Traveling-Wave Tube Noise Figure

Abstract

The reduction of shot noise by space charge at high frequencies in a parallel-plane diode was predicted by Rack. Neglecting space charge and circuit loss, Pierce has employed Rack's method in estimating the noise figure of the traveling-wave tube. In the present paper, use is made of the Hahn-Ramo space-charge waves together with Pierce's theory to show that the noise convection current has maxima and minima along the stream. Cutler and Quate have described an experiment in which these space-charge waves of noise were observed experimentally. The present analysis shows that there is an optimum location of the traveling-wave tube circuit entrance with respect to the space-charge waves for minimum noise figure. The magnitude of this minimum and the optimum position are presented graphically as functions of space charge and circuit loss. It is then shown that passing the electron stream through a short nonresonant gap which suddenly accelerates the electrons causes a further reduction in the tube noise figure if the gap is placed at one of the noise-convection current minima between the electron gun and the circuit entrance. It is also possible to use two properly placed voltage jumps for noise-figure reduction. An experimental tube of the latter type is described which verifies the theory. The data obtained from this tube agree reasonably well with the theoretical predictions.