Correction of phase and amplitude imbalance of I/Q modulator

An arrangement for determining correction parameters used for correcting phase and amplitude imbalance of an I/Q (Inphase/Quadrature phase) modulator in a transmitter, which transmitter comprises the I/Q modulator for I/Q-modulating a signal to be transmitted and a corrector for correcting the phase and amplitude imbalance caused by the I/Q modulator according to the correction parameters, the arrangement comprising:

• sampling means for sampling an I/Q-modulated test signal to be transmitted, which is formed from I- and Q-test signals fed into the transmitter, wherein the I- and Q-test signals are sine signals having a phase shift of 90° between them;
• A/D conversion means for A/D-converting the signal samples taken from the test signal to be transmitted;
• I/Q demodulation means for I/Q-demodulating the signal samples digitally into I- and Q-feedback signals;
• means for determining the phase and amplitude imbalance caused by the I/Q modulator, the means being arranged to:
• determine an ellipse formed by one sequence or an average of several sequences of the I- and Q-feedback signals on an I/Q plane;
• determine, on the basis of timing information, the distorted I- and Q-axes corresponding to the I- and Q-feedback signals on the I/Q plane;
• measure the diameters of the ellipse in the direction of the distorted I- and Q-axes;
• determine the amplitude imbalance from the relation between the diameters;
• determine the phase imbalance from the angle between the distorted I- and Q-axes;
• and
• means for determining the correction parameters of phase and amplitude on the basis of the determined phase and amplitude imbalance.