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

Claim 1

A frequency synthesizer including a phase lock loop for conducting, using a phase comparator, a phase comparison of a comparison signal based on an output of a voltage-controlled oscillator with a reference signal based on an output of a reference oscillator, and for submitting a phase difference signal derived from the comparison to a loop filter whose output serves as a control signal of said voltage-controlled oscillator, the frequency synthesizer comprising: a preset circuit provided for quickly charging and discharging a capacitor of said loop filter so that the output frequency of said voltage-controlled oscillator may be switched; and a modifying circuit for modifying a time constant of said loop filter, said phase lock loop being quickly brought into phase lock-in by reducing the time constant of said loop filter when switching said output frequency; wherein the control signal to be input to said voltage-controlled oscillator is set, when switching the output frequency of said voltage-controlled oscillator, to charge or discharge, by means of said preset circuit, said capacitor of said loop filter; and wherein said preset circuit comprises a control portion, a storage portion, a D/A converter and a switch, said storage portion storing data relating to the value of the control signal of said voltage-controlled oscillator, which data corresponds to an output frequency synthesized by the frequency synthesizer, and said control portion connecting, by a switching operation, said storage portion to the terminal of the capacitor of said loop filter via the D/A converter, when switching said output frequency.