As an exercise, Analog is going to look at how signals add constructively and destructively in an image reject architecture. There are two signals: a cosine at the desired frequency, which will be called RF, and a cosine at the undesired frequency, otherwise known as the undesired image (UI). Note that there is no need for there to be any relationship between the RF and UI signals. This image rejection will work even if the RF is not present, or if the UI just happens to be noise.