Aspects of the invention include an equivalent-time sampling oscilloscope that receives a carrier signal, the carrier signal after it has been modulated with a repeating data pattern, and a pattern trigger signal that is synchronous with the data pattern. The carrier signal and the modulation are asynchronous, that is, they are not phase-locked in any way. The oscilloscope simultaneously samples the modulated carrier signal and quadrature phases of the unmodulated carrier signal at a plurality of timebase delays relative to the pattern trigger signal, and a plurality of times at each timebase delay. After collecting this information, the oscilloscope uses the quadrature samples to calculate phases of the unmodulated carrier signal that correspond to the samples of the modulated carrier signal. The oscilloscope then calculates a stationary representation of the modulated carrier signal by selecting samples of the modulated carrier signal that correspond to a carrier signal phase progression that would have been observed if the unmodulated carrier signal had been synchronous with the pattern trigger signal.