Potential-minimum noise in the microwave diode

An analysis of the potential-minimum region in the microwave diode is presented which predicts the amount of noise convection current at the potential minimum under conditions present in guns of low-noise traveling-wave tubes and klystrons. Contrary to previous work by one of the authors, this analysis partially includes the effects of finite transit angle between the cathode and potential minimum. The results indicate that under certain conditions of space-charge limitation, the noise convection current may be larger than full shot noise, whereas under other conditions reduction by as much as a factor of four may be obtained. When the results of this work are applied to the noise-figure theory of traveling-wave tubes and klystrons, it is found that their minimum obtainable noise figure may be in the vicinity of 4 db rather than the previously predicted 6 db with reasonable cathode current densities. For very large current densities or low operating frequencies, the present theory indicates that there is no theoretical limit to the minimum noise figure of such devices, in agreement with the previous work.