Inductive output tube
Encyclopedia
The inductive output tube or IOT is a variety of vacuum tube
which evolved in the 1980s to meet increasing efficiency requirements for high-power RF
amplifier
s. The primary commercial use of IOTs is in UHF
television transmitter
s, where they have mostly replaced klystron
s because of their higher efficiencies (35% to 40%) and smaller size. IOTs are also used in particle accelerator
s.
was later issued for the IOT to Andrew V. Haeff and assigned to the Radio Corporation of America (RCA). During the 1939 New York World's Fair
the IOT was used in the transmission of the first television images from the Empire State Building
to the fair grounds. RCA sold a small IOT commercially for a short time, under the type number 825. It was soon obsoleted by newer developments, and the technology lay more or less dormant for years.
The inductive output tube has reemerged within the last twenty years after having been discovered to possess particularly suitable characteristics (broadband linearity) for the transmission of digital television
and high-definition digital television
.
The power output of the modern 21st century IOTs is orders of magnitude higher than the first IOTs produced by the RCA in 1940/1941 but the fundamental principle of operation basically remains the same. IOT's since the 1970s have been designed with electromagnetic modelling computer software that has greatly improved their electrodynamic performance.
, hence Eimac
's trade name for them, Klystrode. They have a cathode
with a control grid
0.1 mm in front of it like a triode. They then use high voltage DC
and a magnetic lens
to focus a modulated high energy electron beam through a small drift tube like a klystron. This drift tube prevents backflow of electromagnetic radiation. The bunched electron beam passes through a resonant cavity, equivalent to the output cavity of a klystron. The electron bunches excite the cavity, and the electromagnetic energy of the beam is extracted by a coaxial transmission line.
The highest frequency achievable in an IOT is limited by the grid-to-cathode spacing. The electrons must be accelerated off the cathode and pass the grid before the RF electric field reverses direction. The upper limit on frequency is approximately .
Heat radiation from the cathode heats the grid; as a result, low-work-function
cathode material evaporates and condenses on the grid. This eventually leads to a short between cathode and grid, as the material accreting on the grid narrows the gap between it and the cathode. In addition, the emissive cathode material on the grid causes a negative grid current (electrons sent from the grid to the cathode) that may cause problems to the grid power supply if getting too high (causing the grid (bias) voltage to change). Today's IOTs are equipped with coated cathodes that work at very low operation temperatures, and hence have very low evaporation rates, preventing this effect. IOTs, like most linear beam tubes having external tuning cavities to achieve bandwidth, are equipped with arc detectors located in the output cavities, which trigger a crowbar
circuit based on a hydrogen thyratron
in the high-voltage supply.
The latest versions of IOTs achieve even higher efficiencies (60%-70%) through the use of a Multistage Depressed Collector (MSDC) (one manufacturer's version is called Constant Efficiency Amplifier (CEA) or another manufacturer's version ESCIOT (Energy Saving Collector IOT)). The design difficulties of an MSDC on IOTs have been overcome by the use of transformer oil as a combined cooling and insulation media to avoid arcing and erosion between the collector stages and to provide maintenance free cooling operation for the life of the tube. Earlier MSDC versions had to be air cooled (limited power) or used de-inonized water that had to be filtered, regularly exchanged and provided no freezing and corrosion protection.
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
which evolved in the 1980s to meet increasing efficiency requirements for high-power RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s. The primary commercial use of IOTs is in UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
television transmitter
Television transmitter
A television transmitter is a device which broadcasts an electromagnetic signal to the television receivers. Television transmitters may be analog or digital.- Types of transmitters :There are many types of transmitters depending on* The system standard...
s, where they have mostly replaced klystron
Klystron
A klystron is a specialized linear-beam vacuum tube . Klystrons are used as amplifiers at microwave and radio frequencies to produce both low-power reference signals for superheterodyne radar receivers and to produce high-power carrier waves for communications and the driving force for modern...
s because of their higher efficiencies (35% to 40%) and smaller size. IOTs are also used in particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s.
