Frequency analysis of holder and sampling unit

In the theoretic analysis of computer control systems we generally treat sampling-holder as the ldquoidealrdquo rather than the ldquopracticalrdquo in order that the difficulty of mathematical treatment might be reduced. There exists difference between ideal sampling and practical sampling. Because sampling is based on the time variable, it canpsilat be described by transfer function, therefore we analysis it in the way of frequency spectrum. The frequency domain characteristics of both cases are analyzed and the equivalent substitute conditions are discussed in this paper. After sampled practically, connect an amplifier 1/ tau or connect a zero-order holder. Then we can see the substitution is feasible. It is equivalent on input and output.