Abstract: A synchronization voltage is formed as the product of the output voltage of a phase-locked loop and the mean value of the magnitude of the network voltage. The synchronization voltage is compared with the network voltage. A first control signal is formed when both voltages are the same. A reversing switch, in a first switching state, conducts the network voltage and, in a second switching state, conducts the synchronization voltage to a reference frequency-input of the phase-locked loop. A second control signal is formed when the phase-locked loop locks or latches with the thereto inputted reference frequency. The reversing switch is controlled by both control signals such that upon absence of the first control signal and the simultaneous presence of the second control signal there is attained the second switching state and otherwise the first switching state.