Spectrum problem of AD9162 output

Question

When AD9162 output 1.9G single tune,the spectrum : 
 1. AD9162 configures: DAC clock=4GHz,FIR85=1,interpolation=1,nco_en=0,complex data on 204b interface(2G samplerate). 
 
 2. AD9162 configures: DAC clock=4GHz,FIR85=1, interpolation=0,nco_en=0,real data on 204b interface(4G samplerate). 
 
 My question is: Why is the amplitude of the image frequency so high? It seems that the suppression of FIR85 has no effect. What are the possible reasons for this phenomenon?

PMH2023 · Answer

Hello,

Likely issue is that you are operating within the FIR85 transition band region. One would expect the image to be significantly suppressed if a 1.25 GHz digital tone was injected as the digital data.

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Spectrum problem of AD9162 output