IOT: an old dream, now come true

by Nicola Faccioni

The Inductive Output Tube (IOT) is a hybrid device combining some aspects of the design and operation of both gridded and beam tubes (Klystrons). It consists in an electronic gun whose beam is focused by a magnetic field and its density modulated at the input circuit RF frequency. RF power is extracted from the density modulated beam by a simple resonant output cavity. The introduction of the IOT technology in the field of television signal transmission provides a significant jump in quality, from the technological innovation point of view. Even if it is not a totally new idea, this technology, reviewed and supported by the most powerful computer systems available, makes it possible to obtain a higher efficiency device when compared with other contemporary technologies used in broadcast transmitters.

Historical background

During the 1930's the state of the art in transmitting and receiving tubes had attained a maturity and cost­effectiveness that served most needs of contemporary radio communications and industrial heating. However, interest in higher frequencies was growing and in all Western countries several people, fascinated by the challenges implicit in vacuum tube development, worked toward improved performance. Some designers began to look for newer approaches, turning away from the classical "ridded density­modulated tube. Other scholars, among them ANDREW V. HAEFF, continued to do research on gridded tubes.

Haeff perceived that a major limitation to power output was the power that could be dissipated (thus wasted!) by the grids and anodes of conventional tubes. He saw that non-intercepting electrodes, such as apertures, rather than wire grids, could be used if a magnetic field were deployed coaxially with the electron stream, producing more efficient, high level output power. He also perceived that power could be removed from an electron beam by passing it through a resonant cavity in which the kinetic energy of the electrons could be converted to electromagnetic energy without the necessity of collecting the electrons on the walls of the cavity. The inductive output cavity (IOT) invented by Haeff was first proposed as an RF source for radar. He described it in 1939 and again, in cooperation with L. NERGAARD, in 1940.

The Haeff tube achieved 100 W at 450 MHz, with a 10 dB power gain and 35% efficiency, which was very remarkable at the time. The improvement in performance of the present IOT tubes over the original Haeff tube, about 1000 times in power output, is due to the microwave beam tube technology as well as certain proprietary improvements in design.

However, during World War 11, VHF transmitting tubes for radar pulse service were triodes and, at microwave frequencies, cavity magnetrons. Alter the war, velocity­modulated (V/M) tubes, blossomed and flourished. For this reason no interest was placed on the IOT and disappearance of this device was due to tremendous emphasis on V/M in the fifties and sixties. Haeff himself made innventions and major contributions to the understanding and development of V/M. In the 1960's when European UHF TV was starting, broadcasting engineers were surprised and delighted when transmitting a few kilowatts of decent quality signal from a Klystron tube initially designed for troposcatter communications. No one worried about the costs this kind of device involved. All this changed very rapidly. The initial focus change concerned the provision of high gain Klystron, optimized for UHF TV, so that system reliability could be improved by Solid State RF drive systems.

Political and military actions in the Middle East around the seventies soon caused huge increases in oil prices which rapidly led to ever­increasing electricity prices. Suddenly, the UHF TV Klystron needed much higher efficiency and, since Solid State devices had greatly improved, gain was no longer so important. UHF TV Klystron efficiency was improved by 50% often at the expense of gain, over early tubes by the revision of RF body design using newly­available computer based, design techniques. But the energy price pressure did not relax and the broadcasters needed even lower energy bills.

Electronic techniques had meanwhile improved and it became possible to pulse UHF TV Klystrons so that the beam current used could be reduced from its maximum value. This gave another 50% of efficiency improvement but at the price of a more complicated and expensive transmitter which was more difficult to set up for optimum performance and long term stability.

Since electronic conversion efficiency in UHF TV Klystrons had been squeezed near the limit of physics, the search for an alternative UHF TV family became a main target. The UHF Tetrode was carefully examined. It has excellent efficiency and adequate gain but a designer has to be bold to suggest pushing the power output per tube much more above 20 kW because of power dissipation problems within an inevitably very small volume. What if a tube could be devised which would combine the small size and excellent efficiency of the Tetrode with the long life, high power output, and reliability of the Klystron?

The first tube to attempt this was the Klystrode, soon to be followed by the Inductive Output Tube (IOT). The Klystrode uses an input cavity system which uses internal feedback to enhance the available gain, while the IOT has an input cavity system which inherits the unconditional stability of the Klystron input system. Both tubes use a conventional Klystron RF output extraction system, modified to meet the transmission bandwidth requirements.

A new family of energy saving UHF TV devices was born, and the ideas of A.V. Haeff after 50 years, finally took place.

IOT Characteristics and Working Principles

Structurally the device consists in an electron gun (taken from a conventional Klystron along with its heater structure) containing the tungsten matrix cathode, a pyrolithic carbon grid, an RF interaction region and a Klystron­like collector. The circuit contains an input cavity for coupling the RF signal into the tube via the grid, and a double output cavity system which extracts the power. Magnetic coils, for focusing the electron beam, are also included.

