0.3-THz SiGe-Based High-Efficiency Push-Push VCOs With > 1-mW Peak Output Power Employing Common-Mode Impedance Enhancement

We present a novel method of maximizing the output power and efficiency of millimeter-wave and terahertz signal sources, which are based on the push-push topology. In this method, the common-mode impedance of a differential Colpitts oscillator operating in the odd mode is maximized by introducing a fixed-valued capacitor (C-r) at the common-base node. This capacitor is designed to introduce a common-mode parallel resonance at the desired second harmonic, boosting the common-mode voltage swing and subsequently its output power. The proposed method is analyzed using a high-frequency even-mode pi-model. Analytical expressions of input impedance are derived and are used for calculating the common-mode resonance frequency and the required value of C-r. Two 0.3-THz voltage-controlled oscillators (VCOs) are implemented in a 130-nm SiGe BiCMOS process. It is shown that by using the proposed technique, the output power is improved by more than 6 dB, as compared with the conventional approaches. The implemented VCOs work from 292 to 318 GHz and 305 to 327 GHz, delivering a peak output power of 0.6 and 0.2 dBm, with a dc-to-RF efficiency of 0.8% and 0.9%, and can achieve a phase noise of -108 and -105 dBc/Hz at 10-MHz offset, respectively. As compared with the prior state-of-the-art Si-based tunable signal sources and arrays working above 270 GHz, this paper shows the lowest phase noise and the best figure-of-merit, while having an excellent output power, a tuning range, and a dc-to-RF efficiency.