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Any crystal detector that incorporates a very thin, resilient wire to make contact with the crystal is popularly known as a cat whisker detector.
Precision crystal detector with iron pyrite crystal, used in large wireless stations, early 1900s. The crystal is inside the metal capsule under the vertical needle (right). The leaf springs and thumbscrew allow fine adjustment of the pressure of the needle on the crystal.

A crystal detector, used in early 20th century technology radio receivers, is a device that provides a physical contact between a crystalline mineral and a metal or between two minerals for the purpose of rectifying radio frequency alternating current so as to permit the radio program to be heard in the headphones.[1] [2] The asymmetric electrical conduction across a contact between a crystalline mineral and a metal was first discovered in 1874 by Karl Ferdinand Braun. The first crystal detectors were used in wireless research by Braun, and microwave optics research in 1894 by Jagadish Chandra Bose[citation needed]. Development of this type of detector began to significantly advance in 1902 when Greenleaf Whittier Pickard also observed that two different materials in physical contact, with no local power source in the circuit, would function as a detector.[3] Pickard than commenced research on the performance of many crystalline and metallic materials as detectors. Crystal detectors of the configuration commonly referred to as the cat whisker detector type became very commonly used in crystal radios.[4] The crystal detector was an early predecessor of semiconductor diodes, and one of the first semiconductor electronic devices (after photoconductors), although semiconductor principles were not known at the time these detectors were developed.

Contents

FunctionEdit

The crystal detector extracts the audio signal from the radio frequency:

How a crystal detector works in a radio receiver [1][5]
(A) The amplitude modulated radio signal from the receiver's tuning section. 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. This signal cannot be converted to sound by the earphone, because the audio excursions are the same on both sides of the axis, averaging out to zero, resulting in no net motion of the earphone's diaphragm.(B) 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) A bypass capacitor across the earphone smooths the waveform, removing the radio frequency carrier pulses, leaving the audio signal.

HistoryEdit

 
The modern circuit symbol for a diode originated as a schematic drawing of a crystal detector.[citation needed]

The crystal detector was the most successful of many detector devices that were used in the early days of radio. It replaced earlier electrolytic, magnetic, and particularly coherer detectors in radio receivers around 1906. Later, when AM radio transmission was developed to transmit sound, around World War I, crystal detectors proved able to receive this transmission as well.

The "unilateral conduction" of crystals, as it was then called, was discovered by Ferdinand Braun, a German physicist, in 1874 at the University of Würzburg, before radio had been invented.[6] Bengali-Indian scientist Jagadish Chandra Bose was the first to use a crystal to detect radio frequency waves, in his pioneering experiments with microwaves in 1894, applying for a patent on a galena detector in 1901.[7] The crystal detector was developed as a practical device for use in wireless communication mainly by G. W. Pickard. His first detector, which used a silicon crystal, was patented in 1906.[8] At nearly the same time, Henry Harrison Chase Dunwoody,[9] a retired general in the U.S. Army Signal Corps, patented the silicon carbide (carborundum) detector,[10] an artificial substance created accidentally during attempts by Edward Acheson to create diamonds. Pickard tested more than 30,000 combinations of crystal and wire contacts and developed several types of detectors that saw wide use.[11] One variation consisted of a pair of different crystals with their faces touching, such as zincite touching bornite or chalcopyrite. Pickard named this the Perikon detector, from "PERfect pIcKard cONtact". Other detectors patented by Pickard employed the common crystal iron pyrite.

(top) Cartridge carborundum detector from 1925. Crystal detectors were also used to a limited extent in vacuum tube radios because they were more sensitive than vacuum tube detectors. The carborundum detector was used, since it did not require adjustment and so was made in the form of cartridges
(bottom) The carborundum detector required a DC bias of several volts, provided by a battery and potentiometer.
 
A crystal detector made in the 1960s for antique reproduction crystal sets.

The temperamental, unreliable action of the crystal detector was a barrier to its acceptance as a standard component in commercial radio equipment[12] and was one reason for its rapid replacement by vacuum tubes after 1920. Frederick Seitz, a later semiconductor researcher, wrote:

Such variability, bordering on what seemed the mystical, plagued the early history of crystal detectors and caused many of the vacuum tube experts of a later generation to regard the art of crystal rectification as being close to disreputable.[13]

From the earliest wireless telegraphy days of radio, well into the age of commercial AM broadcasting, unamplified radio receivers were powered only by the radio energy they picked up through their antennas. The crystal radio was the most widely used of these. Manufactured and homemade by the millions, it helped introduce radio to the public, contributing to the development of radio from an experimental hobby to an entertainment medium around 1920. After about 1920, receivers using crystal detectors were largely superseded by the first amplifying receivers, which used vacuum tubes. These did not require the fussy adjustments that crystals required, were more sensitive, and also were powerful enough to drive loudspeakers. Nevertheless, the expense of the early vacuum tubes and the batteries needed to run them meant that the crystal detector remained in commercial and military use for almost a decade more. However, by the late 1920s, radios using crystal detectors were relegated to use by hobbyists and youth groups and have been used by them as educational devices to the present day.

