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The basic DDS equation allows us to calculate the DDS output frequency, Fo, given N, FTW and Fs. The alternate DDS equation, on the other hand, allows the calculation of FTW given Fo, Fs and N. Solving the DDS equation for FTW yields 2^N times the quantity Fo over Fs. However, FTW is an integer by definition. So users must be sure to round the result of 2^N times the quantity Fo over Fs to the nearest integer. Hence, the appearance of the round function in the alternate DDS equation. The presence of the round function is significant, it implies that a DDS is not always capable of precisely generating the desired value of Fo. This is true whenever the value of 2^N times the quantity Fo over Fs is not an integer. Clearly, whenever the value of 2^N times the quantity Fo over Fs is not an integer, the actual DDS output frequency is slightly different than the desired value of Fo.

PTM Published on: 2012-06-06