If a signal is sampled, the spectrum is replicated at every harmonic of the sampling frequency (see the figure). For example, if the spectrum is sampled at 9.2 GHz, the spectrum will repeat at 9.2-GHz intervals, out to plus and minus infinity. As a consequence, the frequency at 4 GHz is indistinguishable from the signal at +13.2 GHz, +22.4 GHz, etc. The opposite is also true—signals at +13.2 GHz and +22.4 GHz, sampled at 9.2 GHz, will appear at 4 GHz.
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