Patent ID: 8421542

Claim:
An apparatus, comprising: a voltage-controlled oscillator (VCO) having a first capacitor unit and a second capacitor unit, the VCO being configured to generate an oscillating signal having a frequency based on a first capacitance of the first capacitor unit and a second capacitance of the second capacitor unit; a detector module configured to generate a voltage signal as a function of the oscillating signal and a reference signal, the voltage signal being used to control the first capacitor unit to stabilize the frequency of the oscillating signal; and a ramp module configured to generate a ramp signal based on the voltage signal, the ramp signal being used to control the second capacitor unit to ramp the second capacitance from a first value to a second value to keep the voltage signal in a predetermined range, range, the ramp module including: a monitor configured to monitor the voltage signal, and generate a trigger signal when the voltage signal is out of the predetermined range; and a counter configured to generate a digital ramp signal by counting in response to the trigger signal.