A calibration apparatus for calibrating at least one of a signal generator and a signal analyzer is described, wherein the calibration apparatus comprises an input terminal for establishing a connection with the signal generator, an output terminal for establishing a connection with the signal analyzer, a reference signal source for providing a reference signal, and a combiner circuit for combining signals received. The combiner circuit has a first input connected to the reference signal source and a second input assigned to the input terminal, the combiner circuit having an output assigned to the output terminal. Further, a calibration system and a method for calibrating at least one of a signal generator and a signal analyzer are described.