History
The inductive output tube (IOT) was invented in 1938 by Andrew V. Haeff. A patentPatent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
was later issued for the IOT to Andrew V. Haeff and assigned to the Radio Corporation of America (RCA). During the 1939 New York World's Fair
1939 New York World's Fair
The 1939–40 New York World's Fair, which covered the of Flushing Meadows-Corona Park , was the second largest American world's fair of all time, exceeded only by St. Louis's Louisiana Purchase Exposition of 1904. Many countries around the world participated in it, and over 44 million people...
the IOT was used in the transmission of the first television images from the Empire State Building
Empire State Building
The Empire State Building is a 102-story landmark skyscraper and American cultural icon in New York City at the intersection of Fifth Avenue and West 34th Street. It has a roof height of 1,250 feet , and with its antenna spire included, it stands a total of 1,454 ft high. Its name is derived...
to the fair grounds. RCA sold a small IOT commercially for a short time, under the type number 825. It was soon obsoleted by newer developments, and the technology lay more or less dormant for years.
The inductive output tube has reemerged within the last twenty years after having been discovered to possess particularly suitable characteristics (broadband linearity) for the transmission of digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
and high-definition digital television
High-definition television
High-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...
.
The power output of the modern 21st century IOTs is orders of magnitude higher than the first IOTs produced by the RCA in 1940/1941 but the fundamental principle of operation basically remains the same. IOT's since the 1970s have been designed with electromagnetic modelling computer software that has greatly improved their electrodynamic performance.
Description
IOTs have been described as a cross between a klystron and a triodeTriode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
, hence Eimac
Eimac
Eimac is a trade mark of Eimac Products, part of the Microwave Power Products Division of Communications & Power Industries. It produces vacuum tubes for radio frequency applications such as broadcast and radar transmitters.-History:...
's trade name for them, Klystrode. They have a cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
with a control grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
0.1 mm in front of it like a triode. They then use high voltage DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
and a magnetic lens
Magnetic lens
A magnetic lens is a device for the focusing or deflection of charged particles, such as electrons or ions. Charged particles are deflected based upon the strength of the magnetic field, which can be varied by controlling the current flowing through several electromagnets...
to focus a modulated high energy electron beam through a small drift tube like a klystron. This drift tube prevents backflow of electromagnetic radiation. The bunched electron beam passes through a resonant cavity, equivalent to the output cavity of a klystron. The electron bunches excite the cavity, and the electromagnetic energy of the beam is extracted by a coaxial transmission line.
The highest frequency achievable in an IOT is limited by the grid-to-cathode spacing. The electrons must be accelerated off the cathode and pass the grid before the RF electric field reverses direction. The upper limit on frequency is approximately .
Heat radiation from the cathode heats the grid; as a result, low-work-function
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
cathode material evaporates and condenses on the grid. This eventually leads to a short between cathode and grid, as the material accreting on the grid narrows the gap between it and the cathode. In addition, the emissive cathode material on the grid causes a negative grid current (electrons sent from the grid to the cathode) that may cause problems to the grid power supply if getting too high (causing the grid (bias) voltage to change). Today's IOTs are equipped with coated cathodes that work at very low operation temperatures, and hence have very low evaporation rates, preventing this effect. IOTs, like most linear beam tubes having external tuning cavities to achieve bandwidth, are equipped with arc detectors located in the output cavities, which trigger a crowbar
Crowbar (circuit)
A crowbar circuit is an electrical circuit used to prevent an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. It operates by putting a short circuit or low resistance path across the voltage source, much as if one dropped a tool of the same name...
circuit based on a hydrogen thyratron
Thyratron
A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
in the high-voltage supply.
The latest versions of IOTs achieve even higher efficiencies (60%-70%) through the use of a Multistage Depressed Collector (MSDC) (one manufacturer's version is called Constant Efficiency Amplifier (CEA) or another manufacturer's version ESCIOT (Energy Saving Collector IOT)). The design difficulties of an MSDC on IOTs have been overcome by the use of transformer oil as a combined cooling and insulation media to avoid arcing and erosion between the collector stages and to provide maintenance free cooling operation for the life of the tube. Earlier MSDC versions had to be air cooled (limited power) or used de-inonized water that had to be filtered, regularly exchanged and provided no freezing and corrosion protection.
External links
- http://www.bext.com/iot.htm
- http://www.ebu.ch/departments/technical/trev/trev_273-heppinstall.pdf
- http://www.davidsarnoff.org/kil-chapter03.html
- http://www.allaboutcircuits.com/vol_3/chpt_13/11.html
- http://www.harris.com/view_pressrelease.asp?act=lookup&pr_id=2037
- http://epaper.kek.jp/p95/ARTICLES/TAQ/TAQ02.PDF