How can a circuit make a repeating signal without a repeating input?

An electronic oscillator turns energy from a steady power supply into a repeating electrical signal. It does not need someone to feed in a complete wave for every cycle. Instead, a circuit repeatedly exchanges or redirects stored energy, while an active device replaces losses and controls the growing motion.

One useful model combines an inductor and a capacitor. A charged capacitor stores energy in an electric field. When it discharges through the inductor, current builds a magnetic field. As the capacitor's voltage reaches zero, the current can continue because the inductor resists an abrupt change in current. Charge then accumulates with the opposite polarity. Energy moves back and forth between electric and magnetic forms through successive parts of the cycle.

A real passive LC circuit loses energy through resistance and other effects, so its oscillation fades. An active circuit can use supply energy to compensate. A portion of the output returns through a frequency-selective feedback network with a phase that reinforces the motion near the intended frequency. Small disturbances, including noise when the circuit starts, provide a beginning. At startup the reinforcing effect can exceed the losses, allowing the signal to grow.

Growth cannot continue indefinitely. Device nonlinearities or a dedicated amplitude-control mechanism reduce the effective reinforcement as the oscillation becomes larger. In steady operation, the average energy added each cycle balances the energy lost. A simplistic claim that feedback must always have gain exactly one misses the distinction between starting and sustaining an oscillation. Real designs must also avoid unwanted modes and distorted waveforms.

Other oscillators use different timing mechanisms. A relaxation oscillator repeatedly charges a capacitor to a threshold and then changes state to discharge it. A crystal oscillator uses the mechanical resonance of a piezoelectric crystal within an electrical feedback system. Their waveforms and frequency stability differ, but each obtains energy from a supply and uses an internal process to establish repetition. Temperature, component tolerances and noise can disturb timing.

Clocks, radios and measurement instruments rely on these repeating signals as references or carriers. An oscillator is not a perpetual-motion machine: disconnecting the supply eventually stops a practical sustained oscillation. Its achievement is organizing steady input energy into regular change. The rhythm comes from the circuit's dynamics, while the energy to maintain that rhythm comes from outside.