Frequency synthesizer having quick frequency pull in and phase lock-in

U.S. Patent Number: 5389899

Abstract: A frequency synthesizer, including a phase-locked loop. The phase locked loop effects phase comparison between a comparison signal based on an output from a voltage-controlled oscillator and a reference signal based on an output from a reference oscillator. The resultant phase difference signal is submitted to a loop filter whose output serves as a control signal of the voltage-controlled oscillator. The frequency synthesizer includes a preset circuit for switching the output of the voltage-controlled oscillator by quickly charging or discharging a capacitor of the loop filter and a modifying circuit for modifying the time constant of the loop filter. The phase-locked loop is brought to phase lock at a high speed by decreasing the time constant of the loop filter when switching the output frequency.

Inventors: Yahagi; Shinya (Sendai, JP), Komatsu; Noriyoshi (Sendai, JP), Kibayashi; Toshimitsu (Kawasaki, JP), Toda; Yoshifumi (Kawasaki, JP)

Assignee: Fujitsu Limited (JP)

Application Number: 08/050,151

Issued: 1995-02-14

Expired: 2007-02-14

Classes: 331/10 ; 331/14; 331/16; 331/17; 331/176; 331/25

Field of search: 331/10,14,16,17,25,176

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