An adaptive digital technique for compensating for analog quadrature modulator/demodulator impairments

Analog quadrature modulators and demodulators have three major impairments: gain imbalance, phase imbalance, and dc-offset. A digital technique is presented for compensation of these modulator and demodulator impairments. Part of the RF signal is fed to an envelope detector. The detector output, along with the baseband quadrature components, is used to estimate the impairment values. The estimated impairment values are then used to compensate for the impairments. Simulation results show that spurious signals can be suppressed by more than 30 dB using this technique. The effect of modulator/demodulator impairments on RF power amplifier linearization techniques is also discussed. The quadrature modulator and demodulator impairment compensator can decrease intermodulation distortion products by 12 dB for the complex gain predistorter and by about 5 dB for the Cartesian coordinate negative feedback method of RF power amplifier linearization