Patent ID: 7180376

Claim:
A frequency synthesizer used for modulating an input signal, comprising: a phase locked loop circuit, comprising a phase frequency detector for generating a first signal, a low pass filter for outputting a control signal derived from the received first signal, a voltage control oscillator, generating an output signal with a first frequency based on the control signal, a frequency divider, dividing the first frequency and for output to the input terminal of the phase frequency detector; a modulator, coupled to the frequency divider; a pre-emphasis filter, receiving and filtering the input signal for output to the modulator, wherein the frequency response of the phase locked loop circuit is substantially inverse to the frequency response of the pre-filter; and an auto loop gain calibration circuit, receiving the control signal, and calculating a current gain of the control signal in accordance with the voltage of the control signal, to compensate for the frequency response mismatch between the pre-emphasis filter and the frequency synthesizer.