Traveling wave tube
In a traveling wave tube (English Traveling Wave Tube , abbreviation TWT ; German also running wave tube ), electrical signals are amplified by free electrons releasing part of their kinetic energy and thereby amplifying the electrical signal. To make this possible, the signal field must be slowed down so that the electron beam and signal have approximately the same speed. Traveling wave tubes are used for linear and low-noise signal amplification in the frequency range 0.3 to around 50 GHz. The power gain is between 1,000 and 1,000,000 (30 to 60 dB), and a high degree of efficiency of up to 70 percent can be achieved. Traveling wave tubes are therefore superior to semiconductor amplifiers above 10 GHz.
Layout and function
The most important parts of a traveling wave tube are shown in the two figures.
Electrons are released via a heated cathode and the electron beam travels along the axis of a helically shaped wire called a helix. The coupled signal is amplified in the helix and the amplified signal is coupled out again at the opposite end. At the end of the tube, the electron beam is collected by a collector, which recovers energy. Only this feature enables the high efficiency of traveling wave tubes.
First, electrons are released in a hot cathode , which are accelerated by electrical high voltage fields (between 1 and approx. 20 kV) and bundled into an electron beam. This part of the traveling wave tube is also called the electron gun and is located in the schematic drawing and in the photo on the left (1). The electron beam then passes through the tube to the collector (8) on the right-hand side. Without an externally generated magnetic field (3) directed axially within the tube, however, the electrons would not reach the collector, since they repel each other - the electron beam would expand. The electrons would then hit the helix, which could damage it, but above all the gain would decrease significantly. The magnetic field in the tube is generated either by permanent magnets or electromagnets. Its value is in the order of 0.1 Tesla.
The signal to be amplified is coupled to the helix (5) through a coaxial contact (2). The electrons from the beam transfer their energy to the wave in the helix, if their speed is slightly higher than the speed of the signal, more precisely the phase speed of the signal. The phase velocity in a straight conductor is between 60 and 70 percent of the speed of light. Therefore, relativistic electron velocities would be necessary to achieve an amplification. The acceleration voltage of the electron guns would then have to be 100 kV due to the kinetic energy of the electrons. Therefore, the path that the signal has to travel is lengthened compared to the path that the electrons travel by impressing the signal on a conductor coiled around the electron beam. In the figure, a turn is about nine times as long as the direct path inside the tube. This makes it possible to reduce the operating voltage to as little as 1 kV.
The amplification in a traveling wave tube takes place through the so-called Cherenkov effect . This occurs when the speed of the electrons exceeds the phase speed of the electromagnetic field. Initially, the electrons experience a speed modulation due to the signal and, during their runtime, form bunches that coherently reinforce the signal through influence on the filament. This effect is self-regulating over a wide range - electrons that are too fast are slowed down by the high-frequency field of the filament and give off energy to it. Another explanation for the amplification considers the mutual electrical induction of the signal to be amplified and the waves of the modulated electron beam. The high amplification of the traveling wave tube can also lead to self-excitation due to the high frequency running back on the filament. To prevent this, there is an attenuator (4) in the middle of the tube. This also prevents the wave from moving forward on the filament - in the middle, however, the signal is already imprinted on the electron beam and this has to cover a certain distance anyway before power can be transferred from it to the filament.
At the end of the tube, the amplified signal is separated from the electron beam. The electrons slowed down by the interaction with the RF wave are caught in the collector and the signal is either coupled out into a coaxial cable or a waveguide.
Comparison with other electron vacuum tubes
In contrast to klystrons , traveling wave tubes are very broadband. The bandwidth essentially depends on the coupling and decoupling method into the helix. The following coupling methods are common:
- Direct connection of the Helix to a coaxial connection (see picture above); Advantage: broadband, disadvantage: poor standing wave ratio due to the high line impedance of the helix. You can make an adjustment with a cavity resonator connected to it, but at the expense of the bandwidth.
- Introducing the terminating helix ends into an adapted waveguide; The lower bandwidth is also a disadvantage here. This method is used for the all-glass traveling wave tubes shown below.
