A 45 Gb/s D-band Hybrid Star-QAM-OOK Transmitter Using a Quad-Harmonic Modulator with Constant Impedance Balanced Architecture in 90nm SiGe BiCMOS

This work presents a D-band reconfigurable Quadrature Second Harmonic modulator. The proposed modulator features a quadrature frequency doubler with integrated shunt switches at the common-emitter node, and with balanced switching mechanism inspired by photonic interferometer, enabling reconfigurable 4-APSK (Star QAM) and OOK modulation. The design maintains a nearly constant input and output impedance across all modulation states, minimizing Error Vector Magnitude (EVM) variation over the bandwidth and enabling robust performance for next-generation short-range high-speed connectivity. Developed using a 90 nm SiGe BiCMOS process and heterogeneously integrated with a fused silica antenna, the modulator achieves a data rate of up to $45 \mathrm{~Gb} / \mathrm{s}$ with EVM better than -19 dB and less than $\pm 1.6 \mathrm{~dB}$ variation across a 20% bandwidth spanning 132 GHz to 158 GHz. The chip occupies an area of $1.1 \mathrm{~mm} \times 1.75 \mathrm{~mm}$, consumes an average $P_{D C}$ of 100 mW, and delivers $-\mathbf{3. 6 ~ d B m ~} P_{\text {sat }}$ with a spectral efficiency of $3.3 \mathrm{pJ} / \mathrm{bit}$.