A downconverter mixes a first comb spectrum with a local oscillator signal to generate a second comb spectrum in a lower frequency range. The first comb spectrum comprises frequency components separated from each other according to a frequency spacing interval, and the LO signal has a frequency offset relative to the first comb spectrum, where the frequency offset is a rational fraction of the frequency spacing interval. The second comb spectrum comprises lower and upper sideband responses corresponding to respective lower and upper sideband signals of the first comb spectrum. The lower and upper sideband responses in the second comb spectrum can be distinguished from each other based on the frequency offset.