Abstract: A spectral characterization method accurately characterizes an applied signal spectrum that has multiple, spectrally-separated signal peaks. The method detects signal peaks within the spectrum and measures the spectral content of that spectrum using a first measurement bandwidth that is sufficiently wide to encompass each of the signal peaks and accurately measure a signal component of the spectrum. Spectral content of the spectrum measured using a narrow measurement bandwidth that accurately characterizes a noise component of the spectrum. Signal-to-noise ratio for the signal peaks can be calculated based on the measurements performed using the first and second measurement bandwidths. The spectral characterization method is implemented using a spectral measurement instrument having an adjustable measurement bandwidth.