Which of the following is used to convert a signal from one frequency to another?
Why A mixer is a nonlinear stage that combines an incoming signal with a local oscillator signal and produces outputs at the sum and difference of the two frequencies. Selecting one of those products with a filter moves the signal to a new frequency, which is how superheterodyne receivers convert an incoming signal down to an intermediate frequency and how transmitters move a low-frequency signal up to the operating band. Transverters and frequency multipliers rely on the same mixing principle.
Watch out An amplifier is tempting because it is also a signal-processing stage, but it only increases amplitude and leaves the frequency unchanged. A phase splitter produces two outputs of opposite phase, and an inverter reverses polarity or converts DC to AC.
Mix two frequencies, get sum and difference. Mixing changes frequency; amplifying only changes size.