An apparatus for modulating a signal is described. The apparatus comprises a first modulation unit, a second modulation unit and at least one local oscillator providing a local oscillator signal with an initial frequency. Each modulation unit has a main modulation member and an auxiliary modulation member. The local oscillator is connected with the first and second modulation units via a first local oscillator sub path and a first local oscillator path as well as a second local oscillator sub path and a second local oscillator path respectively. The local oscillator sub paths process down-converted local oscillator signals. The main modulation member of each modulation unit is connected with one of the local oscillator sub paths. The auxiliary modulation member of each modulation unit is connected with one of the local oscillator sub paths and one of the local oscillator paths. Further, a method for modulating a signal is described.