Zenith Electronics, LLC is a research and development company that develops ATSC and digital rights management technologies. It is owned by the South Korean company LG Electronics. Zenith was an American brand of consumer electronics, a manufacturer of radio and television receivers and other consumer electronics, was headquartered in Glenview, Illinois. After a series of layoffs, the consolidated headquarters moved to Illinois. For many years, their famous slogan was "The quality goes in before the name goes on." LG Electronics acquired a controlling share of Zenith in 1995. Zenith was the inventor of subscription television and the modern remote control, the first to develop High-definition television in North America. Zenith-branded products were sold in North America, Thailand, Laos, Vietnam and Myanmar; the company was co-founded by Ralph Matthews and Karl Hassel in Chicago, Illinois, as Chicago Radio Labs in 1918 as a small producer of amateur radio equipment. The name "Zenith" came from ZN'th, a contraction of its founders' ham radio call sign, 9ZN.
They were joined in 1921 by Eugene F. McDonald, Zenith Radio Company was formally incorporated in 1923; the fledgling company soon became known for electronic innovations. Zenith introduced the first portable radio in 1924, the first mass-produced AC radio in 1926, push-button tuning in 1927, it added automobile radios in the 1930s with its Model 460, promoting the fact that it needed no separate generator or battery, selling at US$59.95. The first Zenith television set appeared in 1939, with its first commercial sets sold to the public in 1948; the company is credited with having invented such things as the wireless remote control and FM multiplex stereo. In fact, Zenith established one of the first FM stations in the country in 1940, among the earliest FM multiplex stereo stations, first broadcasting in stereo in June 1961; the station was sold in the early 1970s and is now WUSN. Zenith pioneered in the development of high-contrast and flat-face picture tubes, the multichannel television sound stereo system used on analog television broadcasts in the United States and Canada Zenith was one of the first companies to introduce a digital HDTV system implementation, parts of which were included in the ATSC standard starting with the 1993 model Grand Alliance.
They were one of the first American manufacturers to market a home VCR, selling a Sony-built Betamax video recorder starting in 1977. The 1962 Illinois Manufacturers Directory lists Zenith Radio Corporation as having a total of 11,000 employees of which at least 6,460 were employed in seven Chicago plants; the corporate office was in plant number 1 located at 6001 West Dickens Avenue where 2,500 workers made radio and television sets and Hi-Fi stereophonic phonographs. Plant number 2 was located at 1500 North Kostner Ave. where 2,100 employees made government electronics and television components and hearing aids. Plant number 3 was located at 5801 West Dickens Ave. where 300 employees made electronics and servicing. Plant number 4 was located at 3501 West Potomac Ave.. Plant number 5 located at 6501 West Grand Ave. employed 500-600 workers who made government hi-fi equipment. A subsidiary of Zenith, the Rauland Corporation, located at 4245 North Knox Avenue, employed 850 workers who made television picture tubes.
The other Zenith subsidiary in Chicago was Central Electronics, Incorporated located at 1247 West Belmont Ave. where 100 employees made amateur radio equipment and performed auditory training. The other Central Electronics plant was located at State Route 133 and Grandview in Paris, Illinois where 500 employees made radio receivers, with the total Zenith work force in Illinois being thus at least 6,960. In December, 1970, National Union Electric sued most of the Japanese television manufacturers for violation of the Anti-Dumping Act and a conspiracy which violated American antitrust laws. During the pendency of that suit, Zenith Radio Corporation encountered increasing financial difficulty as their market share progressively went to Japanese companies. Concerned about losing market share to Japanese companies, Zenith filed suit in federal court in Philadelphia in 1974 against the major Japanese television and electronic manufacturers charging violation of the United States Antitrust Laws and the Anti-Dumping Act of 1916.
Zenith joined two United States companies Sears, Roebuck and Co. and Motorola, Inc. as co-plaintiffs. The NUE suit was transferred to the Eastern District of Pennsylvania and the two suits were consolidated for pretrial proceedings and trial; the suit, styled In re Japanese Electric Products Antitrust Litigation, sought $900,000,000.00 in damages. By the end of 1983, Zenith had spent millions of dollars in connection with the litigation. In 1981, the trial court entered summary judgment on the antitrust and antidumping claims and dismissed the lawsuits. Plaintiffs appealed and the appellate court affirmed the summary judgment for Sears, Roebuck and Co. Motorola, Inc. and Sony. The case was appealed, in March 1986 the Supreme Court of the United States ruled in favor of the defendants on Zenith’s antitrust claims. Zenith's hopes to salvage a
In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are
In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio. Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy; the transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers had only to distinguish between the presence or absence of a radio signal; the device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology. After sound transmission began around 1920, the term evolved to mean a demodulator, which extracted the audio signal from the radio frequency carrier wave.
