Crystal detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers. It consists of a piece of crystalline mineral that rectifies an alternating current radio signal. It was employed as a detector to extract the audio modulation signal from the modulated carrier, 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's 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 discovered crystal rectification in 1902 and found hundreds of crystalline substances that could be used in forming rectifying junctions. The physical principles by which they worked were not understood at the time they were used, but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern semiconductor electronics.
The unamplified radio receivers that used crystal detectors are called crystal radios. The crystal radio was the first type of radio receiver that was used by the general public, and became the most widely used type of radio until the 1920s. It became obsolete with the development of vacuum tube receivers around 1920, but continued to be used until World War II and remains a common educational project today thanks to its simple design.
Operation
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 well 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 from the radio frequency carrier wave. An AM demodulator which works in this way, by rectifying the modulated carrier, is called an envelope detector. 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.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.
Image:Amplitude modulation detection.png|thumb|upright=1.2|Diagram showing a waveform and its envelope as processed by a crystal detector.
As shown in the diagram on the right, ' shows an 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 is contained in the slow variations 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. ' 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. shows the current which passes through the earphone. A bypass capacitor across the earphone terminals, in combination with the intrinsic forward resistance of the diode, creates a low-pass filter that smooths the waveform by removing the radio frequency carrier pulses and leaving the audio signal. When this varying current passes through the earphone piezoelectric crystal, it causes the crystal to deform, deflecting the earphone diaphragm; the varying deflections of the diaphragm cause it to vibrate and produce sound waves. If instead a voice-coil type headphone is used, the varying current from the low-pass filter flows through the voice coil, generating a varying magnetic field which pulls and pushes the earphone diaphragm, causing it to vibrate and produce sound.
Types
The crystal detector consisted of an electrical contact between the surface of a semiconducting crystalline mineral and either a metal or another crystal. Since at the time they were developed no one knew how they worked, crystal detectors evolved by trial and error. The construction of the detector depended on the type of crystal used, as it was found different minerals varied in how much contact area and pressure on the crystal surface was needed to make a sensitive rectifying contact. Crystals that required a light pressure like galena were used with the wire cat whisker contact; silicon was used with a heavier point contact, while silicon carbide could tolerate the heaviest pressure. Another type used two crystals of different minerals with their surfaces touching, the most common being the "Perikon" detector. Since the detector would only function when the contact was made at certain spots on the crystal surface, the contact point was almost always made adjustable. Below are the major categories of crystal detectors used during the early 20th century:Cat whisker detector
Patented by Karl Ferdinand Braun and Greenleaf Whittier Pickard in 1906, this was the most common type of crystal detector, mainly used with galena but also other crystals. It consisted of a pea-size piece of crystalline mineral in a metal holder, with its surface touched by a fine metal wire or needle. The contact between the tip of the wire and the surface of the crystal formed a crude unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode. The wire whisker is the anode, and the crystal is the cathode; current can flow from the wire into the crystal but not in the other direction.Only certain sites on the crystal surface functioned as rectifying junctions. The device was very sensitive to the exact geometry and pressure of contact between wire and crystal, and the contact could be disrupted by the slightest vibration. Therefore, a usable point of contact had to be found by trial and error before each use. The wire was suspended from a moveable arm and was dragged across the crystal face by the user until the device began functioning. In a crystal radio, the user would tune the radio to a strong local station if possible and then adjust the cat whisker until the station or radio noise was heard in the radio's earphones. This required some skill and a lot of patience. An alternative method of adjustment was to use a battery-operated electromechanical buzzer connected to the radio's ground wire or inductively coupled to the tuning coil, to generate a test signal. The spark produced by the buzzer's contacts functioned as a weak radio transmitter whose radio waves could be received by the detector, so when a rectifying spot had been found on the crystal the buzz could be heard in the earphones, at which time the buzzer was turned off.
The detector consisted of two parts mounted next to each other on a flat nonconductive base: a crystalline mineral forming the semiconductor side of the junction, and a "cat whisker", a springy piece of thin metal wire, forming the metal side of the junction
Crystal
The most common crystal used was galena, a widely occurring ore of lead. Varieties were sold under the names "Lenzite" and "Hertzite". Other crystalline minerals were also used, the more common ones being iron pyrite molybdenite, and cerussite Not all specimens of a crystal would function in a detector. Often several crystal pieces had to be tried to find an active one. Galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it.A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit. The electrical contact between the cup and the crystal had to be good, because it was important for this contact not to act as a second rectifying junction, creating two back-to-back diodes which would prevent the device from conducting at all. To make good contact with the crystal, it was either clamped with setscrews or embedded in solder. Because the relatively high melting temperature of tin-lead solder could damage many crystals, a fusible alloy with a low melting point, well under, such as Wood's metal was used. One surface was left exposed to allow contact with the cat-whisker wire.