During a digital transmission with AM transmitters non-linear distortions occur also in the RF branch due to the band limitation in the amplitude branch, said distortions leading to undesired interference radiation. ITU requirements call for a constant curve of amplitude and phase response or propagation time in the amplitude branch as well as in the RF branch and for an equalization of the propagation time difference between the two branches. The required bandwith can be achieved in the RF branch by suitably selecting the quality of the oscillating circuits. In order to compensate for the non-linearities in the amplitude branch, a pre-equalization with inverse transmit function is used. According to an embodiment of the invention, the amplitude and propagation times are measured and stored. The transmit function is determined for the equalization network by inverse Fourier transform of the measured values. The network is connected in series upstream of the amplitude branch. As far as the propagation times of the amplitude branch and the RF branch are constant, the propagation time difference can be equalized by delaying the signal having the shorter propagation time in a digital modulator. The equalization of the non-linearities by pre-equalization with inverse transmit function can also be made for the RF branch.