What circuit causes a transmitter to automatically transmit when an operator speaks into its microphone?

VOX stands for Voice-Operated Transmit (or Exchange). It is a circuit that senses the audio from your voice when you speak into the microphone and automatically switches the radio from receive to transmit mode, allowing for hands-free operation without needing a PTT (Push-To-Talk) button. The distractors are all related to frequency generation, not transmit switching. VFO (Variable Frequency Oscillator), VCO (Voltage-Controlled Oscillator), and VXO (Variable Crystal Oscillator) determine the radio's operating frequency.