What type of reference clock is recommended for your DDS devices?

Crystal or SAW oscillators with low phase noise do a very good job for a REF CLK. The performance of the DDS system is limited by the quality of the oscillator. The better the performance of the oscillator, the better the DDS performance. Some DDS devices offer internal PLLs, which can be used to multiply or increase the reference clock frequency. For more details on using the clock multiplier see "Why does the DDS spectral performance degrade when using large values of multiplication in the clock multiplier"?