Patent ID: 6975148

Claim:
A spread spectrum clock generation circuit comprising: a frequency phase comparator for detecting a difference in phase between a reference clock and a generated clock; a charge pump for generating a charge/discharge signal in accordance with the difference in phase detected by the frequency phase comparator; a loop-filter for generating a differential signal in accordance with the charge signal; a spread spectrum modulation circuit for generating a spread spectrum modulation signal by modulating the differential signal so that the differential signal changes with an amplitude, which is smaller than that of the differential signal, and with a spread spectrum modulation period which is sufficiently longer than that of the generated clock; and a clock generator for generating the generated clock with a frequency in accordance with the spread spectrum modulation signal, wherein the spread spectrum modulation period of the spread spectrum modulation signal changes so as to have multiple different periods, wherein the clock generator is a voltage control oscillator, wherein the spread spectrum modulation circuit comprises an analog modulator for generating a spread spectrum analog voltage signal the period of which changes so as to take multiple different periods, and a voltage addition circuit for adding the spread spectrum analog voltage signal to the differential signal, wherein the analog modulator comprises a plurality of different capacitors, a plurality of switches for selecting one of the plurality of different capacitors, a constant current source for supplying a constant current to the selected capacitor or making the constant current flow out of the selected capacitor, a hysteresis comparator for detecting the fact that the voltage of the selected capacitor reaches first and second predetermined voltages and a switch control circuit for switching the selected plurality of switches when the hysteresis comparator detects the fact that the first and second predetermined voltages are reached.