An 8-Channel Varactor-Less 28-GHz Front End with 7-Bit Resolution 340° RTPS for 5G RF Beamformers

An 8-way power combined, high gain, very low power varactor-less front end in a 45 nm bulk CMOS process is presented in this brief. The fully digital near-360° seven-bit reflection type phase shifter (RTPS) utilises a novel varactor-less tuning network in a triple resonating load along with negative resistance generators to reduce the insertion loss. A two-stage passive gm boosted CG-CS low noise amplifier (LNA) with series peaking delivers 32dB gain with a very low DC power consumption. The 8:1 lumped quarter wave combiner/splitter utilizes transformer coupling to achieve a post layout efficiency of 67%, with a 2X reduction in area when compared to a spiral inductor-only implementation. Post-layout EM simulations on the RTPS predict a 340° phase shift with insertion loss of 9± 5dB at a very high resolution of 2.8°. The front end yields an overall gain of 21± 5dB, with a worst-case noise figure (NF) of 4.7dB while consuming only 10mW per channel. With an inter channel isolation greater than 22 dB, the amplitude and phase mismatch of the front end are kept within 0.3dB and 3° respectively. The impact of module and PCB non-idealities on the beamformer performance have also been highlighted with support from initial system level simulations. To the best of the authors' knowledge, this is the first implementation of a fully digital near-360° RF beamforming front end with simultaneous high gain and phase resolution.