Page "Variable-frequency oscillator" Paragraph 3

In addition to the desired IF signal and its unwanted image ( the mixing product of opposite sign above ), the mixer output will also contain the two original frequencies, and and various harmonic combinations of the input signals.

These undesired signals are rejected by the IF filter.

If a double balanced mixer is employed, the input signals appearing at the mixer outputs are greatly attenuated, reducing the required complexity of the IF filter.