Notes for Image Rejection
This test measures a receiver's ability to reject signals that are products of the mixing process. Any given mixer will produce outputs on the IF frequency from two different RF inputs. One input is the sum of the LO and IF frequencies, and the other is the difference between the LO and IF frequencies. One of these is the "desired" RF frequency, and the other is the "image" frequency. To avoid interference from stations operating on the image frequency, a receiver's front end filters must greatly attenuate the image frequency.

The test uses one signal generator connected to the receiver's antenna port and a true RMS AC voltmeter connected to the receiver's AF output. The signal generator is set at the image frequency, and then the power is cranked up until the AC voltmeter shows a 3 dB increase in the noise.

Theoretically, it should be easy to filter signals at the image frequency, because they are far removed from the frequency of interest. However, these days the filtering in receiver front ends is frequently rudimentary, and there are plenty of powerful shortwave stations that could be problematic.