What device superimposes the audio frequency signal into the carrier wave signal in a transmitter?

The modulator is the device in a transmitter that superimposes the audio frequency signal onto the carrier wave signal. This process is essential in radio communications, as it allows the audio signal, which carries the information (such as voice or music), to be transmitted effectively over various distances using the carrier wave.

In modulation, the characteristics of the carrier wave, such as its amplitude, frequency, or phase, are altered in accordance with the audio frequency signal. This manipulation enables the transmitted signal to be detected and demodulated back into its original audio form at the receiving end, making communication possible.

The other devices listed serve different functions in the realm of avionics and communications. For instance, a demodulator retrieves the audio signal from the modulated carrier wave upon reception. An amplifier increases the power of a signal without altering its characteristics, and an oscillator generates a continuous signal at a specific frequency. These functions are crucial in signal processing but do not perform the task of superimposing the audio frequency onto the carrier wave.