This is its current meaning, although modern detectors consist of semiconductor diodes, transistors, or integrated circuits. In a superheterodyne receiver the term is sometimes used to refer to the mixer, the tube or transistor which converts the incoming radio frequency signal to the intermediate frequency; the mixer is called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector. In microwave and millimeter wave technology the terms detector and crystal detector refer to waveguide or coaxial transmission line components, used for power or SWR measurement, that incorporate point contact diodes or surface barrier Schottky diodes. One major technique is known as envelope detection; the simplest form of envelope detector is the diode detector that consists of a diode connected between the input and output of the circuit, with a resistor and capacitor in parallel from the output of the circuit to the ground to form a low pass filter.
If the resistor and capacitor are chosen, the output of this circuit will be a nearly identical voltage-shifted version of the original signal. An early form of envelope detector was the crystal detector, used in the crystal set radio receiver. A version using a crystal diode is still used in crystal radio sets today; the limited frequency response of the headset eliminates the RF component, making the low pass filter unnecessary. More sophisticated envelope detectors include the grid-leak detector, the plate detector, the infinite-impedance detector, transistor equivalents of them and precision rectifiers using operational amplifiers. A product detector is a type of demodulator used for AM and SSB signals, where the original carrier signal is removed by multiplying the received signal with a signal at the carrier frequency. Rather than converting the envelope of the signal into the decoded waveform by rectification as an envelope detector would, the product detector takes the product of the modulated signal and a local oscillator, hence the name.
By heterodyning, the received signal is mixed with a signal from the local oscillator, to give sum and difference frequencies to the signals being mixed, just as a first mixer stage in a superhet would produce an intermediate frequency. Product detector circuits are and so ring modulators or synchronous detectors and related to some phase-sensitive detector circuits, they can be implemented using something as simple as ring of diodes or a single dual-gate Field Effect Transistor to anything as sophisticated as an Integrated Circuit containing a Gilbert cell. AM detectors can not demodulate PM signals because both have a constant amplitude; however an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are used.
A phase detector is a nonlinear device whose output represents the phase difference between the two oscillating input signals. It has two inputs and one output: a reference signal is applied to one input and the phase or frequency modulated signal is applied to the other; the output is a signal, proportional to the phase difference between the two inputs. In phase demodulation the information is contained in the amount and rate of phase shift in the carrier wave; the Foster-Seeley discriminator is a used FM detector. The detector consists of a special center-tapped transformer feeding two diodes in a full wave DC rectifier circuit; when the input transformer is tuned to the signal frequency, the output of the discriminator is zero. When there is no deviation of the carrier, both halves of the center tapped transformer are balanced; as the FM signal swings in frequency above and below the carrier frequency, the balance between the two halves of the center-tapped secondary is destroyed and there is an output voltage proportional to the frequency deviation.
The ratio detector is a variant of the Foster-Seeley discriminator, but one diode conducts in an opposite direction, using a tertiary winding in the preceding transformer. The output in this case is taken between the sum of the diode voltages and the center tap
Radio is the technology of signalling or communicating using radio waves. Radio waves are electromagnetic waves of frequency between 300 gigahertz, they are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, received by a radio receiver connected to another antenna. Radio is widely used in modern technology, in radio communication, radio navigation, remote control, remote sensing and other applications. In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking and satellite communication among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal in the transmitter. In radar, used to locate and track objects like aircraft, ships and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, the reflected waves reveal the object's location. In radio navigation systems such as GPS and VOR, a mobile receiver receives radio signals from navigational radio beacons whose position is known, by measuring the arrival time of the radio waves the receiver can calculate its position on Earth.
In wireless remote control devices like drones, garage door openers, keyless entry systems, radio signals transmitted from a controller device control the actions of a remote device. Applications of radio waves which do not involve transmitting the waves significant distances, such as RF heating used in industrial processes and microwave ovens, medical uses such as diathermy and MRI machines, are not called radio; the noun radio is used to mean a broadcast radio receiver. Radio waves were first identified and studied by German physicist Heinrich Hertz in 1886; the first practical radio transmitters and receivers were developed around 1895-6 by Italian Guglielmo Marconi, radio began to be used commercially around 1900. To prevent interference between users, the emission of radio waves is regulated by law, coordinated by an international body called the International Telecommunications Union, which allocates frequency bands in the radio spectrum for different uses. Radio waves are radiated by electric charges undergoing acceleration.
