US Patent 7298190 Phase locked loop having enhanced locking characteristics

A phase locked loop (PLL) integrated circuit includes a voltage-controlled oscillator (VCO) configured to generate a clock signal at an output terminal thereof. The VCO is further configured to improve the frequency response of the PLL by varying a capacitance of the output terminal concurrently with changing a frequency of the clock signal. The VCO may include a control signal generator, which is configured to generate a plurality of control signals in response to UP and DOWN pumping signals, and an oscillator, which is configured to generate the clock signal in response to the plurality of control signals. The oscillator may be a ring oscillator, which is responsive to the plurality of control signals.