A Low Phase Noise Circular-Coupled Quad-Core Oscillator Employing Dual-Path Synchronization Technique

This article presents a millimeter-wave (mm-Wave) quad-core oscillator utilizing a circular transformer and dual-path synchronization technique to achieve low phase noise (PN). We analyze the mechanism of the PN degradation induced by frequency mismatch between individual cores in the conventional quad-core oscillator utilizing a circular inductor, which reveals that the frequency mismatch between the non-adjacent cores will induce an off-resonance issue, which could significantly degrade the PN. This PN degradation induced by the off-resonance is acerbated in the quad-core oscillator utilizing a circular transformer. Based on the analysis, a dual-path synchronization technique is proposed to eliminate the off-resonance issue by providing a direct synchronization path for either two cores in a circular-coupled quad-core oscillator. The proposed technique can be applied to both inductor- and transformer-based circular-coupled oscillators, preventing PN degradation due to the frequency mismatch. Fabricated in 65-nm CMOS, our quad-core oscillator prototype measures a frequency tuning range of 18.2%, from 22.4 to 26.8 GHz. At the 25.8 GHz carrier frequency, the oscillator achieves a low PN of$\!-\!138$dBc/Hz at 10 MHz offset while consuming 19.7 mW, corresponding to an excellent FoM of 193.3 dBc/Hz.