A method of frequency tuning an electrical generator for supplying electrical power to a plasma, wherein the method comprises a pulsed mode, the pulse mode comprising at least a high power pulse comprising a high power level and a low power pulse comprising a low power level different to zero power, the method comprising the steps of: - providing RF-power with a high power starting frequency set, wherein the high power starting frequency set comprises at least a first and a second high power starting frequency (502, 504, 506) wherein the first and the second high power starting frequency (502, 504, 506) are adapted to load impedances at the high power level which are determined by the high power pulse with a predefined high power pulse shape; - providing RF-power with a low power starting frequency set, wherein the low power starting frequency set comprises at least a first and a second low power starting frequency (512, 514) wherein the first and the second low power starting frequency (512, 514) are adapted to load impedances at the low power level which are determined by the low power pulse with a predefined low power pulse shape; - determining a reflected high power at the first or the second high power starting frequency (502, 504, 506); - tuning the first and the second high power starting frequency (502, 504, 506) to a different first and second high power frequency of a first high power frequency set if the reflected high power exceeds a high power threshold value such that the reflected high power decreases below the high power threshold value; - determining a reflected low power at the first or the second low power starting frequency (512, 514); - tuning the first and the second low power starting frequency (512, 514) to a different first and second low power frequency of a first low power frequency set if the reflected low power exceeds a low power threshold value such that the reflected low power decreases below the low power threshold value. - providing RF-power with a high power starting frequency set, wherein the high power starting frequency set comprises at least a first and a second high power starting frequency (502, 504, 506) wherein the first and the second high power starting frequency (502, 504, 506) are adapted to load impedances at the high power level which are determined by the high power pulse with a predefined high power pulse shape; - providing RF-power with a low power starting frequency set, wherein the low power starting frequency set comprises at least a first and a second low power starting frequency (512, 514) wherein the first and the second low power starting frequency (512, 514) are adapted to load impedances at the low power level which are determined by the low power pulse with a predefined low power pulse shape; - determining a reflected high power at the first or the second high power starting frequency (502, 504, 506); - tuning the first and the second high power starting frequency (502, 504, 506) to a different first and second high power frequency of a first high power frequency set if the reflected high power exceeds a high power threshold value such that the reflected high power decreases below the high power threshold value; - determining a reflected low power at the first or the second low power starting frequency (512, 514); - tuning the first and the second low power starting frequency (512, 514) to a different first and second low power frequency of a first low power frequency set if the reflected low power exceeds a low power threshold value such that the reflected low power decreases below the low power threshold value.