Quadrature oscillator is a core for modern wireless systems, yet its phase accuracy and FOM traditionally trade off against each other: the quadrature voltage-controlled oscillators require quadrature coupling for frequency pulling to achieve high phase accuracy, causing resonant Q-factor degradation and phase noise deterioration, and thus creating an inherent trade-off between FOM and phase accuracy; while rotary travelling-wave oscillators(RTWOs) suffer from the current phase misalignment and noise degradation in their negative resistance units due to the Möbius ring resonator constraints in planar chip fabrication.
Spoof surface plasmon polaritons (Spoof SPPs) can mimic the optical SPPs, and hence the SPP metamaterials enable precise electromagnetic wave manipulations at microwave and terahertz frequencies. Spoof SPPs combine the advantages of planar circuit configurations with the unique properties including strong field confinement, tunable dispersion, field enhancement, and low crosstalk. Our breakthrough published in Light: Science & Applications harnesses spoof SPPs to redefine RTWOs. The key of this innovation is a meta-RTWO chip built around a spoof SPP Möbius ring resonator with uniformly distributed negative resistance units. Below is how we cracked the code.
To achieve perfect oscillation with high phase accuracy and figh FOM, we firstly established a critical physical requirement: every transmission line segment in the Möbius ring must maintain exactly 90° phase shifts at the oscillation frequency. This ensures synchronized current alignment throughout the resonator – the foundation of our design. We engineer the resonator by three non-crossing spoof SPP transmission lines seamlessly connected to a single-edge crossing jumper structure. The proposed architecture delivers exceptional phase controls, and measurements confirm that four distinct spoof SPP transmission lines exhibit nearly identical phase responses across 0-100 GHz. At 25.5 GHz, the maximum phase difference is just 0.16°, dwarfing the 2.81° variation seen in the conventional microstrip lines. This precision ensures every segment of the Möbius ring maintains exactly 90° phase shifts at the target frequency, enabling perfect current synchronization throughout the resonator. Adjacent spoof SPP units show minimal characteristic impedance deviation (≤2.1 Ω at 25.5 GHz), reducing the signal reflection to a negligible 0.5% energy loss.
Fabricated in standard 65nm CMOS process, the meta-RTWO chip achieves the excellent performance that once seemed impossible: the phase error of 0.21° and FOM of 188.5 dBc/Hz. Remarkably, these results surpass the recent designs with the traditional microstrip lines using advanced 28nm technology. The implications are profound: this meta-RTWO technology could become the backbone for real-time calibration in 6G massive MIMO systems, enable sub-picosecond synchronization for terahertz quantum communications, and revolutionize beamforming in the next-generation intelligent networks. Sometimes, the smallest chips with trigger the biggest revolutions.