Crossed-field Backward-wave Amplifier Noise-figure Studies†

†Communicated by Dr. R. Kompfner. This research is supported by the U.S. Army Electronics Research and Development Laboratory under Contract DA 36-039 AMC-02164(E)

Abstract

In the past only a few reports of noise-figure for crossed-field amplifiers have appeared in the literature. The reported measurements were spot cheeks and did not reveal the dependence on operating parameters of the amplifier. An extensive set of measurements discussed in this paper yielded continuous and repeatable data giving the dependence on parameters. For a variation of beam voltage from 107 v to 367 v and a corresponding variation in beam current from 15 mA to 49·5 mA, gain improvement from 8·5 dB to 15·5 dB and noise-figure improvement from 27 dB to 22·5 dB have resulted with the beam centred between the sole and circuit electrodes. A novel experimental technique was used to achieve repeatable measurements and is described here. Over the past decade a great deal has been said in the literature about inherent excess noise in crossed-field amplifiers. Furthermore, numerous papers have shown that much more noise exists when the cathode is space-charge-limited rather than temperature-limited—the opposite of the counterpart longitudinally focused amplifiers. The noise-figure measurements reported in this paper have shown that neither of these generally accepted assertions need be true in all crossed-field amplifiers. In particular, for a beam voltage of 367 v a significant reduction in noise-figure of about 5 de has resulted in space-charge-limited operation as compared to the temperature-limited operation. Experimental results showing an improvement of 10 dB in noise-figure as the beam is moved close to the circuit from its midway position between the sole and circuit electrodes are discussed. Finally the variation of noise-figure with pressure is reported. By combining optimum focusing, space-charge smoothing, and beam displacement, a noise figure of 10·5 dB is obtained without using noise transducers.

Notes

†Communicated by Dr. R. Kompfner. This research is supported by the U.S. Army Electronics Research and Development Laboratory under Contract DA 36-039 AMC-02164(E)