The structure of noise in crossed-field amplifiers

Summary form only given, as follows. The results of a series of detailed tests on a pulsed forward wave CFA show that its noise output is highly structured in the frequency domain. The noise structure results from the CFA acting as a delay-line oscillator while it is amplifying the normal drive signal. The CFA oscillations occur at frequencies that are multiples of the reciprocal group delay through the tube. Typical spectrograms of the structured noise are shown as plots of spectral amplitudes as a function of time and frequency. They illustrate the following features. The noise is concentrated in a series of lines evenly spaced around the drive frequency. The lines are noisy in amplitude and phase. The locations of noise lines as the drive frequency is tuned over a few hundred megahertz is shown. The locations of the lines are constant as the drive frequency is changed. A diagram of a CFA model is shown. To achieve a low noise CFA, the conditions necessary for causing delay line oscillations must be removed. This requires that the electronic (active) and electric feedback (passive) paths have sufficient attenuation to prevent oscillation over the entire band where the CFA has small signal gain. The parameters of these feedback paths require considerable attention during the initial design of the CFA.