| 摘要 |
PURPOSE: To set the free-running oscillation frequency of the VCO of a phase locked loop(PLL) circuit by incrementing or decrementing the output frequency of a voltage controlled oscillator(VCO) according to the size of a count value stored in a register and an expected count value. CONSTITUTION: A comparator 34 compares an actual count value M of a VCO output pulse stored in a register 35 with an expected count value R of the preset VCO output pulse in response to the previous strobe pulse when conditioned by a strobe pulse. Then, when M<R in this comparison, the output of the comparator 34 increments a counter 32, and when M>R, it decrements the counter 32. Thus, it is not necessary to operate the laser trimming of a VCO 12 for setting a VCO free-running oscillation frequency, and the free- running oscillation frequency of the VCO 12 is held in the lock range of a phase locked loop(PLL) circuit. |
