A Compact and High-Conversion-Gain W-Band Differential Distributed Frequency Doubler With Over-Neutralizing Second-Harmonic Feedback

This article presents a compact W-band distributed frequency doubler that achieves high-conversion gain (CG) by employing over-neutralized second-harmonic (2f₀) feedback. The proposed two-stage distributed doubler consists of two push–push frequency doublers (PPFDs). A transmission line with a 90° electrical length is inserted between the two PPFD stages to generate in-phase and quadrature-phase signals. Compared to conventional quadrature-coupler-based topologies, this network achieves a smaller chip area and reduced insertion loss. Furthermore, cross-coupled capacitors (Ccc) are connected from the drain of one PPFD to the gate of the other. By choosingCcclarger than the parasitic gate-to-drain capacitor, over-neutralization is achieved. This converts the inherent negative feedback into positive feedback, enabling constructive summation at the drain. As a result, the doubler exhibits enhanced CG, total efficiency, saturated2f₀output power, and fundamental (f₀) rejection. In addition, the matching network of the doubler is designed to operate as an input matching network atf₀and as a reflector at2f₀, compensating the phase variation of the positive-feedback path to ensure perfect constructive summation. The measured peak CG of the proposed doubler is 0.1 dB. The saturated output power is 6.7 dBm, and the peak total efficiency is 6.2% at a DC power consumption of 49 mW. The measured 3-dB bandwidth is 13.9 GHz, which is from 74.8 to 88.7 GHz, and within this bandwidth, the fundamental rejection ratio (FRR) exceeds 36 dBc. The core area is only 0.05 mm2, demonstrating the compactness of the proposed design.