page_title: Crystal detector
section_title: How it works
hierarchical_section_title: Crystal detector / How it works
caption_attribution_description: English: Graphs of the detection (demodulation) process for amplitude modulated (AM) radio signals, adapted from a 1922 radio book. (A) The amplitude modulated signal applied to the detector. (B) The signal at the output of the detector. The detector allows current in only one direction, stripping off the excursions of the signal on one side. (C) After filtering. The filter smoothes the output, removing the radio frequency carrier pulses, leaving the audio frequency signal (modulation). Alterations to image: added vertical axis to top drawing, and colored labels.
context_page_description: A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal and was employed as a detector to extract the audio modulation to produce the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices. The most common type was the so-called cat whisker detector, which consisted of a piece of crystalline mineral, usually galena, with a fine wire touching its surface. The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his microwave experiments. Bose first patented a crystal detector in 1901. The crystal detector was developed into a practical radio component mainly by G. W. Pickard, who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions.
context_section_description: The contact between two dissimilar materials at the surface of the detector's semiconducting crystal forms a crude semiconductor diode, which acts as a rectifier, conducting electric current in only one direction and resisting current flowing in the other direction.  In a crystal radio, it was connected between the tuned circuit, which passed on the oscillating current induced in the antenna from the desired radio station, and the earphone.  Its function was to act as a demodulator, rectifying the radio signal, converting it from alternating current to a pulsing direct current, to extract the audio signal (modulation) from the radio frequency carrier wave.  The audio frequency current produced by the detector passed through the earphone causing the earphone's diaphragm to vibrate, pushing on the air to create sound waves.  This diagram shows a simplified explanation of how it works:
  (A) This graph shows the amplitude modulated radio signal from the receiver's tuned circuit, which is applied as a voltage across the detector's contacts.  The rapid oscillations are the radio frequency carrier wave.  The audio signal (the sound) is contained in the slow variations (modulation) of the size of the waves.  If this signal were applied directly to the earphone, it could not be converted to sound, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the earphone's diaphragm.
  (B)  This graph shows the current through the crystal detector which is applied to the earphone and bypass capacitor.  The crystal conducts current in only one direction, stripping off the oscillations on one side of the signal, leaving a pulsing direct current whose amplitude does not average zero but varies with the audio signal.
  (C) This graph shows the current which passes through the earphone.  A bypass capacitor across the earphone terminals smooths the waveform, removing the radio frequency carrier pulses, leaving the audio signal.  When this varying current passes through the earphone voice coil, it creates a varying magnetic field which pulls on the earphone diaphragm, causing it to vibrate and produce sound waves.
  Crystal radios had no amplifying components to increase the loudness of the radio signal; the sound power produced by the earphone came solely from the radio waves of the radio station being received, intercepted by the antenna. Therefore, the sensitivity of the detector was a major factor determining the sensitivity and reception range of the receiver, motivating much research into finding sensitive detectors.
  In addition to its main use in crystal radios, crystal detectors were also used as radio wave detectors in scientific experiments, in which the DC output current of the detector was registered by a sensitive galvanometer, and in test instruments such as wavemeters used to calibrate the frequency of radio transmitters.
caption: Diagram showing how a crystal detector works
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