Switchable capacitors are increasingly intensively used in modern LC oscillators, as they suffer less on the process variation in comparison to the conventional MOSFET-based varactors with the technology shrinking. In the design of an LC oscillator, the structure of the switchable capacitor is carefully chosen because it serves as the main contributor to the overall quality factor of the LC tank that has a significant impact on the phase noise performance of the oscillator. In order to study the characteristics of recently popular switchable capacitor structures, this paper introduces a design methodology using large-signal simulations, i.e. periodic steady-state (PSS) and periodic AC analysis (PAC), to better calculate the difference between on- and off-state capacitance as well as the quality factor. The influence of the structures of the switchable capacitor on the performance of the LC oscillator is verified using a 10 GHz complementary class-B digitally controlled oscillator designed in a 28-nm CMOS technology.