Study And Design Of Low Phase Noise Ring Oscillator

Ring oscillators have the advantages of large tuning range,small occupied area,multiphase output,simple circuit and easy integration.They are widely used in phase-locked loop,frequency divider,analog-to-digital converter,etc.However,the phase noise of ring oscillators is relatively large,so low phase noise ring oscillators have gradually become one of the hotspots and difficulties of research.In order to reduce the phase noise of the ring oscillators,this paper analyzes the influence of the noise in the delay cell on the output waveform,studies the influence of the noise elimination structure on the output voltage of the delay cell and the effect of reducing the phase noise of the ring oscillators,and proposes a differential structure delay cell with noise cancellation structure.The delay cell is composed of the main path,the auxiliary path,and the active feedback.The input signal forms an differential signal in the different polarity at the output through the main path and the auxiliary path respectively;a portion of the main noise source goes directly to one end of the differential output through the main path,the other returns to the input through the feedback loop,and then reaches the other end of the differential outputs through the auxiliary path,which forms a common-mode signal in the same polarity.When the noise cancellation condition is satisfied,the polarity and amplitude of the noise voltage at both ends of the output are the same,so the noise voltages cancel each other out.The delay cell provides an additional phase shift by introducing a mirror pole,enabling a single delay cell to provide a 90 ° phase shift and a gain greater than or equal to one at the oscillation frequency.Therefore,only two delay cells are needed to meet the conditions of oscillation,which greatly reduces the number of noise sources and can provide an output waveform with the orthogonal characteristic.Because the noise voltage at the output of the differential structure delay cell is closely related to the clock jitter of the oscillator,and there is a certain amount of relationship between the phase noise and the clock jitter,the two-stage ring oscillator with noise cancellation structure can effectively reduce the clock jitter and phase noise.In this paper,a two-stage ring oscillator with noise cancellation structure is designed based on the cadence platform using TSMC 0.18 ?m CMOS technology,and compared with the traditional three-stage ring oscillator under the same simulation conditions and center frequency.The comparison results show that the two-stage ring oscillator with noise cancellation structure has a lower phase noise,which is 11.7 d Bc/Hz@1 MHz less than the traditional three-stage ring oscillator.Finally,the circuit layout drawing and post-simulation were completed under Cadence Virtuoso,and the layout is optimized according to the post-simulation results.The final post-simulation results show that when the power supply voltage is 1.8 V,the oscillation frequency of the two-stage ring oscillator with noise cancellation structure is 800 MHz,the phase noise is-117.38 d Bc/Hz at 1 MHz offset,and the frequency tuning range is 280 MHz-1.35 GHz.