AD6688 Data Capture in 1.2 GHz BW Mode

I am capturing data using the AD6688 Eval Board and ACE software with the following  settings :

NCO frequency set to 1 GHz 

 I apply 4.1 GHz , 10 dBm tone signal. Against my expectation the  the signal has a component at the image frequency (- 100 MHz).  Following is the DDCFFT of the capured data: 

What is it that I am doing wrong?

Thank you