Phase Coherent FSK Modulator (cn0186)
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 (see Figure 2, for example). However, the circuit shown in Figure 1 demonstrates how to configure the AD9958/AD9959 multichannel DDS for a robust phase coherent FSK (frequency shift keying) modulator by summing the outputs of the multichannel DDS together.
A multichannel DDS virtually eliminates temperature and timing issues between channels compared to synchronizing multiple single channel devices for the same application. For instance, multichannel DDS outputs, though independent, share the same system clock edges in the chip. Consequently, the system clock edges across multiple chips would not track as well over temperature and power supply deviations compared to an integrated multichannel DDS. As a result, a multichannel DDS is better suited for producing a closer to ideal phase coherent frequency transition at the summed output.
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