A Beamformer with constant-gm vector modulators and its spatial intermodulation distortion

Spatial interference rejection in analog adaptive beamforming receivers can improve the distortion performance of the circuits following the beamforming network, but is susceptible to the non-linearity of the beamforming network itself. This paper presents an analysis of intermodulation product cancellation in analog active phased array receivers and verifies the distortion improvement in a 4-element adaptive beamforming receiver for low power applications in the 1.0 to 2.5 GHz frequency band. In this architecture, a constant-Gm vector modulator is proposed which produces an accurate equidistance square constellation, leading to a sliced frontend design that is duplicated for each antenna element. By moving the transconductances to RF, a four-fold reduction in power is achieved, while simultaneously providing input impedance matching. The 65-nm implementation consumes between 6.5 and 9 mW per antenna element, and shows a +1 to +20 dBm in-band, out-of-beam IIP3 due to intermodulation distortion reduction.