They are generated artificially by time varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna. In transmission, a transmitter generates an alternating current of radio frequency, applied to an antenna; the antenna radiates the power in the current as radio waves. When the waves strike the antenna of a radio receiver, they push the electrons in the metal back and forth, inducing a tiny alternating current; the radio receiver connected to the receiving antenna detects this oscillating current and amplifies it. As they travel further from the transmitting antenna, radio waves spread out so their signal strength decreases, so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, antenna radiation pattern, receiver sensitivity, noise level, presence of obstructions between transmitter and receiver. An omnidirectional antenna transmits or receives radio waves in all directions, while a directional antenna or high gain antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction.
Radio waves travel through a vacuum at the speed of light, in air at close to the speed of light, so the wavelength of a radio wave, the distance in meters between adjacent crests of the wave, is inversely proportional to its frequency. In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent is converted by some type of transducer to a time-varying electrical signal called the modulation signal; the modulation signal may be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal consisting of a sequence of bits representing binary data from a computer. The modulation signal is applied to a radio transmitter. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier wave because it serves to "carry" the information through the air; the information signal is used to modulate the carrier, varying some aspect of the carrier wave, impressing the information on the carrier.
Different radio systems use different modulation methods: AM - in an AM transmitter, the amplitude of the radio carrier wave is varied by the modulation signal. FM - in an FM transmitter, the frequency of the radio carrier wave is varied by the modulation signal. FSK - used in wireless digital devices to transmit digital signals, the frequency of the carrier wave is shifted periodically between two frequencies that represent the two binary digits, 0 and 1, to transmit a sequence of bits. OFDM - a family of complicated digital modulation methods widely used in high bandwidth systems such as WiFi networks, digital television broadcasting, digital audio broadcasting to transmit digital data using a minimum of radio spectrum bandwidth. OFDM has higher spectral efficiency and more resistance to fading than AM or FM. Multiple radio carrier waves spaced in frequency are transmitted within the radio channel, with each carrier modulated with bits from the incoming bitstream
A regenerative circuit is an amplifier circuit that employs positive feedback. Some of the output of the amplifying device is applied back to its input so as to add to the input signal, increasing the amplification. One example is the Schmitt trigger, but the most common use of the term is in RF amplifiers, regenerative receivers, to increase the gain of a single amplifier stage; the regenerative receiver was invented in 1912 and patented in 1914 by American electrical engineer Edwin Armstrong when he was an undergraduate at Columbia University. It was used between 1915 and World War II. Advantages of regenerative receivers include increased sensitivity with modest hardware requirements, increased selectivity because the Q of the tuned circuit will be increased when the amplifying vacuum tube or transistor has its feedback loop around the tuned circuit because it introduces some negative resistance. Due to its tendency to radiate interference when oscillating, by the 1930s the regenerative receiver was superseded by other TRF receiver designs and by another Armstrong invention - superheterodyne receivers and is considered obsolete.
Regeneration is still used in other areas of electronics, such as in oscillators, active filters, bootstrapped amplifiers. A receiver circuit that used larger amounts of regeneration in a more complicated way to achieve higher amplification, the superregenerative receiver, was invented by Armstrong in 1922, it was never used in general commercial receivers, but due to its small parts count it was used in specialized applications. One widespread use during WWII was IFF transceivers, where single tuned circuit completed the entire electronics system, it is still used in a few specialized low data rate applications, such as garage door openers, wireless networking devices, walkie-talkies and toys. The gain of any amplifying device, such as a vacuum tube, transistor, or op amp, can be increased by feeding some of the energy from its output back into its input in phase with the original input signal; this is called positive regeneration. Because of the large amplification possible with regeneration, regenerative receivers use only a single amplifying element.
In a regenerative receiver the output of the tube or transistor is connected back to its own input through a tuned circuit. The tuned circuit allows positive feedback only at its resonant frequency. In regenerative receivers using only one active device, the same tuned circuit is coupled to the antenna and serves to select the radio frequency to be received by means of variable capacitance. In the regenerative circuit discussed here, the active device functions as a detector. A regeneration control is provided for adjusting the amount of feedback, it is desirable for the circuit design to provide regeneration control that can increase feedback to the point of oscillation and that provides control of the oscillation from small to larger amplitude and back to no oscillation without jumps of amplitude or hysteresis in control. Two important attributes of a radio receiver are selectivity; the regenerative detector provides sensitivity and selectivity due to voltage amplification and the characteristics of a resonant circuit consisting of inductance and capacitance.