The tube and its input and output cavity system are supported on a low trolley which would also carry the magnetic coils. The trolley has small wheels to allow the whole unit to be easily pushed into and released from the transmitter.

The RF input voltage is applied between the cathode and a grid which allows extra electrons to be drawn from the cathode into a low quiescent current electron beam according to the instantaneous RF voltage appearing between grid and cathode. The resulting density modulated beam is then passed into the Klystron­like RF output interaction region of the tube.

The grid is fixed in front of the cathode, supported on a metal cylinder and isolated from the cathode by a ceramic insulator. This is the grid­to­cathode ceramic through which the RF energy from the input circuit enters the tube to apply the RF voltage to the grid.

A second ceramic insulator supports the complete gridded electron gun at the correct distance from the grounded anode. This ceramic completes the vacuum envelope and holds off the full beam voltage of around 30 kV. The ceramic used contains a high percentage of alumina. This makes it possible to obtain:

  1. better performance and optimal electrical insulation between the different electrode terminals
  2. perfect vacuum tightness
  3. good mechanical stability at high operating temperatures

Moreover, a good thermal conductivity leads to an optimal resistance to thermal shocks.

The grid­to­cathode space of the electron gun forms the end of a long and complex RF transmission line from the RF input connector of the input cavity system. This has a strong effect on the final frequency range of the IOT input cavity system. So, the grid­to­cathode distance became crucial to the physics (and performance too!) of IOT. Great care has been taken to keep the grid and cathode at the required distance when the cathode is raised to its operating temperature of about 1000°C. At this temperature the grid is very stressed, so it must be mechanically and electrically stable. This is made by means the adoption of a pyrolytic graphite grid. This material has the advantage that its strength increases with temperatures up to above 2500°C whereas the strength of pure metals decreases as the temperature rises. This gives the designer the ability to produce a thin grid, with fine grid wires which may be accurately positioned yet will retain their position and shape when raised to operating temperatures in the region of 1000°C.

In order to provide the instantaneous bandwidth needed for UHF TV, the IOT uses a double­tuned output cavity system. This is because the IOT lacks the intermediate cavities which, in the conventional Klystron, are stagger­tuned to provide the bandwidth. The output cavity system has only two tuning and two coupling controls, which are fitted with either digital indicators or scales so that a tuning set­up for a given channel can be predicted and repeated. The IOT tube represents a very innovative device in UHF transmission. The small size of the tube, the very high peak power capability and its bandwidth make it very attractive from both a signal quality and cost standpoint. Moreover, this linear and stable device is ready and suitable for new and emerging technologies. These include multiple channel sound services, signal compression, HDTV, digital information transmission, paging

A step towards future, the Itelco IOT transmitter

Itelco has been the first company to believe in this technology and to carry on a TX which represents the best of its quality. Since the end of July six T674K 40 kW IOT TV UHF Itelco Transmitters were installed in Sweden, another big goal made by Itelco: Advidsjaur, Kiruna, Lycksele, Pajala, Storuman, Overkalix, are the stations in which the T674K are installed. Midnight Sun Project is the name we have chosen for a project involving the installation of new T674K UHF TV Itelco transmitters in Sweden, near the Arctic Circle.

These are very demanding climatic conditions for a high technology transmitter which currently represents the state­of­the­art in the broadcasting world. This project is the result of excellent cooperation between Swedish, English and Italian technicians. As a matter of fact, the entire project was developed at Itelco with the cooperation of Swedish Teracom's technicians, and the English staff of E.E.V.: T674K ­ a transmitter made in Europe to meet the demands of customers from all over the world.

The "Midnight Sun Project" immediately gave us optimal results considering that right other countries had faith in the IOT technology and opened orders.

The increasing demand for quality, the knowledge of the importance nowadays in the broadcast world of being able to best meet the request for high technology, easy use, low consumption and easy maintenance of the transmitter pushed the Itelco engineers and technicians to choose the IOT as the only device which meets these needs. These requests led to the development of a transmitter which, for its small size and extremely low energy consumption, is at the forefront of high power UHF TV transmission.

The development of the Itelco IOT transmitter involved a new design activity in which the customer worked with us to illustrate aspects which only a highly experienced user can highlight. This design activity allows us to have a really customer oriented transmitter, easy to run and easy to maintain, able to obtain the best performance from the IOT device running in common amplification mode.

The final stage amplification is performed by an IM7360 IOT and thanks to EEV's cooperation, the company which supplies the tube, it was possible to optimize the functioning conditions. The main functions of the transmitter are managed by a new computerized control system. The primary control unit directs the transmitter secondary systems through the Data Acquisition Interface, DAI. Each interface controls secondary units (as sensors, alarms, etc.) and data are spread through optical fibers with a 40 kbit /sec speed.

