| 摘要 |
PROBLEM TO BE SOLVED: To provide a frequency synthesizer that can improve the frequency switching speed, while keeping the S/N characteristic of FM modulation satisfac tory. SOLUTION: In the frequency synthesizer including a reference signal circuit 11, a frequency divider 12 that frequency-divide a reference signal into a comparison frequency, a phase comparator 13, a low-pass filter circuit 14 including a loop filter, a voltage-controlled oscillator 15, and a variable frequency divider 16 for frequency-dividing the voltage-controlled oscillator, when the frequency is much varied, a switching signal from a control section 9 fixes a smaller frequency division ratio of a prescaler 17, a residue arithmetic operation is ignored, and the frequency divider for a reference signal is witched to enhance the comparison frequency. Synchronously with the operation, the frequency is matched with the comparison frequency, to enhance the band characteristic of the low-pass filter circuit 14 with a witching signal from the control section 9. After the lapse of a prescribed time, by restoring normal state the frequency switching speed is improved, while keeping the S/N characteristic satisfactory. |
