a dds remarkably increased in the number of frequencies which can be output while maintaining the phase coherency , and an nmr instrument using such a dds are provided . a dds including phase accumulators and a phase - to - amplitude modulator is provided with a plurality of phase accumulators operating with fixed phase implements which are equal to powers of 2 , a controller for outputting each bit of a frequency tuning word as control data , a plurality of switches for outputting an output of an associated one of the phase accumulators when an associated one of the control data supplied from the controller is 1 and outputting 0 when the associated one of the control data is 0 , and an adder for adding up outputs of the switches .