 With coherent sampling one is assured that the signal power in an FFT is contained within one FFT bin, assuming single input frequency.The condition for coherent sampling is given by 

Where formula_4 value must be a power of 2 and formula_3 value must be an odd or prime number.If we have formula_8 and formula_9 and we want an input frequency close to formula_10, let's say formula_11, then formula_12 which is close to an integer, so we could round it down to formula_13 and we would get formula_14.