A fast turn on compensation system for a synthesized signal source comprising: the synthesized signal source coupled to a power supply and configured to generate a phase stable radio frequency output signal; a mute amplifier coupled to the synthesized signal source and the power supply, wherein the mute amplifier is arranged to receive the phase stable radio frequency output signal at an input of the mute amplifier and provide an amplified signal at an output; and a mute controller coupled to the mute amplifier; characterised by a dummy amplifier coupled to the power supply and having an input coupled to the synthesized signal source to receive the phase stable radio frequency signal therefrom; and the mute controller coupled to the dummy amplifier, the mute controller being responsive to an enable signal and configured to enable the dummy amplifier and disable the mute amplifier if no radio frequency output signal is required at the output of the mute amplifier and disable the dummy amplifier and enable the mute amplifier if the radio frequency output signal is required at the output of the mute amplifier, wherein a current drawn by the dummy amplifier is selected to match a current drawn by the mute amplifier such that the transient effect on the power supply seen by the synthesized signal source as a result of enablement of the mute amplifier is offset by the complementary disablement of the dummy amplifier, and in which a combined impedance of the mute amplifier and the dummy amplifier presented to the synthesized signal source is configured to be constant before, during, and after enabling the mute amplifier. the synthesized signal source coupled to a power supply and configured to generate a phase stable radio frequency output signal; a mute amplifier coupled to the synthesized signal source and the power supply, wherein the mute amplifier is arranged to receive the phase stable radio frequency output signal at an input of the mute amplifier and provide an amplified signal at an output; and a mute controller coupled to the mute amplifier; characterised by a dummy amplifier coupled to the power supply and having an input coupled to the synthesized signal source to receive the phase stable radio frequency signal therefrom; and the mute controller coupled to the dummy amplifier, the mute controller being responsive to an enable signal and configured to enable the dummy amplifier and disable the mute amplifier if no radio frequency output signal is required at the output of the mute amplifier and disable the dummy amplifier and enable the mute amplifier if the radio frequency output signal is required at the output of the mute amplifier, wherein a current drawn by the dummy amplifier is selected to match a current drawn by the mute amplifier such that the transient effect on the power supply seen by the synthesized signal source as a result of enablement of the mute amplifier is offset by the complementary disablement of the dummy amplifier, and in which a combined impedance of the mute amplifier and the dummy amplifier presented to the synthesized signal source is configured to be constant before, during, and after enabling the mute amplifier.