A system for measuring a full spectrum of a modulated output signal provided by a device under test (DUT) includes a signal generating device that provides a radio frequency (RF) stimulus signal to the DUT, and generates a trigger signal, where the DUT outputs the modulated output signal in response. A receiver, having an intermediate frequency (IF) bandwidth less than a total bandwidth of the modulated output signal, includes mixers, an analog-to-digital converter (ADC) triggered by the trigger signal, and a local oscillator (LO) that consecutively generates multiple LO signals having different LO frequencies. A harmonic phase reference (HPR) generator generates a repetitive HPR signal having an HPR bandwidth wider than the total bandwidth of the modulated output signal and a modulation repetition rate the same as that of the RF stimulus signal. The mixers consecutively mix the LO signals with the HPR signal and the modulated output signal to provide IF signals input to channels of the ADC to form ADC data records, respectively, that capture the full spectrum of the modulated output signal.