Design of novel hybrid - digitally controlled oscillator for ADPLL

Abstract

Digitally Controlled Oscillators (DCOs) are an integral part of All Digital Phase Locked Loops (ADPLLs). It is used to generate output frequency corresponding to the applied digital input. But, due to practical limitations in circuit design, DCOs resolution will be highly limited. Delta Sigma Modulator (DSM) is generally used in DCOs/ADPLLs to obtain a greater resolution. However, using DSM will lead to introduction of frequency spurs in the output spectrum. In this work we propose an alternate method to increase the frequency resolution of DCO without introducing frequency spurs. Novel Hybrid DCO architecture is proposed in this work to increase the resolution. Hybrid DCO proposed in this work has both digital and analog control for tuning frequency. Digital control input of the proposed DCO provides coarse frequency control and the analog control input provides fine frequency control. It has been demonstrated in this work that by integrating Hybrid DCO and DSM with an analog low pass filter (LPF), resolution can be greatly increased, without introducing spurs in the spectrum. As a proof of concept, two Hybrid Digitally controlled Ring Oscillators (DCROs) are designed in 90 nm CMOS process, and their period Jitter performance is compared.