Direct conversion techniques commonly used for the transmitter in basestation rely on analog performance of the quadrature modulator. In this paper, simulated results show that imperfections in analog quadrature modulation--gain and phase error and dc offset--have a devastating effect on digital predistortion for power amplifier linearization circuits. We construct compensation circuit to remove these errors, and conduct an experiment. The imbalance errors are compensated by using the output spectrum of modulator. The image spurious (-56.74 dBm) with gain, phase and the dc offset spurious (-43.83 dBm) are reduced to -93.21 dBm and -93.56 dBm, respectively. These experimental results illustrate that modulation errors can be eliminated.