Phase Coherent FSK Modulator Using AD9959 Direct Digital Synthesizer. The standard single channel direct digital synthesizer (DDS) does not switch between frequencies in a phase coherent manner. By design, DDS frequency transitions are phase continuous. However, the circuit demonstrates how to configure the AD9959 multi-channel DDS for a robust phase coherent FSK (frequency shift keying) modulator by summing the outputs of the multi-channel DDS together