The point-contact semiconductor detector was subsequently resurrected around World War II because of the military requirement for microwave radar detectors. Vacuum-tube detectors do not work at microwave frequencies. The small area of the point contact minimized minority carrier storage and capacitance, making these diodes fast enough to function at radar frequencies. Silicon and germanium point-contact diodes were developed. Wartime research on p-n junctions in crystals paved the way for the invention of the point-contact transistor in 1947.

The germanium diodes that became widely available after the war proved to be as sensitive as galena and did not require any adjustment, so germanium diodes replaced crystal detectors in the few crystal radios still being made, largely putting an end to the manufacture of this antique radio component. Although crystal detectors are obsolete, modern point-contact silicon detectors are still commercially produced.[14] Thus, the point-contact method used to make these first semiconductor diodes 100 years ago is still being used today.

TypesEdit

Historically, many minerals and compounds have been used as crystal detectors,[15] the most important being silicon, iron pyrite ("fool's gold", iron disulfide, FeS2), galena, molybdenite (MoS2), and silicon carbide (carborundum, SiC). Some were used with gold or graphite cat whiskers. The term "cat whisker detector" has been popularly applied to any crystal detector that incorporates a small gauge, resilient, formed length of wire to contact the crystal with only a small amount of force.[16] Another type had a crystal-to-crystal junction instead of a cat whisker, with two crystals mounted facing each other. One crystal was moved forward on an adjustable mount until the crystal faces touched. The most common of these was a zincite–chalcopyrite junction trade-named Perikon,[17] but zincite–bornite (ZnO-Cu5FeS4), silicon–arsenic, and silicon–antimony junctions were also used. To increase sensitivity, some of these junctions such as silicon carbide were biased by connecting a battery and potentiometer across them to provide a small constant forward voltage across the junction.[18]The goal of researchers was to find junctions that were not as sensitive to vibration and unreliable as galena and pyrite[citation needed]. Some of these other junctions, particularly carborundum, were stable enough that they were equipped with a more permanent spring-loaded contact rather than a cat whisker.[19] For this reason, carborundum detectors were preferred for use in large commercial wireless stations and military and shipboard stations that were subject to vibration from waves and gunnery exercises. Another quality desired was the ability to withstand high currents without damage, because in communication stations the fragile detector junction could be "burned out" by atmospheric electric charge from the antenna or high radio frequency current leaking into the receiver from the powerful spark-gap transmitter during transmissions. Carborundum detectors, which used large-area contacts, were also particularly robust in this regard.

 
Foxhole radio from World War II

The oxide layers that form on many ordinary metal surfaces have semiconducting properties, and detectors for crystal radios have been improvised from a variety of everyday objects such as rusty needles and corroded pennies. The foxhole radio [5] was a crystal radio receiver improvised by soldiers during World War II without access to conventional sets. It used a razor blade and a safety pin or pencil lead to form a demodulating junction. Much patience was required to find an active detecting site on the blade.







Silicon detectorEdit

The silicon detector was the first crystal detector type that widely took the place of coherers and electrolytic detectors in wireless receivers[citation needed]. A much larger metal point size and greater force is usually used with the silicon crystal detector than with some other minerals.[3] The silicon detector is more sensitive than the carborundum detector, but less sensitive than the galena detector.[20]

DescriptionEdit

The junction consists of silicon on one side and the rounded or pointed end of a metal cylinder or screw on the other. It is preferred that the contact point be of platinum.[17] In a common implementation of this detector, the crystal is placed, without fastening, upon a flat metal surface that is centered under the rounded or pointed contact.[21] The crystal may be readily re-positioned under the point by sliding on the flat surface. Better sensitivity is obtained if the crystal is set into low melting point alloy to form the non-rectifying connection to the crystal.[22]

CrystalEdit

In many implementations, the silicon crystal is polished to a smooth, flat face on the side that the adjustable point will contact.[22]

OperationEdit

The pointed contact is brought to bear against the surface of the silicon crystal while the operator listens for the desired sound from the headphones. Different locations on the surface of the crystal are tried until one is found that produces the desired sound[citation needed].


