Analysis and Design of Broadband Terahertz Push–Push Frequency Doublers With Second Harmonic Source Tuning

This article presents the analysis and design of push-push frequency doublers (PPFDs) with second harmonic source tuning for broadband terahertz (THz) generation. Based on a simplified large-signal transistor model, we study the frequency-doubling mechanism of THz PPFDs and reveal that the input second harmonic, tuned by the common-mode (CM) source impedance, makes a profound impact on circuit performances, especially the bandwidth. Hence, to enable broadband THz PPFDs, we develop a source-tuning-based design scheme. In the scheme, a mode-decoupling harmonic source/load-pull strategy is used to identify the broadband optimal CM source and second harmonic load impedances, and a slotline-based second harmonic source-tuning technique then helps to realize the source impedance and simultaneously implements the broadband input matching. As a proof-of-concept, the proposed design scheme is applied in a full H-band amplifier-doubler chain in 0.13- $\mu $ m SiGe BiCMOS. The fabricated chain demonstrates up to 4.7-dBm saturated output power, 1.13% dc-to-RF efficiency, and 37-dBc fundamental suppression with only 3.6-dB power fluctuation from 200 to 350 GHz.