What is the function of the Variable Frequency Oscillator (VFO) circuit in a transceiver?
Why A VFO is the tunable oscillator that establishes the operating frequency of a radio. In the receiver it feeds the mixer to select which incoming signal gets converted to the IF, and on transmit it sets where the signal is radiated, so the same circuit determines both receive and transmit frequency in a transceiver. When you turn the main tuning knob, you are changing the VFO frequency (in modern rigs usually a synthesizer or DDS doing the same job).
Watch out The choice about injecting a variable frequency for CW reception describes the BFO (beat frequency oscillator), which is a separate fixed oscillator near the IF that turns CW and SSB into audible audio. Automatic frequency control is a correction circuit that nudges tuning to stay on signal, not the primary tuning source.
VFO = the tuning knob. Variable Frequency Oscillator sets where you listen and where you talk; BFO is the one that makes CW audible.