Which of the following is used to convert a signal from one frequency to another?

In radio electronics, a mixer is a circuit that takes two different input frequencies and combines them to produce new output frequencies. Specifically, it outputs the sum and the difference of the two original frequencies. This frequency conversion process is the heart of most modern radio designs, particularly the superheterodyne receiver, which shifts incoming high-frequency signals down to a lower intermediate frequency (IF) for easier processing. The other options serve completely different purposes. An amplifier increases the strength of a signal without changing its frequency, an inverter reverses polarity or logic state, and a phase splitter creates two out-of-phase signals from a single input.