This control circuit adjusts two signal waveforms to a phase difference of exactly 90.degree. and to exactly equal amplitudes of the respective frequency components. It is a digital circuit containing a phase control stage (pr) and an amplitude control stage (ar). The control circuit (r) delivers first and second quadrature signals (q1, q2) which are multiplied together to obtain a phase control value (p) which is multiplied by one of the digitized quadrature analog signals and added vectorially to the second digital signal. The two signals thus brought to a phase difference of exactly 90.degree. are the two phase-corrected digital signals (p1, p2). In the amplitude control stage (ar), one of the two phase-corrected digital signals (p2, p1) is multiplied by the amplitude control value (g) from the amplitude comparator (av1, av2), and the product is the amplitude- and phase-corrected quadrature signal (q2, q1) belonging to the other quadrature signal (q1, q2).