AD9122

ADF4350+AD9122+ADL5375

I use ADF4350 generate 2600MHZ LO single_end to ADL5375 , the input of I and Q for AD9122 is a constant, then generate 1MHZ NCO low sideband  to modulate a the constant  , that is , the output of AD9122 is 1MHZ single frequency ,

my question is  :

the output signal frequency of ADL5375 should be 2599MHZ , but in the Analyzer the frequency both 2599MHZ and 2600MHZ exist  ,  so why ?  how to remove the LO frequency ?

thanks