Galena detectorEdit

The galena detector[23] has also been widely known as a "cat's whisker detector".[24]

DescriptionEdit

The physical construction of the galena detector commonly includes a mounting base of insulating material, two binding posts or plugs for electrical connection, a metal cup that holds a galena crystal, and a metal bracket or hardware that retains a moveable metal structure equipped with a knob and a thin wire to contact the crystal with controllable spring pressure.

CrystalEdit

A galena mineral crystal forms the semiconductor side of the junction. Galena (PbS, lead sulfide), is a naturally occurring ore of lead. It is a semiconductor with a small bandgap of about 0.4 eV and is used without treatment directly as it is mined. However, not all galena crystals would function in a detector; galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it from galena samples with poor detecting properties. A rough pebble of detecting mineral about the size of a pea is mounted in a metal cup, which forms one side of the circuit. The electrical contact between the cup and the crystal has to be good, because this contact must not act as a second rectifying junction, which will prevent the device from functioning. To make good contact with the crystal, it is either clamped with setscrews or mounted in low melting point alloy metal. Because the relatively high melting temperature of tin–lead solder can damage many crystals, a low-melting-point (well under 200 °F (93 °C)) alloy such as Wood's metal is used. One surface is left exposed to allow contact with the cat whisker.

Cat whiskerEdit

The "cat whisker"[23][25][26] (also "catwhisker"[27] or "catswhisker"[28]), a springy piece of thin metal wire, forms the metal side of the junction. Phosphor bronze wire of about 30 gauge is commonly used because it has the optimal amount of springiness. It is mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal can be probed from many directions to find the most sensitive spot. Cat whiskers in simple detectors may be straight or curved, but most manufactured cat whiskers include a coiled section in the middle that serves as a spring.[29] The crystal requires just the right gentle pressure by the wire; too much pressure causes the device to conduct in both directions.

OperationEdit

 
A small portable crystal radio with a crystal detector visible at top

The tip of the wire contacting the surface of the crystal forms an unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode. This junction conducts electric current in only one direction and resists current flowing in the other direction. In a crystal radio, its function is to rectify 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 metal whisker is the anode, and the crystal is the cathode; current flows from the whisker into the crystal but not in the other direction.

Only certain sites on the crystal surface function as rectifying junctions. The device is very sensitive to the exact geometry and pressure of contact between wire and crystal. It is therefore made adjustable, and a usable point of contact is found by trial and error before each use. The wire is suspended from a moveable arm and is dragged across the crystal face by the operator until the device begins functioning. In a crystal radio, the operator tunes the radio to a strong local station if possible and then adjusts the cat whisker until the station or static is heard in the radio's earphones. This requires some skill and a great deal of patience; even then, a good contact is easily lost by the slightest vibration. An alternative method of adjustment is to use a battery-operated buzzer to generate a test signal. The spark produced by the buzzer's contacts functions as a weak radio transmitter whose radio waves are received by the detector. When a rectifying spot has been found on the crystal, the buzz is heard in the earphones. The buzzer is then turned off.


The first crystal detector commercially produced, Pickard's silicon detector, from 1906.
A silicon-arsenic crystal detector, manufactured by the Wireless Specialty Apparatus Company, used in wireless receivers around the first decade of the 20th century.
Precision detector using leaf spring and thumbscrew to control pressure
Pyrite detector
Carborundum (silicon carbide) detector. This detector uses a robust metal clamp contact with the crystal, so carborundum detectors were used in military and shipboard stations where vibration could be expected.

