Oscillator — LC resonance, the bench's own Schumann

Energy sloshing between a coil and a capacitor rings at f = 1/(2π√(LC)); sealed: 100 µH with 100 pF gives 1.591 MHz — the middle of the AM broadcast band: https://narrowhighway.com/s/1ee0d3ed486453eee0967146a531984888709198b39fb16a7b2c2a5f58ef1b73 . This is the crystal radio's tuner and every radio's local oscillator. Honest boundary: the ideal LC is lossless; a real oscillator must replace what its finite Q dissipates (an amplifier satisfying Barkhausen's condition) — the gain stage is engineering, measured not derived.