltage input to the voltage controlled oscillator 5, the frequency oscillated from the voltage controlled oscillator 5 can be finely controlled.

  The present invention is not limited to the configuration of the above embodiment, and various modifications can be made. In the above embodiment, the configuration of the video clock correction circuit 1 is shown in FIG. 1, and the configuration of the voltage controlled oscillator 5 is shown in FIG. 2, but these configurations are merely examples, and the video clock correction circuit according to the present invention is shown. 1. The configuration of the voltage controlled oscillator 5 is not limited to the above configuration.

It is a block diagram which shows the basic composition of the video clock correction circuit which concerns on one Embodiment of this invention. It is a figure which shows schematic structure of a voltage controlled oscillator.

Explanation of symbols

1 Video Clock Correction Circuit 21 Reference Frequency Generation Circuit 22 Frequency Divider 3 Phase Comparator 4 Low Pass Filter 5 Voltage Controlled Oscillator 51 Ceramic Oscillator 51 Capacitor 53 Capacitor 54 Varicap Diode 55 Resistor 56 Inverter 57 Resistor 58 Capacitor 6 Second Component Circulator

Claims (3)

A video clock correction circuit of a laser beam scanning optical system in a tandem color image forming apparatus,
A reference frequency generation circuit for generating a reference frequency;
An oscillation circuit for oscillating a frequency according to the control signal;
A phase difference detection circuit for detecting a phase difference between a reference frequency generated by the reference frequency generation circuit and a frequency to be compared;
A control signal generation circuit that generates a control signal to be input to the oscillation circuit based on the phase difference detected by the phase difference detection circuit;
Switching means for switching to open loop control in the non-image forming area and closed loop control in the image forming area,
Correction for generating a control signal for correction based on the control signal generated by the control signal generation circuit when the switching means is in an open loop control in order to change the frequency oscillated by the oscillation circuit A control circuit generating circuit, and a PLL circuit comprising:
A video clock correction circuit of a laser beam scanning optical system in which a ceramic oscillator is used for the oscillation circuit.   The oscillation circuit includes a varicap diode connected to the ceramic oscillator, and changes the frequency at which the ceramic oscillator oscillates in accordance with a change in voltage applied to the varicap diode, whereby the control signal or 2. The video clock correction circuit for a laser beam scanning optical system according to claim 1, wherein a frequency corresponding to the correction control signal is oscillated.   The video clock correction circuit of the laser beam scanning optical system according to claim 1, wherein the non-image forming area is a time interval between sheets.

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