What type of circuit varies the tuning of an amplifier tank circuit to produce FM signals?

A phase modulator creates frequency modulation (FM) indirectly by varying the phase of the carrier signal. When the phase of an RF signal is shifted back and forth, its instantaneous frequency also changes, resulting in a phase-modulated signal that can be demodulated by standard FM receivers. Varying the resonant frequency of an amplifier tank circuit using a varactor diode or a similar reactive component is a classic method of achieving phase modulation. This changing reactance shifts the phase of the passing signal in step with the audio. In contrast, a balanced modulator is used to produce Double Sideband Suppressed Carrier (DSB-SC) signals for SSB transmission, and an audio modulator is a generic term that doesn't describe this specific RF tuning action.