If a function _x_ ( _t_ ) contains no frequencies higher than _B_ hertz
, it is completely determined by
giving its ordinates at a series of points spaced 1/(2 _B_ ) seconds apart.
换句话说就是，采样的频率F1和所传输信号的频率F2的关系如果满足，F1 >=2F2，那么该传输信号可与等同于采样后生成的信号。
如果F1<F2，这种情况被称为Undersampling，这样会导致测试最后生成的波形的频率比F2小，
如果F1>> 2F2，这种情况被称为Oversampling，这样可以提高测试精度。
所以为了找到一个合适的采样频率和合适的采样次数，引人Coherent sampling（周期信号取样）
UTP is a time interval over which unique information is spread（用来确保采样的唯一性）
假设：
被测device是：8-bit 21MHz ADC Test
采样频率Fs是： 20MHz
被测试信号的频率Fin：5MHz
采样次数Msamples是：N ≧ 2的9次方 =512
带入公式的Ncycles=512/4=128
因为“This integer number should be chosen carefully. We have three possible
types of integers, even, odd, and prime. Even is not a good idea since we
would hit the same code every M samples, where M can be much less than N. Odd
is a better idea since it takes longer to hit the same code.”，
所以我们将Ncycles改成129，然后计算出新的Fin=5.03MHz