As I know Quadrature mixer only keep positive frequecy, e.g., if frf>flo, the result is fif=frf-flo, and the other portion image, flo+fif, will be depressed, that is called image injection, however, if I want to keep frequecy, frf+flo, and depress flo+if, is it can be implimentated?
If the RF input is a real signal then using a complex mixer with I and Q you get a complex IF output with both frequency ranges. Then you could apply a image rejection filter. It is called polyphase filter. It could reject the unwanted image to some degree.
If you have a complex RF and a complex LO you mix down to IF a true single sideband. Both mixer outputs have to be added together.