All-Digital Adaptive Module for Automatic Background IIP2 Calibration in CMOS Downconverters With Fast Convergence

An all-digital adaptive module for input-referred second-order intercept point (IIP2) calibration in CMOS downconverters is proposed. The proposed automatic background calibration scheme utilizes a modified least-mean-square algorithm with a variable update step to achieve fast convergence. An easily acquired and highly second-order-intermodulation-distortion-correlated reference input is used with delay estimation/optimization to improve the convergence efficiency and reduce the implementation complexity. The proposed all-digital adaptive module is implemented on a field-programmable gate array to calibrate the IIP2 of a 2.62-GHz downconverter fabricated in a 0.18-μm CMOS process. With the proposed automatic background IIP2 calibration, the measured IIP2 of the CMOS downconverter is improved by 21 dB from 62 to 83 dBm, while the IIP2 calibration converges in less than 0.1 ms with a 10-MHz clock, ten times faster than the conventional adaptive algorithm.