DAC Nyquist Zones, Zero Order Hold, and Images
why do the Fout images exist in every Nyquist zone? What is a Nyquist Zone?
why do the Fout images decrease in amplitude? The answer is due to frequency response of the zero-order hold function. In Figure below, a zero order hold function is shown in the time domain and frequency domain. The frequency domain plot shown in Figure is also the sinc function (sin(x)/x). The sinc function has a decreasing amplitude with frequency due to the denominator x
原文：http://www.cnblogs.com/hiramlee0534/p/5878766.html