The regenerative voltage amplification u o is u o = u / where u is the non-regenerative amplification and a is the portion of the output signal fed back to the L2 C2 circuit. As 1 − u a becomes smaller the amplification increases; the Q of the tuned circuit without regeneration is Q = X L / R where X L is the reactance of the coil and R represents the total dissipative loss of the tuned circuit. The positive feedback compensates the energy loss caused by R, so it may be viewed as introducing a negative resistance R r to the tuned circuit; the Q of the tuned circuit with regeneration is Q r e g = X L /. The regeneration increases the Q. Oscillation begins when | R r | = R. Regeneration can increase the detection gain of a detector by a factor of 1,700 or more; this is quite an improvement for the low-gain vacuum tubes of the 1920s and early 1930s. The type 36 screen-grid tube had a non-regenerative detection gain of only 9.2 at 7.2 MHz, but in a regen
Plate detector (radio)
In electronics, a plate detector is a vacuum tube circuit in which an amplifying tube having a control grid is operated in a non-linear region of its grid voltage versus plate current transfer characteristic near plate current cutoff in order to demodulate an amplitude modulated carrier signal. This differs from the grid leak detector, which utilizes non-linearity of the grid voltage versus grid current characteristic for demodulation, it differs from the diode detector, a two terminal device. Plate detector circuits were used from the 1920s until the start of World War II. In 1927, the advent of screen grid tubes permitted much more radio frequency amplification prior to the detector stage than practically possible; the used grid leak detector was less suited to the higher radio frequency signal level than the plate detector. Diode detectors became popular during the 1920s because, unlike plate detector circuits, they could provide automatic gain control voltage for the radio frequency amplifier stages of the receiver.
However, the dual-diode/triode and dual-diode/pentode tubes used for detection/A. V. C. Circuits had bulk wholesale costs that were as much as twice the cost of the tubes used as plate detectors; this made plate detector circuits more practical for low-priced radios sold during the depths of the Great Depression. Negative bias is applied to the grid to bring the plate current to cutoff; the grid is connected directly to the secondary of a radio frequency or intermediate frequency transformer. An incoming signal will cause the plate current to increase much more during the positive 180 degrees of the carrier frequency cycle than it decreases during the negative 180 degrees; the plate current variation will include the modulation envelope. The plate current is passed through a plate load impedance chosen to produce the desired amplification in conjunction with the tube characteristics. A capacitor of low impedance at the carrier frequency and high impedance at audio frequencies is provided between the tube plate and cathode, to minimize amplification of the carrier frequency and remove carrier frequency variations from the recovered modulation waveform.
The allowable peak 100% modulated input signal voltage is limited to the magnitude of the bias voltage, corresponding to an unmodulated carrier peak voltage of half the bias voltage magnitude. Either fixed bias or cathode bias may be used for the plate detector; when cathode bias is implemented, a capacitor of low impedance at the carrier frequency and high impedance at audio frequencies bypasses the cathode resistor. Cathode bias reduces the amplification obtainable. Plate detector circuits do not produce A. V. C. Voltage for the radio frequency stages of the receiver. In these receivers, volume control is accomplished by providing variable cathode bias of one or more stages prior to the detector. A potentiometer is used to implement the variable cathode bias; the most common connection of the potentiometer is as follows: One end of the potentiometer is connected to the antenna coupling component. F. amplifier or the cathode of the converter and/or the I. F. amplifier. To set a limit on the ability of the volume control to reduce the bias on the stages that it controls, the potentiometer is equipped with a mechanical rotation limit facility that prevents the resistance from being reduced below a specific amount.
Other volume control circuits in non-A. V. C. Receivers include: A potentiometer where the high end and center wiper are connected as above, but where the low end is connected to the control grid of audio output tube.. F. amplifiers. Because the volume control in non-A. V. C. Receivers adjusts R. F. signal levels rather than A. F. signal levels, the volume control must be manipulated while tuning the radio in order to find weak signals.'01A, 1H4G, 6C6, 6J7, 6SJ7, 12F5, 12J5, 12J7, 12SF5, 12SJ7, 24, 24A, 27, 30, 36, 37, 56, 57, 76, 77, 201A, 301A In the Infinite-Impedance detector, the load resistance is placed in series with the cathode, rather than the plate, the demodulated output is taken from the cathode. The circuit is operated in the region where grid current does not occur during any portion of the carrier frequency cycle, thus the name "Infinite Impedance Detector". An example schematic diagram of an implementation using a field effect transistor is shown; as with the standard plate detector, the device is biased completely off.
