Brief answer
e.g. Channel1 freq. = 900MHz
Channel2 freq. = 900.025MHz
Channel step = 25Khz
IF = 21.4MHz
Therefore VCO = 878.6MHz ( low side injection )
or VCO = 921.4MHz ( high side injection )
ACR : The ability of receiver to avoid the influrence of adjacent channel interference.
e.g. Rx receiving signal at Ch1 900MHz. Then Ch2, 900.025MHz is the adjacent channel. ( because channel step is 25KHz )
We hope Ch2 has no influrence on Ch1 received signal quality.
Image rejection : a measure of the ability to reject undesired signal at the image frequency
e.g. VCO freq. = 878.6MHz and channel frequency = 900MHz ( IF=21.4MHz ). Then image freq is at 857.2MHz. Image rejection depends on how good is the front end bandpass filter attenuation at this freq.
Of course, a clean VCO ( e.g. low phase noise ) is always needed.
Rogerynt