See alsoEdit

ReferencesEdit

  1. ^ a b Marx & Van Muffling (1922) Radio Reception, pp.46-49
  2. ^ The Principles Underlying Radio Communication, 2nd Ed., Radio pamphlet no. 40. USA: Prepared by US National Bureau of Standards, United States Army Signal Corps. 1922. p. 434. 
  3. ^ a b G. W. Pickard, "How I Invented the Crystal Detector". Electrical Experimenter, vol. VII, no. 4, p. 325, Aug. 1919
  4. ^ P. W. Harris, Crystal Receivers for Broadcast Reception, London: Wireless Press LTD., 1922
  5. ^ a b Campbell, John W. (October 1944). "Radio Detectors and How They Work". Popular Science. New York: Popular Science Publishing Co. 145 (4): 206–209. Retrieved 2010-03-06. 
  6. ^ Braun, F. (1874), "Ueber die Stromleitung durch Schwefelmetalle" [On current conduction through metal sulfides], Annalen der Physik und Chemie (in German), 153 (4): 556–563, doi:10.1002/andp.18752291207 
  7. ^ US 755840, Bose, Jagadis Chunder, "Detector for electrical disturbances", published September 30, 1901, issued March 29, 1904 
  8. ^ US 836531, Pickard, Greenleaf Whittier, "Means for receiving intelligence communicated by electric waves", published August 30, 1906, issued November 20, 1905 
  9. ^ Some biographical information on General Henry H.C. Dunwoody is available at: Arlington National Cemetery.
  10. ^ US 837616, Dunwoody, Henry H. C., "Wireless-telegraph system", published March 23, 1906, issued December 4, 1906 
  11. ^ Lee, Thomas H. (2004). Planar Microwave Engineering: A practical guide to theory, measurements, and circuits. UK: Cambridge University Press. pp. 297–300. ISBN 978-0-521-83526-8. 
  12. ^ Braun, Ernest; Stuart MacDonald (1982). Revolution in Miniature: The history and impact of semiconductor electronics, 2nd Ed. UK: Cambridge Univ. Press. pp. 11–12. ISBN 978-0-521-28903-0. 
  13. ^ Riordan, Michael; Lillian Hoddeson (1988). Crystal fire: the invention of the transistor and the birth of the information age. USA: W. W. Norton & Company. pp. 19–21. ISBN 0-393-31851-6. 
  14. ^ For example, Advanced Semiconductor Inc. (North Hollywood, California, USA) is selling Si point-contact detectors which will cover from UHF (ultra high frequency) to 16 GHz.
  15. ^ P. E. Edelman, "Experimental wireless stations", New York: N. W. Henley, 1920, p. 258
  16. ^ E. E. Bucher, The Wireless Experimenters Manual, New York: Wireless Press, 1920 p. 167
  17. ^ a b A. P. Morgan, Wireless Telegraph Construction for Amateurs, 3rd ed. New York: D. Van Nostrand Co., 1914, p. 136
  18. ^ Pender, Harold; William Arthur del Mar (1922). Handbook for Electrical Engineers, 2nd Ed. New York: John Wiley & Sons. p. 1268. 
  19. ^ The Principles Underlying Radio Communication, 2nd Ed., Radio pamphlet no. 40. USA: Prepared by US National Bureau of Standards, United States Army Signal Corps. 1922. p. 435. 
  20. ^ A. P. Morgan, 1914, p. 118
  21. ^ A. P. Morgan, 1914, p. 134
  22. ^ a b A. P. Morgan, 1914, p. 135
  23. ^ a b A. P. Morgan, 1914, p. 199
  24. ^ Kumar, Shukla, 2014, Concepts and Applications of Microwave Engineering, p.211
  25. ^ H. V. Johnson, A Vacation Radio Pocket Set. Electrical Experimenter, vol. II, no. 3, p. 42, Jul. 1914
  26. ^ Signal Corps U.S. Army, The Principles Underlying Radio Communication, 2nd ed. Washington, DC: U.S.G.P.O., 1922, p. 587
  27. ^ A. A. Ghirardi, Radio Physics Course, 2nd ed. New York: Rinehart Books, 1932, p. 375
  28. ^ J. F. Corrigan and S. O'Connor The P.W. Crystal Experimenter's Handbook, London: Popular Wireless, 1925, Climax Radio advertisement inside front cover
  29. ^ Sievers, Maurice L. (2008). Crystal Clear: Vintage American Crystal Sets, Crystal Detectors, and Crystals. Sonoran Publishing. p. 6. ISBN 1-886606-01-3. 

External linksEdit

Patents
  • U.S. Patent 906,991 - Oscillation detector (multiple metallic sulfide detectors), Clifford D. Babcock, 1908
  • U.S. Patent 912,613 - Oscillation detector and rectifier ("plated" silicon carbide detector with DC bias), G.W. Pickard, 1909
  • U.S. Patent 912,726 - Oscillation receiver (fractured surface red zinc oxide (zincite) detector), G.W. Pickard, 1909
  • U.S. Patent 933,263 - Oscillation device (iron pyrite detector), G.W. Pickard, 1909
  • U.S. Patent 1,118,228 - Oscillation detectors (paired dissimilar minerals), G.W. Pickard, 1914