In an FM transmitter, which stage produces a useful harmonic?

A frequency multiplier intentionally creates a strong harmonic (a multiple of the input frequency) to reach a higher operating band. By deliberately overdriving this stage, it produces a distorted waveform rich in harmonics, and a tuned circuit at the output selects the desired "useful" harmonic to pass on to the next stage. In contrast, other stages like the speech amplifier or power amplifier strive for low distortion. Unwanted harmonics produced by the power amplifier are considered spurious emissions and must be filtered out before they reach the antenna to prevent interference.