Digitally controlled oscillator with high frequency resolution using novel varactor bank

An LC tank based digitally controlled oscillator (DCO) with high frequency resolution by using a novel varactor bank is presented. The proposed varactor exploits the opposite C-V characteristics of the PMOS and NMOS and achieves unit capacitance that is an order of magnitude less than a simple PMOS or NMOS varactor. A prototype DCO is designed in 65 nm CMOS process, and simulation results show that the DCO achieves a frequency range of 4.5- 5.5 GHz while having 1.4 kHz of minimum frequency step, which corresponds to 0.8 aF of unit capacitance.