The positive-going 180 degrees of the carrier input signal causes substantial increase of cathode or source current above the amount set by the bias, the negative going 180 degrees of the carrier cycle causes little decrease of cathode current below the level set by the bias. C2 is charged to a dc voltage determined by the carrier amplitude. C2 can only be discharged via R1, the circuit acts as a peak detector at the carrier frequency; the C2 R1 time constant is much shorter than the period of the highest modulating frequency, permitting the voltage across C2 to follow the modulation envelope. Negative feedback takes place at the recovered modulation frequencies
In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode, heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode; the heating element is an electrical filament heated by a separate electric current passing through it. Hot cathodes achieve much higher power density than cold cathodes, emitting more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment, do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons. From the 1920s to the 1960s, a wide variety of electronic devices used hot-cathode vacuum tubes. Today, hot cathodes are used as the source of electrons in fluorescent lamps, vacuum tubes, the electron guns used in cathode ray tubes and laboratory equipment such as electron microscopes.
A cathode electrode in a vacuum tube or other vacuum system is a metal surface which emits electrons into the evacuated space of the tube. Since the negatively charged electrons are attracted to the positive nuclei of the metal atoms, they stay inside the metal and require energy to leave it; this energy is called the work function of the metal. In a hot cathode, the cathode surface is induced to emit electrons by heating it with a filament, a thin wire of refractory metal like tungsten with current flowing through it; the cathode is heated to a temperature that causes electrons to be'boiled off' of its surface into the evacuated space in the tube, a process called thermionic emission. There are two types of hot cathodes: Directly heated cathode In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes. Today, they are used in most high-power transmitting vacuum tubes. Indirectly heated cathode In this type, the filament is not the cathode but rather heats a separate cathode consisting of a sheet metal cylinder surrounding the filament, the cylinder emits electrons.
Indirectly heated cathodes are used in most low power vacuum tubes. For example, in most vacuum tubes the cathode is a nickel tube, coated with metal oxides, it is heated by a tungsten filament inside it, the heat from the filament causes the outside surface of the oxide coating to emit electrons. The filament of an indirectly heated cathode is called the heater; the main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament, allowing vacuum tubes to use alternating current to heat the filament. In a tube in which the filament itself is the cathode, the alternating electric field from the filament surface would affect the movement of the electrons and introduce hum into the tube output, it allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source though the cathodes they heat may be at different potentials. To improve electron emission, cathodes are treated with chemicals, compounds of metals with a low work function.
These form a metal layer on the surface. Treated cathodes require less surface area, lower temperatures and less power to supply the same cathode current; the untreated thoriated tungsten filaments used in early vacuum tubes had to be heated to 2500 °F, white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature, so they only have to be heated to 800–1100 °F. The most common type of indirectly heated cathode is the oxide-coated cathode, in which the nickel cathode surface has a coating of alkaline earth metal oxide to increase emission. One of the earliest materials used for this was barium oxide. More modern formulations utilize a mixture of strontium oxide and calcium oxide. Another standard formulation is barium oxide, calcium oxide, aluminium oxide in a 5:3:2 ratio. Thorium oxide is used as well. Oxide-coated cathodes operate at orange-hot, they are used in most small glass vacuum tubes, but are used in high-power tubes because the coating is degraded by positive ions that bombard the cathode, accelerated by the high voltage on the tube.
For manufacturing convenience, the oxide-coated cathodes are coated with carbonates, which are converted to oxides by heating. The activation may be achieved by microwave heating, direct electric current heating, or electron bombardment while the tube is on the exhausting machine, until the production of gases ceases; the purity of cathode materials is crucial for tube lifetime. The Ba content increases on the surface layers of oxide cathodes down to several tens of nanometers in depth, after the cathode activation process; the lifetime of oxide cathodes can be evaluated with a stretched exponential function. The survivability of electron emission sources is improved by high doping of high‐speed activator. Barium oxide reacts with traces of silicon in the underlying metal; this layer has high electrical resistance under discontinuous current load, acts as a resistor in series with the cathode. This is undesirable for tubes used in computer applications, where they can stay without conducting current for extended periods of time.