Backward wave oscillator

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A backward wave oscillator (BWO), also known as a carcinotron , is a backward wave tube and generally a time-of- flight tube for generating microwaves . Compared to reflex klystrons and magnetrons, for example, it has the advantage of being electrically tunable over a large frequency range.

The tube is mainly used in research and in the military sector. BWOs were developed in the early 1950s and manufactured industrially since the 1960s.

With BWOs, frequencies from around 1 G Hz up to the terahertz range can be generated, depending on the type . The output power for the common designs is a few milli watts and decreases when the frequency is increased. Output powers of up to 100 kW can be achieved with special designs.

Layout and function

BWOs are oscillator tubes that, like the traveling wave tubes that are only used for amplification, consist of a helix as a delay line. A wave runs on this and interacts with an electron beam . In BWOs, however, the generated wave runs backwards on the helix, i.e. H. opposite to the direction of the electron beam. This phenomenon, which is undesirable in a traveling wave tube, is exploited here. The feedback of the oscillator is realized by the electron beam in that the speed changes induced in it at the output become changes in density due to the transit time, which stimulate the end of the helix through their alternating electric field. Oscillations are thus generated when there is a phase dispersion between the electron speed and the phase speed of the wave on the delay line.

The longitudinal magnetic field of a permanent magnet is used to focus or guide the electron beam.

The frequency of the generated wave can be varied over a wide range with the acceleration voltage (anode voltage) of the electron beam.

Carcinotron

In terms of its technical design, a BWO consists of the following parts:

  • the electron gun, usually designed as a tetrode beam system
  • the delay line with decoupling
  • the assembly unit with magnet and base

To avoid unwanted vibrations, the end of the delay line is dampened by vapor deposition of a thin iron layer.

application

The BWO is used under the name Carcinotron as a high frequency generator in older radar devices . BWOs were built, for example, by the US company Watkins-Johnson .

literature

  • Hewlett Packard Company: How a helix Backwars-wave tube works in HP Application Note, No. 12, Palo Alto, USA, 1959
  • Hewlett Packard Company: Sweep Oscillator Model 8690, Operating and service manual, Palo Alto, USA, 1970