A Digitally Controlled Oscillator for ADPLL using combinational frequency multiplier and novel variable delay inverter

Abstract

In this article, a novel digitally controlled Oscillator (DCO) for All Digital Phase Locked Loop (ADPLL) is presented. In the proposed DCO, the concept of a combinational frequency multiplier is used to double the frequency range of a conventional digital ring oscillator. However, conventional combinational frequency multiplier circuits can produce a 50% duty cycle only for a single input frequency. Hence, a novel circuit for frequency multiplication is proposed that can produce an output frequency equal to 2 times the input frequencies with a 50% duty cycle for all the frequencies. The proposed DCO is developed by integrating a digitally controlled ring oscillator with the proposed novel combinational multiplier. Digitally controlled delay inverters are used to design both the DCRO and the novel combinational frequency multiplier. A 5-bit DCO is realized using a 4-bit DCRO and the proposed combinational frequency multiplier in 90nm CMOS technology. The resultant frequency range is verified by simulation.