PURPOSE: To reduce the lockup time at the change of the setting frequency without reducing the purity of an output signal by providing a changeover section short-circuiting a low pass filter only for a prescribed time at changeover of the setting frequency.
CONSTITUTION: In the even of a change of a setting signal fr outputted from a PLL arithmetic section 1 resulting from the revision of a setting frequency data DA< an analog switch 8 is closed based on an input of a strobe signal STB. Thus, an output signal SG1 of a charge pump 4 is outputted directly to a VCO 6 not through an LPF 5. Thus, an output signal SG 3 of the VCO 6 is shifted quickly up to a newly set frequency F2 from the original frequency F1 in a slight lockup time t2. Thus, after the strobe signal STB goes to an L level, an output signal SG 3 without any distortion based on the output signal SG 2 of the LPF 5 is outputted from the VCO 6.
AKIYAMA TAKEHIRO
TAKEGAWA KOUJI
FUJITSU VLSI LTD
| JPS5533554B2 | 1980-09-01 |