Design and analysis of high-resolution CMOS digitally controlled oscillator with tunable transformer

Abstract

A tunable transformer based millimeter-wave digitally controlled oscillator (DCO) is proposed to achieve high frequency resolution with compact area in this paper. An 8-shaped inductor is employed as primary coil of the transformer to generate the opposite magnetic field and achieve a weak coupling to the secondary coil, which is beneficial to enhance the scaling effect of the capacitor to achieve a smaller equivalent capacitance. The equivalent model of the transformer structure is analyzed theoretically. Distributed capacitors are placed under the secondary coil, which can be flexibly adjusted to achieve high frequency resolution. A digitally controlled artificial dielectric (DiCAD) differential transmission line is implemented to achieve coarse and medium frequency tuning. The DCO is implemented in 40-nm CMOS process with a core area of 0.026 mm2. Simulated results demonstrate a minimum frequency tuning step of 381 KHz with a frequency tuning range of 12.3% from 58.60 to 66.28 GHz, and the phase noise at 1-MHz offset is from −91.3 to −94.0 dBc/Hz, corresponding to an FoM range of −179.5 to −182.2 dB, while the power consumption is 8.8 mW.