For an accurate spectral measurement, it is not sufficient to use proper signal acquisition techniques to have a properly scaled, single-sided spectrum. Spectral leakage is the result of an assumption in the FFT algorithm that the time record is exactly repeated throughout all time and that signals contained in a time record are thus periodic at intervals that correspond to the length of the time record. If the time record has a nonintegral number of cycles, this assumption is violated and spectral leakage occurs.
There are two cases in which an integral number of cycles are always acquired. One case is if the sampling synchronously with respect to the signal it measures and can therefore deliberately take an integral number of cycles. Another case is if a transient signal fits entirely into the time record.
In most cases, however, you measure an unknown signal that is stationary; that is, the signal is present before, during, and after the acquisition. In this case, you cannot guarantee that you are sampling an integral number of cycles. Spectral leakage distorts the measurement in such a way that energy from a given frequency component is spread over adjacent frequency lines or bins. You can use windows to minimize the effects of performing an FFT over a nonintegral number of cycles.
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