IQ signal frequency shifter cannot downconvert to DC

[zyb1003]

Hello guys, 

Now I use labview fpga module to generate and acquire IQ signal. The structure is create DC IQ first then use frequency shifter (f) to upconvert frequency to generate AC IQ. To acquire the amplitude and phase, I use frequency shifter (-f) to downconvert the AC signal to get the DC IQ, then I can use the cordic to calculate the amplitude and phase. 

This is my code screenshot:

So now I am sure that the outputs are good quadrature AC signals, but the downconverted I and Q (I_test and Q_test in the screenshot) are still AC. Why would this happens? 

[zyb1003]

Anyone has any ideas? 

Thanks