Feedback and resonance create sustained AC
An oscillator generates AC (a repeating waveform) from a DC power supply using positive feedback and a resonant circuit. The circuit amplifies a signal at its resonant frequency and feeds it back into itself with the right phase and magnitude to sustain oscillation. A Wien bridge oscillator uses an RC network tuned to a frequency where the phase shift is zero; feedback reinforces that frequency and suppresses others. A Colpitts or Hartley oscillator uses an LC tank circuit (inductor and capacitor) at its resonant frequency. A crystal oscillator uses a piezoelectric crystal, which has extremely stable and precise resonance.
Frequency stability and control
Low-cost RC oscillators (phase-shift, Wien bridge) drift with temperature and component tolerance, making them suitable for audio applications where precision is not critical. LC oscillators are better because inductive and capacitive reactances are less temperature-sensitive, but still drift more than crystal oscillators. Crystal oscillators (quartz) are the benchmark for frequency stability, used in digital clocks, radio transmitters, and signal generators. Tunable oscillators use voltage-controlled capacitors (varactors) to adjust frequency across a range, implementing frequency modulation or phase-locked loops for precise synchronization to external signals.