A PHASE LOCKED LOOP FREQUENCY SYNTHESIZER CIRCUIT WITH IMPROVED NOISE PERFORMANCE

摘要

A phase locked loop frequency synthesizer comprises a voltage controlled oscillator; a loop filter for supplying a control voltage to the oscillator; a phase frequency detector arranged to detect a phase difference between a reference signal and a feedback signal generated from the oscillator signal and generate pulses on detector signals (UP/DN) dependent on the sign of the phase difference; and a charge pump (61) comprising current generating means and controlled switches (64, 65) arranged to convert pulses on the detector signals to current pulses from a reference voltage (Vdd') to a common terminal (Vloop) connected to the loop filter or to current pulses from the common terminal to ground. The current generating means comprises at least one resistor (62, 63) connected between the common terminal and the switches, and the charge pump comprises an operational amplifier (66) coupled to keep the reference voltage at twice the voltage at the common terminal.