An 18–50 GHz RF-CMOS Transmitter Front-End for a Digital Phased Array System

An ultra-wideband 18–50 GHz millimeter-wave (mm-wave) transmitter is presented. The proposed mm-wave transmitter is designed for compact size and low-power consumption, and it is suitable for an element-level digital phased array system. The fully-integrated transmitter includes buffers, cascaded baseband amplifiers with sharp roll-off, a local oscillator (LO) driver, a broadband coupled line coupler, and a stacked active mixer fabricated in a 45-nm SOI CMOS process. The cascaded Sallen-Key filters are implemented to provide sharp roll-off to remove the aliasing tones from the DACs and have moderate gain to reduce the gain required of the following mixer and PA stages. The LO driver is paired with a differential broadside coupler to generate the differential in-phase (I) and quadrature (Q) LO signals locally to ensure abrupt switching of the mixer. The phase imbalance of the I and Q LO signals can be tuned out by the tunable resistance at the isolation port of the broadside coupler. The stacked double-balanced active mixer mitigates the requirement for bulky inductors between transmitter building blocks. This allows the whole transmitter to be implemented within a limited area much smaller than the array spacing to allow area for other chip functions. The resulting transmitter has a compact chip area of 0.55x0.85 mm ² including pads. The prototype demonstrates a maximum 14.8-dB power gain and excellent gain flatness across the frequency range of interest.