Design of a Low-Power Varactor-Based DCO Using NMOS Switching Network as a Digital Control Technique

This paper introduces a new design of a varactor-based Digitally Controlled Oscillator (DCO). The hybrid ring-type DCO is designed using three different delay stages. Each delay stage consists of a varactor-based load element which is controlled digitally using an NMOS-based switching network. A 4-bit three-stage DCO and a 4-bit five-stage DCO have been designed, both utilizing the MOS varactor as a load element. The 4-bit three-stage DCO exhibits a frequency variation from 2.092 to 1.750 GHz while consuming 0.643 mW of power. In the case of a 4-bit five-stage DCO, the output frequency is in the range of 1.080–0.920 GHz, accompanied by a power consumption of 1.099 mW. The control bits are systematically changed from [0000] to [1111]. The effects of varying supply voltage on the output frequency and power consumption are also measured and recorded. Results are obtained in TSMC 0.18 µm CMOS process technology.