A high linearity of the transmitter's characteristic curve, both in amplitude and phase, has been obtained thanks to a modular IF pre-corrector unit which represents the state of the art in video signal processing. The choice of modularity as a fundamental aspect of the transmitter offered remarkable advantages such as the possibility of matching the setup to all television standards or the ease in correcting the various parameters by working on one group independently from the others. Another advantage is offered by the fact that wide band correction is used with the possibility of introducing AM/ AM, AM/PM selective elements to compensate frequency distortions. It must be emphasized that the pre-corrector introduces a very slight amount of noise so it is practically transparent in respect with the transmitted signal.

Last but not least, the pre-corrector is arranged so it can correct the linearity of a transmission device up to a level of 2.6 dB over the sync peak power. This characteristic is particularly useful when the pre-corrector is used with HDTV modulated signals.

The water cooling system represents an other important characteristic . When this system is in small dimensions and put on an other rack, it makes the transmitter an independent unit from the cooling system, because they are linked through a flexible pipe.

The water circulation is performed by a pumping system in "Passive stand­by" configuration which improves the final result of the cooling system itself. Anyway, the Itelco capability to offer an innovative and confortable product doesn't stop on the transmitter itself; it is well known that Itelco has a remarkable possibility to offer sophisticated active and passive transmission systems. All over the world there are hundreds of systems installed.

With the IOT active system it is possible to have high transmission powers with lower and lower installation and management costs. A high power IOT transmitter, in a passive configuration and as main unit, represents the best solution as for quality, confort and lower costs. When we put together this latter and a medium solid state transmitter as a reserve, there is no doubt that we have a very confortable system. It is evident that with such configuration the most important broadcast technologies are combined together in order to assure a high efficiency, low maintenance rate and remarkable performances.

This is the reason for our proudness.

Conclusions

IOT represents an optimal device for Dual Sound transmission and for future HDTV signals, supplying adequate bandwidth theoretically increaseable up to 20MHz) and the right amplification class (AB). The HDTV signal is essentially a broadband signal with a constant average power and a large peak to average ratio; in other words, a low duty cycle. The peak power of the HDTV signal occurs randomly in time and frequency. Therefore, this signal is easily damaged by nonlinear distortion in a transmitter power amplifier stage. Distortions that create intermodulation products would be particularly damaging because, in a high power amplification stage of a HDTV transmitter, they would reduce the transmitter effective range, while in current standard transmitters the distortion only affects picture quality.

For these reasons, a HDTV transmitter will be able to manage a low average power but it will be also capable of linear amplification of short randomly occurring peaks of perhaps ten times the average output power. At this moment, most of high power UHF transmitters are equipped with some Klystron. But the difference between current standard signal and HDTV signal is significant. A transmitter for today's standard must handle a high average power signal compared with the HDTV one. Several studies about Klystrons use in HDTV amplifiers concluded that it may technically provide HDTV service, but at high price in energy consumption, size, complexity and cost. The class A nature of the Klystron makes it very energy inefficient when amplifying a signal with low duty cycle like HDTV one.

It has also been shown that the best efficiency in HDTV could be achieved when a class AB device draws power as a function of the average signal level and thus, its power consumption will reflect the low average power of the HDTV signal. IOT is, like tetrodes and transistors, an AB class device.

Furthermore, its capability makes the IOT device already available for all the future I digital applications of TV transmissions. All these features make IOT technology the prime candidate to become the industry standard for the next years.

With the realization of its IOT transmitter, Itelco succeeded in a big target and is proud of it. Itelco is proud of the small dimensions obtained by using the most sophisticated technology available today and of the high performance the transmitter achieved.

This transmitter is another milestone in the Itelco history. A history made by men, people of different countries, who closely cooperated to reach this goal.

Differential Phase, differential gain, without precorrection Differential Phase, differential gain, precorrection included
Intermodulation products with -8, -16 (3.25 MHz), -20 tone, without precorrection Intermodulation products with -8, -16 (3.25 MHz), -20 tone, precorrection included
Linearity, without precorrection Linearity, precorrection included
Cross modulation, without precorrection Cross modulation, precorrection included

REFERENCES

T. CLAYWORTH, H.P. BOHLEN, R. HEPPINSTALL, "Some exciting adventures in the IOT business" Technical report presented at NAB Engineering Conference 1992, EEV 1992

MERRALD, DONALD H. PRIEST, "The Klystrode, an old idea with new applications" Varian Associates Palo Alto CA

GEORGE M. BADGER, "The Klystrode, a new high efficiency UHF TV power amplifier " Technical report presented at NAB Engineering Conference l 985

M.B. SHRADER, D.H. PREIST RF "Sources for particle accelerators" Technical report, Varian, Eimac division, San Carlos CA

DONALD H. PREIST, MERRALD B. SHRADER, "The Klystrode, an unusual transmitting tube with potential for UHF­TV" proceedings of the IEEE, Vol. 70 No. 11 November 1982