Disclosed is a method of measuring frequency distortions characteristics of a device under test, the device configured convert an input signal in an input frequency range to an output signal in a different output frequency range. The method includes, for each test frequency fi, where i=1, . . . , N and N a positive integer, in a selected frequency range, providing a corresponding test signal with multiple frequency components having a measurement component with a frequency fi, a first reference component with a frequency fA, and a second reference component with a frequency fB; inputting the test signals into the device under test; measuring output test signals at the output of the device under test corresponding to the input test signals; and determining, for each test frequency fi, information representative of frequency distortions based on the corresponding input test signal and the corresponding output test signal.