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-μm 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.