A suitable method for low power is to adapt a coaxial connection by means of a coupling winding around the beginning and end of the helix.
application
Traveling wave tubes are used to amplify weak signals in radars, satellite communications and radio astronomy. They are used in both transmitters and receivers.
Due to their broadband nature, traveling wave tubes can, for example, cover the entire C-band (3.4 to 4.2 GHz) used for satellite downlink and deliver around 50 watts of output power with an efficiency of up to 71 percent.
Traveling wave tubes can also be modulated or their gain controlled. For this purpose, the electron flow of the electron gun can be controlled by means of its Wehnelt cylinder .
TWTs can also as a reinforcing mixer at the heterodyne detection are used (superheterodyne).
care
Traveling wave tubes require a heating voltage (a few volts) for the hot cathode, an axial magnetic field generated by permanent magnets or electromagnets, and an operating voltage in the kilovolt range. In addition, there are control and focusing voltages for the electron gun and, in power applications, cooling of the collector and often the helix.
In satellite technology, this electrical supply unit for traveling wave tube amplifiers (TWTA) is called Electronic Power Conditioner (EPC), and the unit consisting of EPC and amplifier is also called Microwave Power Module (MPM).
Traveling wave tubes compared to semiconductor amplifiers
Semiconductor amplifier Solid state power amplifiers (SSPA) have an efficiency of 25 to 30 percent in the maximum frequency range of 30 GHz, compared to 50 to 70 percent for traveling wave tube amplifiers. Traveling Wave Tube Amplifier (TWTA). The linearity is slightly lower than with TWTA. SSPA are robust against mechanical stress, but sensitive to cosmic rays. The failure rate in space is greater than that of TWTA. The ratio of useful power and weight is more favorable with TWTA with a power consumption of around 200 W than with SSPA. SSPAs are superior to TWTAs only for small services. They are cheaper and do not require heavy magnets and supply units for cathode heating and high voltage.
History and present
The TWT was invented in Great Britain during the Second World War by Rudolf Kompfner and later perfected by him together with John R. Pierce at Bell Labs . Pierce contributed the theoretical representation of the traveling wave tube, which was indispensable for the targeted further development of the complicated component.
Around 1960 the efficiency of the traveling wave tube was 10 to 20 percent, compared to 70 percent today.
Until the 1980s it was used extensively, for example. B. as Telefunken TL6 in broadband radio systems as a transmitter output stage.
The early specimens were still made with a glass cover (see picture), while today u. a. a metal-ceramic construction is also preferred due to the increased performance.
Used today u. a. the company TESAT-Spacecom traveling wave tubes; it also produces power supplies for it. As the largest supplier in Europe, the Thales Group manufactures traveling wave tubes. Boeing Defense, Space & Security manufactures traveling wave tubes, etc. a. for radars and satellite communications.
Concept history
The German word "traveling wave tube" was first used in 1949 in the publication "Investigations on self-excited vibrations in the traveling wave tube" by Herbert Schnittger and Dieter Weber. Some authors reject the term “ traveling wave tube ” as unsuitable because the term “ traveling wave ” arouses completely different associations in high voltage technology. In 1957, the Telecommunications Society chose the designation traveling wave tube.
See also
Web links
Literature / sources
- ^ John R. Pierce : Traveling Wave Tubes . D. van Nostrand Co., 1950.
- ^ Frederick L. Gould: Radar for Technicians - Installation, Maintenance, and Repair; McGraw-Hill Professional Publishing, 277 pages, ISBN 0-07-024062-0 , 1995; Page 64ff
- ↑ Boeing Introduces Lightweight, High Efficiency C-Band Amplifier ( Memento from December 2, 2008 in the Internet Archive )
- ↑ Telefunken tubes and semiconductor messages The TL 6 traveling wave tube as a power amplifier for 4 GHz radio links
- ^ Warner: Historical dictionary of electrical engineering, information technology and electrophysics . German, 2007, ISBN 978-3-8171-1789-5 , pp. 392–393 (accessed November 8, 2012).
- Pierce, John R. (1950). Traveling wave tubes. D. van Nostrand Co.
- Kompfner, Rudolf (1964). The Invention of the Traveling-Wave Tube. San Francisco Press.