10-GSymbols/s Supply-Modulated APSKC SiGe HBT Transmitter RF Front End for Operation at 250 GHz With 6.8-dBm Peak Modulated Power

This article presents a 250-GHz supply-modulated SiGe heterojunction bipolar transistor (HBT) ( $f_{T}$ and $f_{\max }$ of 350 and 450 GHz, respectively) transmitter RF front end that integrates a transformer-coupled four-stage power amplifier (PA) whose fourth stage is supply-modulated at a symbol rate of 10 GSymbols/s utilizing a high-speed low-dropout regulator (LDO). An integrated LO chain with a frequency multiplier generates a 121-GHz LO signal. Additionally, on-chip bipolar current-mode logic circuits are used to decode 1- or 2-bit PRBS data stream(s) and to drive the LDO that implements two- or four-level supply modulation. The transmitter supports 16-customized-APSK and 32-customized-APSK constellations, achieving instrumentation-limited data rates of 40 and 50 Gb/s with the peak modulated output power of 6.8 dBm and error vector magnitudes (EVMs) of −18.9 and −20.4 dB, respectively, while consuming approximately 1 W of dc power. The use of supply modulation with the APSK modulations increases both the peak modulated output power and RF-to-dc efficiency by $3{\times }$ and $2{\times }$ , respectively, compared to when conventional quadrature amplitude modulation (QAM) modulation with power backoff is employed. The measured 6.8-dBm peak modulated power is the highest for a transmitter IC operating at this frequency range.