Prototype implementation and RF performance measurements of DSP based transmitter I/Q imbalance calibration

Communication transceivers utilizing the inphase/ quadrature (I/Q) up- and down-conversion principle face a common problem of amplitude and phase mismatch. These mismatches result in the generation of an image signal and thus, cause mirror-frequency interference. Moreover, quadrature modulation/demodulation errors also affect the performance of pre-distortion based power amplifier linearization techniques. This article addresses the problem of frequency-dependent I/Q mismatch in direct-conversion radio transmitters and investigates the performance of a DSP-based compensation algorithm with simulations and practical RF measurements. We first review the analytical equations to describe the impact of gain and phase imbalance on the transmitted RF signal. Then, a digital predistortion structure coupled with a technique to efficiently estimate the I/Q mismatch and the needed pre-distortion parameters are discussed. It is shown with simulations and practical measurements that the compensation algorithm produces very good calibration results.