Spread spectrum clock generator, spread spectrum clock generating method, and circuit, image reading device and image forming apparatus using the spread spectrum clock generator

A method for generating a spread spectrum clock signal, comprising:

• detecting phase difference between an input clock signal and a feedback clock signal while outputting a phase difference signal;
• supplying a current depending on the phase difference signal;
• converting the output current to a voltage while smoothing the voltage;
• generating a spread spectrum clock signal having a frequency depending on the smoothed voltage;
• generating a modulation signal having an amplitude depending on a modulation width set value;
• detecting the modulation width of the spread spectrum clock signal having a frequency depending on the smoothed voltage;
• comparing the detected modulation width with a predetermined modulation width target value to update the modulation width set value in such a manner that difference between the detected modulation width and the modulation width target value is narrowed;
• feeding back the updated modulation width set value for generating the modulation signal;
• adding the modulation signal generated depending on the updated modulation width set value and the smoothed voltage so that the spread spectrum clock signal is generated according to the added signal;
• and
• dividing the spread spectrum clock signal to feed back the signal as the feedback clock signal.

A spread spectrum clock generator comprising:

• means for detecting phase difference between an input clock signal and a feedback clock signal while outputting a phase difference signal;
• means for supplying a current depending on the phase difference signal output from the phase difference detecting means;
• means for converting the current output from the current supplying means to a voltage while smoothing the voltage;
• oscillating means for generating a spread spectrum clock signal having a frequency depending on the smoothed voltage;
• means for generating a modulation signal having an amplitude depending on a modulation width set value;
• means for inputting a signal obtained by adding the modulation signal generated by the modulation signal generating means and the smoothed voltage output from the current converting means to the oscillating means so that the oscillating means generates the spread spectrum clock signal according to the input signal;
• means for dividing the spread spectrum clock signal output by the oscillating means to feed back the signal to the detecting means as the feedback clock signal;
• and
• means for detecting a modulation width of the spread spectrum clock signal generated by the oscillating means while comparing the detected modulation width with a predetermined modulation width target value to update the modulation width set value in such a manner that difference between the detected modulation width and the modulation width target value is narrowed, followed by feeding back the updated modulation width set value to the modulation signal generating means.