| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide an oscillation frequency control circuit which can keep an oscillation frequency stably even in case of free running by correcting its own frequency, and can oscillate by a control voltage produced by making an external fixed voltage variable. <P>SOLUTION: The oscillation frequency control circuit includes a voltage controlled oscillator 15, a frequency divider 16, a phase comparator 12, a loop filter 14, a detection circuit 17, a PWM circuit 22, a memory 21, a control voltage variable circuit 26, a selection switch 13 for turning connection between the phase comparator 12 and the loop filter 14 on/off and outputting a control voltage from the control voltage variable circuit 26 selectively, and a CPU 20 for outputting the control voltage preferentially and selectively in response to a designation for selecting the control voltage, turning the selection switch 13 on if the level of an external reference signal detected by the detection circuit 17 falls within a proper range when there is no designation, and turning the selection switch 13 off if the level deviates from the proper range and outputting the information of pulse generation stored in the memory 21 to the PWM circuit 22. <P>COPYRIGHT: (C)2009,JPO&INPIT |
