Noise near the carrier in microwave oscillators

Abstract

This experimental study aimed at observing and whenever possible verifying the sources of noise near the carrier in microwave oscillators. The noise was detected as equivalent amplitude and frequency modulations; these and other signals of interest were analyzed by a spectrum analyzer resolving 25 bands of about \frac{1}{2} octave each, covering 14 cps to 34 Kc. The frequency domain across spectra (correlations) were also measured. The correlation measurements greatly facilitated the control of power supplies and environment. The tubes tested were two reflex klystrons and 3 B.W.O.'s, all operated cw at 9 Gc. It was found in all cases that the amplitude noise was negligible, being orders of magnitude lower in sideband energy than the frequency noise. The correlation between beam current and frequency was so small that it could be safely concluded that internally generated beam current fluctuations were a negligible source of frequency noise in the tubes tested. From the shape of the frequency noise spectra, it appeared that a major source of frequency noise was the perturbation of the beam in the interaction region by ions. It is hoped to directly verify this by controlled experiments.