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Excess Noise in Cavity Magnetrons

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1 Author(s)
Sproull, Robert L. ; RCA Laboratories, Princeton, N. J.

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The magnetron oscillator of some frequency‐modulation radars also furnishes the ``local oscillator'' excitation for the crystal mixer of the receiver. Excess noise generation by the magnetron was observed to reduce greatly the receiver sensitivity. This noise exhibited a strong dependence upon anode voltage and current, and changed with time in a perplexing manner. After many experiments, a hypothesis of the cause of excess noise was developed, and further experiments confirmed this hypothesis. The noise is thought to be caused by ionization of atoms of the cathode oxide coating, which atoms are removed from the cathode by electron bombardment. In order to reduce the generation of excess noise, and still preserve the advantages of an oxide‐coated cathode, a special shape of cathode has been developed. The coated regions of this cathode are sheltered from electron bombardment, and the noise is much reduced. Use of cathodes of this general type should also produce magnetrons with longer operating lives than present tubes.

Published in:

Journal of Applied Physics  (Volume:18 ,  Issue: 3 )