610-GHz 4th Harmonic Signal Reactively Generated in a CMOS Voltage Controlled Oscillator Using Differentially Pumped Varactors

A fourth harmonic signal reactively generated in a voltage controlled oscillator (VCO) using differentially pumped varactors is used to demonstrate a compact 610-GHz source in 65-nm CMOS. The nonlinear $C{-}V$ characteristics of accumulation mode varactor pair are used to generate the harmonic power. The circuit also utilizes a technique to boost the varactor $Q$ and dynamic cut-off frequency to increase the harmonic power. The source delivers a peak radiating power of −25.2 dBm, with a frequency tuning range of 604.5–618.6 GHz (2.3%), a 3-dB tuning bandwidth of 607.1–614.3 GHz (1.2%), and a dc-THz efficiency of 0.01% (0.013% excluding the power consumption in the current source). The chip core area is 0.62 mm $\times $ 0.39 mm excluding the area for bond pads.