A 14.5 Gb/s, 2.75 pJ/bit, Direct-Digital, Star-QAM Modulator and Co-Designed Frequency Multiplier Operating at 140 GHz

This article presents a direct-digital, Star-QAM modulator with frequency up conversion (x2). A push-push frequency doubler (PPFD) with integrated common node (CN) switches generate 2nd harmonic amplitude shift keying (H-ASK) modulation, uniquely affording constant load impedance. Four PPFDs, (i) feed by using half-quadrature phase shift network (HQ-PSN), (ii) applied with suitable binary data sequence (Dbits) and (iii) combined (in-phase and quadrature), enable Star-QAM at the output. The circuit exhibits conversion loss of 9.7 dB and maximum output power (Pout,2f) = -5 dBm at 140 GHz. It also shows maximum data rate of 14.5 Gb/s by using Star-QAM, having an error vector magnitude (EVM) of -13 dB without external pulse shaping/equalization. Implemented in Global Foundries 90 nm Silicon Germanium BiCMOS technology, it occupies only 0.8 mm2 of active chip-area, consumes 40 mW of power with bit-efficiency of 2.75 pJ/bit, which exhibits significant advancement in compact, multi-functional and high-data rate modulator design.