A system and method sequentially measure the amplitude and phase of an output signal of a device under test in each of two or more frequency ranges which together span the output signal spectrum, using a local oscillator (LO) signal whose frequency changes for each measurement. The measured phase of the output signal is adjusted for at least one of the frequency ranges to account for a change of phase in the LO signal from measurement of one frequency range to another frequency range, including applying to the measured phase a phase offset determined by measuring the phase of a pilot tone using the LO signal before and after the frequency of the LO signal changes from measurement of one frequency range to another. The phase-adjusted measurements of the output signal in the two or more frequency ranges are stitched together to determine the amplitude and phase of the output signal across the output signal spectrum.