| 摘要 |
PROBLEM TO BE SOLVED: To provide a frequency control system simple in circuit configuration, high in pull-in speed and capable of maintaining frequency at the time of synchronism even if a synchronizing signal is not inputted from outside. SOLUTION: A control micro computer 11 incorporates a counter 15 and a timer 16. The counter 15 counts the output Fs1 of VCO 12. The control micro computer 11 reads the value of the counter 15 at the input timing of the synchronizing signal SYNC when the synchronizing signal SYNC is inputted from outside and at timing generated inside when the synchronizing signal SYNC is not inputted, compares it with an inner target value, outputs an up pulse PU or a down pulse PD to VCO 12 based on the compared result and controls the output frequency of VCO 12. The timer 16 outputs a switch signal SEL for changing over a switch 14 when the synchronizing signal SYNC is not inputted to the control micro computer 11 for prescribed time. |
