Travelling wave tube amplifier
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Categories: Vacuum tubes | Electronic amplifiers
A TWTA (often pronounced "TWEET-uh") or travelling-wave tube amplifier is an electronic device used to produce high-power radio frequency signals. The bandwidth of a broadband TWTA can be as high as one octave, although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The voltage gain of the tube can be of the order of 40 decibels.
The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A solenoid coil wrapped around the tube creates a magnetic field which focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches the length of the tube, finally striking a collector at the other end. (In lower-power devices, the solenoid coil can be replaced by permanent magnets.)
A directional coupler, which can be either a waveguide or an electromagnetic coil, fed with the low-powered radio signal that is to be amplified, is positioned near the emitter, and induces a current into the helix.
The helix acts as a delay line, in which the RF signal travels at the same speed along the tube as the electron beam. The electromagnetic field due to the current in the helix interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation), and the electromagnetic field due to the beam current then induces more current back into the helix (i.e. the current builds up and thus is amplified as it passes down).
A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the helix. An attenuator prevents any reflected wave from travelling back to the cathode.
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Acronym Usage Note
While the travelling wave tube is certainly an amplifier, in industry the above device is usually referred to as a TWT. The acronym TWTA is reserved for a TWT coupled with its protection circuits and regulated power supply (EPC, electronic power conditioner), which may be supplied and integrated by a different manufacturer. The subsequent addition of a linearizer can, by complementary compensation, improve the gain compression and other characteristics of the TWTA; this combination is called a linearized TWTA (LTWTA, "EL-tweet-uh").
Coupled Cavity TWTA
Helix TWTAs are limited in peak RF power by the current handling (and therefore thickness) of the helix wire. As power level increases, the wire can overheat and cause the helix geometry to warp. Wire thickness can be increased to improve matters, but if the wire is too thick it becomes impossible to obtain the required helix pitch for proper operation. Typically helix TWTAs achieve less than 2.5 kW output power.
The coupled cavity TWTA overcomes this limit by replacing the helix with a series of coupled cavities arranged axially along the beam. Conceptually, this structure provides a helical waveguide and hence amplification can occur via velocity modulation. Helical waveguides have very nonlinear dispersion and thus are only narrowband (but wider than klystron). A coupled cavity TWTA can achieve 15 kW output power.
Operation is similar to that of a klystron, except that coupled cavity TWTAs are designed with attenuation between the slow-wave structure instead of a drift tube. The slow-wave structure gives the TWTA its wide bandwidth. See Free electron laser also, which allows higher frequencies.
Uses
TWTAs are commonly used in satellite transponders.
They are also used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems. Typically a control grid is introduced between the electron gun and the slow-wave structure to allow pulsed operation.
See also
Other types of microwave power tubes include: