This thesis reports a novel varactor pairs with small switchable unit capacitance by controlling digital code word. Two discrete capacitances are determined the difference between the accumulation region capacitance and inversion region capacitance of two PMOS varactors. The reliable linear frequency control by a novel varactor pair is verified with high resolution through the implemented digitally controlled oscillator in 0.18 um CMOS process. The novel varactor pairs make much smaller switchable capacitance than those of other approaches, and hence the DCO achieves the high frequency resolution and low phase noise. Also, identical sizes of PMOS varactor make them robust from process variation.