Image Rejection Ratio in ADS

For calculating the image rejection ratio in ADS, we give two signals as system input, "original signal" and the "image signal + 10Khz ".

In my receiver I am down converting my signal to 20Mhz. So for calculating IRR, What i have to do ?
Shall i take peek difference of "20Mhz" and "20Mhz +10Khz" or 0Hz and 10Khz?

plz help !1

20Mhz + (-)10Khz if mixer.....

What is the IF frequency?
Is it single or double down conversion?