Some questions about AD6674

Hi,

I have a questions about AD6674. i find that when i use the MIXER+NCO of the AD6674,the near-end phase noise of the output of AD6674 will become worse then do not use them.

The sample clock of AD6674 is 1GHz(@10KHz, -135dBc/Hz),  the input signal is 749MHz(@10KHz,-135dBc/Hz). DCM=4

The power of input signal is -10dBm,and the power of output data is about -20dBFs. The numbers of sample data are 4194304(the noise floor is low enough).

a):  when i use the MIXER+NCO= 1/4 FS MODE. the output signal frequency is 1MHz in first Nyquist zone. i get the datas output of AD6674 and analyse them in MATLAB, i find the phase noise of output signal is -120dBc/Hz @10KHz. 

b):  Then i change the analog input frequency to 1MHz, the ADC MODE change to MIXER+NCO disable,and the DCM also is 4. And the output data of AD6674 is also 1MHz in first Nyquist zone. I also get the datas of AD6674 and analyse them in MATLAB. i find the phase noise of output signal is -130dBc/Hz @10KHz. 

So why the MIXER+NCO will affect the near-end phase noise?

i can not find any answer in AD6674 DATASHEET. So  i guess it may be have some relationship about the data width of the NCO(12bit may be too low).  

Look forward for your help 

Thanks.

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