LearnTronics
Passive AM Crystal Radio
Recipe #42 — passive am crystal radio
The crystal radio is a beautiful payoff circuit: resonance, diode detection, capacitance, and audio all work together, yet a strong station can sometimes be heard without any battery-powered amplifier.
Several earlier lessons meet here
LC resonance selects a narrow frequency range. Diode rectification removes one polarity of the RF waveform. Capacitance and the earphone/load help reject the carrier while preserving the much slower audio variation.
Why crystal radio is such a remarkable teaching circuit
A strong local AM station can sometimes be heard with no battery at all. The received radio wave itself supplies the tiny amount of energy delivered to the earphone. Early detectors used mineral crystals; modern demonstrations often use a germanium or Schottky diode because of its low forward voltage.
Antenna, ground, and earphone matter greatly
Because no amplifier supplies extra power, losses are critical. A long antenna, effective RF ground, high-Q coil, low-loss tuning capacitor, sensitive high-impedance earphone, and low-forward-drop detector can make the difference between hearing a station and hearing nothing.