On the Estimation and Compensation of IQ Impairments in Direct Conversion Transmitters

Compensating IQ impairments in transmitters is an important prerequisite for linearizing power amplifiers (PA's) by means of digital predistortion of the input signal. As the effects of gain- and phase imbalance and DC offset are increasing with the bandwidth of a transmitter, they must be corrected for because otherwise digital predistortion for PA's becomes ineffective. An important implementation aspect for many impairment correction schemes is the realization of the required feedback loop including the downconversion from the RF- to the baseband domain. We propose an IQ and DC offset estimation and compensation method which uses a simple detector diode in the feedback path. The characteristic of the detector diode needs not to be known, which makes implementation easy. It is shown by simulation and measurements, that the method works excellent for narrowband as well as for wideband signals.