Characterization of relaxation-oscillation noise in continuous-wave traveling wave tubes

Extremely low noise levels are frequently required in radar and communication systems that utilize microwave tubes. A noise waveform frequently found in continuous-wave (CW) linear-beam microwave tubes is a relaxation-type oscillation that has a period typically on the order of several milliseconds. This paper provides an extensive experimental characterization of this relaxation-oscillation phenomenon by recording the corresponding signal modulations on the output of several solenoid-focused CW traveling wave tubes. The characteristics of these amplitude and phase modulations were studied during systematic variations of tube operating parameters that included magnetic field, cathode voltage, depressed collector voltage, heater filament voltage, and input drive level. A measurement technique was also developed with the capability to obtain detailed information on the growth of the relaxation-oscillation noise waveforms along the beam interaction region by comparing reflected and output noise waveforms.