An image response is an undesired input frequency that reaches the same desired IF through another valid sum or difference relationship with the LO. For low-side downconversion with IF = RFwanted - LO, the corresponding image commonly lies at RFimage = LO - IF. For high-side injection, the image moves to the opposite side of the LO and the wanted spectrum may reverse frequency order.
Calculate the complete image band at every RF, IF and LO edge. Then derive required rejection from the maximum credible image or blocker level, allowed converter output interference and downstream gain. Allocate that rejection between the RF preselector, mixer or image-reject/IQ architecture, calibration and any post-conversion filtering. A converter data-sheet image number is not sufficient unless its frequency, gain state, temperature, input level and output reference plane match the project.
Reject the plan when the image overlaps the wanted preselector passband, when quadrature mismatch cannot maintain rejection across production and temperature, or when the required filter transition is physically unrealistic. The RFQ should include wanted and image bands, LO states, blocker levels, preselector response, gain/phase calibration method, residual-image limit and the production verification sweep.