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if you are under-sampling then the phase noise due the sampling clock will increase by 6dB per octave increase in sampling harmonic.
i.e harmonic 1 +6dB, harmonic 3 9.5dB
Phase noise of a source increases by 20*log( f2/f1)
That is just phase noise as biff44 says you need to account for noise folded into your passband by the AD converter