Origin of phase noise
The phase noise of the microwave tube has been paid attention to since its birth, but it has not been clearly explained for a long time, but most studies believe that it is related to the pulsation of the electron beam. The important characteristic of ion noise is that the frequency is very low, generally a few megahertz, and sometimes it can reach low sound frequencies or even ultra-low sounds. Such low frequencies cannot be generated by microwave resonance circuits and are much lower than any plasma in the tube. The oscillation frequency is similar to the Zhang Chi oscillation.
The specific physical process is roughly as follows: the electron beam generates pulsation along the axis due to non-equilibrium Brillouin focusing, and an electrostatic potential well is generated on the axis due to the space charge effect. The positive ions generated by the collision of the electron beam are trapped in the potential well. When filled to a certain extent, the focus state of the electron beam will change. The potential well moves to the cathode, and the ions are released and hit the cathode. This process cycle Repeatedly. During the process of ions being captured, escaped, and recaptured, the electrostatic potential well along the axis should be changed accordingly to change the axial velocity of the electron beam, and then interact with the high frequency field to form noise in the output signal.
Klystrons and traveling wave tubes as high-power microwave vacuum devices are widely used in communications, radar, electronic countermeasures, remote sensing and other fields. But like all vacuum devices, the tube cannot be in an absolute vacuum, always with a small amount of residual gas. Electron beam collisions ionize these background gases, which will generate positive ions. The space charge effect will change the state of the electron beam, which will affect the interaction between the electron beam and the wave, causing periodic disturbances in amplitude and phase (especially phase) of the signal. This disturbance is called ion noise, sometimes called phase noise [1].
With the advancement of communication technology and detection technology, the requirements for the stability of the signal source have also increased accordingly. The research on the phase noise of microwave tubes has become a research hotspot in the field of high-power microwave. Many literatures have made detailed studies on the phase noise caused by ion relaxation oscillations [1 ~ 4], but most of the research has focused on the physical mechanism of ion noise, especially the generation and movement of ions and their effects on the electron beam. However, the research on the process of the wave itself, especially the modulation of low-frequency noise to high-frequency signals is far from perfect. Based on the small signal approximation, starting from the wave theory and the kinematics theory, the effects of ion oscillation on the output signals of the traveling wave tube and the klystron are studied respectively, and the spectral and phase characteristics of the output signal are analyzed.
Analysis of ion noise of traveling wave tube and klystron
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