Abstract: A circuit is provided for producing a control voltage depending on a frequency and/or phase difference between a first and a second periodic signal. The periodic signals are fed to trigger circuits to provide trigger signals. An inhibiting time interval generator receives the trigger signals and produces inhibiting signals. The trigger signals are the inhibiting signals are fed to an inhibiting circuit which inhibits passage of trigger signals depending on the passage of recently passed trigger signals. A first flip-flop circuit is connected to the inhibiting circuit and the signals passed by the inhibiting circuit set and reset the flip-flop. The output of the first flip-flop is fed to an averaging circuit for generating an output control voltage. A comparator connected to the output of the averaging circuit compares the output control voltage with an average output control voltage and its output is connected to the inhibiting circuit.