An LC-tank based DCO for low-power high-speed applications using full-custom nMOS-type varactors

LC-tank based Digital Controlled Oscillators (LC-DCOs) are widely used in digital assisted high-speed data communication systems due to their high quality factor (Q). Typically, LC-DCOs incorporate digital-controlled varactors for frequency control. The varactor structure significantly influences oscillator Q and phase noise performance. In this paper we propose using full-custom varactors built with digitally connected nMOS-type transistors to enhance the varactor Q by reducing its length. However, parasitic resistance from varactor array interconnection with the LC-tank can impact the overall oscillator performance. This paper presents a methodology to systematically include the proposed varactor array and its parasitic effects into LC-DCO design flow. Following the proposed method, an optimal combination of the inductor and varactor array can be accurately determined. As a proof of concept, we designed and manufactured a 23.5-GHz LC-DCO using 28-nm CMOS RF process. The oscillator exhibits a maximum phase noise of -90.1 dBc/Hz at 1-MHz offset with a power consumption of only 1.71 mW at 1-V supply.