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Cyclic-coupled ring oscillators (CCRO) provide several unique features over regular ring oscillators such as availability of multiple sets of phase-shifted outputs and reduced phase noise. Furthermore, when combined with N-push frequency multiplication low phase noise, wide tuning ranges and millimeter-wave (mm-wave) frequencies can be realized. In this work, we propose two N-push CCROs fabricated in a 90 nm digital CMOS process. First, a wideband N-Push/M-Push CCRO is presented which achieves an output frequency range of 3.16-12.8 GHz using a unit cell ring oscillator operating at 1-2.56 GHz. The measured phase noise at 1 MHz offset is -103.4 dBc/Hz and -101.6 dBc/Hz at 3.16 GHz and 12.8 GHz, respectively. Second, an mm-wave N-Push CCRO is presented. It generates an output frequency of 13-25 GHz with a low phase noise performance of -95 dBc/Hz at 1 MHz offset over the output frequency range. The proposed oscillators achieve superior phase noise performance as well as competitive figure-of-merit compared with the state-of-the-art ring oscillators. In addition, the operation of the CCRO and its phase noise is analyzed. We confirm, analytically and experimentally, that the phase noise of an M-stage CCRO improves by 10 M over that of a single ring oscillator. We also show that the phase noise improvement bandwidth is a function of the coupling strength.
Date of Publication: June 2012