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

• A. Modulator
• B. Amplifier
• C. Mixer
• D. Demodulator

Answer: (C)

A mixer is a crucial component in radio communications used to convert a signal from one frequency to another. This process involves taking two input signals, typically referred to as the "local oscillator" signal and the "carrier" signal, and producing outputs that include new frequency components, typically at the sum and difference of the inputs. This frequency conversion is essential in various applications, such as in superheterodyne receivers where signals are moved to an intermediate frequency for easier processing.

The other devices mentioned serve different purposes: a modulator is used to impose information onto a carrier wave, an amplifier is designed to boost the strength of a signal, and a demodulator extracts the information from a modulated carrier wave. Each device plays a vital role in the overall communication system, but the mixer specifically facilitates the frequency conversion process, which is why it is the correct answer in this context.