How does a backward wave oscillator work?

Answer 1

The BACKWARD-WAVE OSCILLATOR (BWO) is a microwave-frequency, velocity-modulated tube that operates on the same principle as the Traveling Wave Tube. However, a traveling wave that moves from the electron gun end of the tube toward the collector is not used in the BWO. Instead, the BWO extracts energy from the electron beam by using a backward wave that travels from the collector toward the electron gun(cathode). Otherwise, the electron bunching action and energy extraction from the electron beam is very similar to the actions in a Travelling Wave Tube.

The typical BWO is constructed from a folded transmission line or waveguide that winds back and forth across the path of the electron beam, as shown in figure.

The folded waveguide in the illustration serves the same purpose as the helix in a Traveling Wave Tube(TWT). The fixed spacing of the folded waveguide limits the bandwidth of the BWO. Since the frequency of a given waveguide is constant, the frequency of the BWO

is controlled by the transit time of the electron beam. The transit time is controlled by the collector

potential.

Thus, the output frequency can be changed by varying the collector voltage, which is a definite advantage. As in the TWT, the electron beam in the BWO is focused by a magnet placed around the body of

the tube.