a very simple receiver in which the signals received from radio stations are not amplified but converted into audio signals (detected) by a contact crystal detector. A crystal receiver usually contains an oscillatory circuit, a crystal detector (a semiconductor diode), a headset, and a blocking capacitor, which are connected in the circuit illustrated in Figure 1.
The oscillatory circuit is tuned to resonance with the carrier frequency that is being received by changing the capacitance of the capacitor C, thus attenuating all signals whose frequency differs from the resonance frequency. A sufficiently loud sound is produced in the headset by placing a steel wire spring at the “sensitive point” (a contact with maximum detecting effect) on the surface of a galena crystal
or a “zincite-chalcopyrite couple,” since they have semiconductor properties. (This type of detector was widely used in the I920’s; later, germanium and other semiconductor diodes with a constant “sensitive point” were used as detectors.) At the output of a crystal detector, currents of high (radio) frequency pass primarily through capacitor C6, and low-frequency (audio) currents pass through the headset. A crystal receiver has no electrical energy source of its own, and all processes take place through the energy of the radio waves that are received. By using a crystal receiver with a high external antenna and proper grounding it is possible to receive powerful broadcast stations at a distance of several thousand kilometers. With the advent of tube receivers, crystal receivers lost their importance.