Origins and minimization of intermodulation distortion in a pseudo-differential CMOS beamforming receiver

Abstract

This paper studies how nonlinear distortion is generated in the combination of an inverter-based low-noise amplifier and a passive mixer. The dominant nonlinearity appears to be the quadratic $$V_{gs}V_{ds}$$ V gs V ds mixing term in the passive mixer that first causes low-frequency IM2 and then upconverts it to IM3. Adding a common-mode feedback (CMFB) cancels the IM2 in a pseudo-differential structure, and hence also reduces the IM3 caused by the cascaded second order nonlinearities significantly. The effect of CMFB gain, bandwidth and linearity were analyzed, and it is concluded that from the linearity point of view, the feedback circuit does not have to be very wideband since the dominant distortion products originate from baseband. Finally, the paper takes a look at the spurious tones rising in the mixing, and how to extend the analysis to include the actual frequency translation effect.