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

We present a digital calibration technique for improving IIP2 by injecting DC offset at the mixer. Most receivers offer DC offset cancellation circuitry, and a targeted non-zero DC offset at mixer output is up-converted to RF carrier frequency due to poor reverse isolation of the mixer switch. By controlling amplitude of the injected DC, IIP2 degradation due to LO leakage at the input of RF amplifiers can be compensated. Mathematical analysis and measurement data for a quad-band GSM/GPRS receiver implemented in 90-nm digital CMOS process are presented. Calibrated IIP2 of 50 dBm is reported at LNA input.