The phase noise of a DDS based system is predominantly related to the residual phase noise of the DDS which is typically very low and the phase noise contribution of the clock driving the DDS. Ignoring phase noise of post amplification my questions are as follows. The residual phase noise of the DDS is usually specified at a particular output frequency. The phase noise of the clock is at a different frequency for example I may care about the phase noise at 200MHz output but the DDS clock is running at 1GHz. Do I scale the 1GHz phase noise to 200MHz and RMS with the residual phase noise to get the system phase noise? This would reduce the clock phase noise contribution by 20log(1000/200)=14dB. Or do I deal with this like a typical mixer application where I directly RMS the two phase noises and ignore the frequency difference? Thanks in advance for your input.