IIP2 Calibration by Injecting DC Offset at the Mixer in a Wireless Receiver

A major contributor to degraded input-referred second-order intercept point (IIP2) in integrated RF systems-on-chips is local oscillator (LO) leakage to the input of RF circuits. In this brief, we present a digital calibration technique for improving IIP2 by injecting controlled dc offset at the mixer output through a three-port network of switched-capacitor filters. The dc offset at mixer output gets up-converted to LO frequency at the input of RF circuits due to poor reverse isolation of the receiver front-end. By controlling the amplitude of the injected dc, IIP2 degradation due to LO leakage at the input of RF amplifiers can be compensated. Mathematical analysis is presented and supported by measurement data from a quad-band GSM/GPRS receiver implemented in 90-nm digital CMOS process. Calibrated IIP2 of 50 dBm is reported for the receiver at low-noise amplifier input.