A 226 GHz Coupled Harmonic VCO with 9.34% Tuning Range Utilizing Three-Coil Transformer with Switched Inductor in 65nm CMOS.

Low-power CMOS process offers a low-cost commercial solution for millimeter-wave and terahertz (THz) system applications. However, their limited maximum oscillation frequency (fmax) remains a bottleneck for the fundamental signal sources with large output power and low phase noise. To tackle these problems, harmonic coupled oscillators are popularly adopted to increase the output power by power combination while reducing the phase noise by 10log 10 N dB (N is the number of coupling cores). Several topologies [1]–[3] of the THz oscillator core and the corresponding coupling schemes are listed and compared in Fig. 1 along with their different features. The oscillator in [1] adopts a source degeneration varactor for tuning, however, due to its self-feeding topology, the transistor gate and drain are biased together, restricting its performance. Also, the $\lambda/4$ transmission lines required for both power supply and grounding increase its area. The oscillator in [2] optimizes the output harmonic power by cascading doublers with the fundamental oscillators, resulting in a larger area and limited tuning range. Reference [3] introduces a concise structure, and achieves a wide tuning range through the biasing voltages, intending to avoid the varactors to preserve the tank quality factor (Q).