CMOS circuits have been proven suitable for sub-millimeter-wave and terahertz frequencies from 300 GHz to 3 THz. To realize a complete terahertz system, however, a challenge still remains in the high-power, tunable signal source. When using LC-resonator-based voltage-controlled oscillators (VCOs), performance begins to degrade beyond 100 GHz. While frequency multipliers solve some of these problems, they require a high-power external source—something undesirable in a fully integrated terahertz source. One alternative could lie in a VCO architecture based on coupled oscillators in a loop configuration, which has been created by Yahya M. Tousi and Ehsan Afshari from Cornell University and Omeed Momeni from the University of California at Davis.
To realize a high-power VCO at the sub-millimeter-wave and terahertz band, three requirements must be met. First, the signal source should be able to generate high harmonic power above the device fmax. The generated power also should be efficiently delivered to the output load. Finally, a frequency-tuning mechanism is needed that will not adversely affect the first two requirements.
In this approach, multiple core oscillators are coupled to generate, combine, and deliver their harmonic power to the output node without using varactor diodes. Leveraging the theory of nonlinear dynamics, the researchers are able to control the coupling between the cores. In doing so, they can set their phase shift and frequency.
Because of the new architecture’s approach to frequency control, the tradeoff between frequency tuning and power generation in conventional VCOs is largely resolved. Frequency tuning can therefore be achieved while maintaining high output power in the sub-millimeter-wave frequency range. The engineers’ approach also provides an effective way to generate and combine the harmonics of the fundamental frequency from multiple core oscillators.
The researchers fabricated two high-power terahertz VCOs in a 65-nm low-power (LP) bulk process. According to measurements, the first one provides 0.76 mW output power at 290 GHz with a 4.5% tuning range. The second VCO puts out 0.46 mW at 320 GHz with a 2.6% tuning range. See “A Novel CMOS High-Power Terahertz VCO Based on Coupled Oscillators: Theory and Implementation,” IEEE Journal Of Solid-State Circuits, Dec. 2012, p. 3032.