Define image frequency. Define image frequency rejection ratio.

 An image frequency is any frequency other than the selected radio frequency carrier that ,if allowed to enter a receiver and mix with the local oscillator ,will produce a cross product frequency that is equal to the intermediate frequency.

The image frequency rejection ratio is the measure of the ability of pre selector to reject the image frequency.

Mathematically ,IFRR is

IFRR =(1+Q²r²)^1/2

Where r= (f_im/f_RF)-(f_RF/f_im)