Abstract: In a radar system, comprising a digital expander, two analog modulation signals are generated with two digital to analog converters and raised to an intermediate frequency range through a quadrature modulator. The amplitude and phase ripple of the output signal occurring in the quadrature modulator due to limited carrier and image frequency suppression are compensated through a multiple regulating circuit. To this end, if necessary calibration signals, of constant amplitude and different phase positions, are generated sequentially by means of the two digital to analog converters and the amplitude of their output signals measured in an amplitude detector. Setting values for the multiple regulating circuit are determined from the averaged amplitude measured values in a phase and amplitude correction unit which via setting elements set the requisite offset and phase and amplitude symmetry values until the deviations of the output signal in amplitude and phase disappear.