Experimental Investigation of Twt Nonlinearities and Distortion Suppression by Signal Injection

Traveling Wave Tube (TWT) amplifiers are high power, often broadband microwave amplifiers used extensively in satellite communications and electronic countermeasures. While the high-power outputs and wide gain-bandwidths make TWTs ideally suited for these purposes, the nonlinearity of these devices results in amplitude, phase and spectral distortion. Nonlinear distortion products appear as harmonics and for multi-carrier operation also as intermodulation products, at the output of the amplifier thus limiting the usable bandwidth of the amplifier and degrading fundamental efficiency. This spectral growth also causes spill-over of one channel’s signal into adjacent frequency channels, as well as distorts the signal within the same frequency band causing inter-symbol interference. Thus, suppression of harmonics and intermodulation frequencies in the output spectra of TWT amplifiers is desirable for reliable and high data-rate multi-carrier communication. This thesis presents an experimental investigation of the nonlinear behavior of a traveling wave tube amplifier. Preliminary computational modeling results are also shown using the LATTE/MUSE/S-MUSE numerical suite. This understanding is crucial to develop and explore linearization techniques for the TWT. In particular, signal injection schemes are investigated for suppressing nonlinear distortion products eg. Second-harmonic suppression by second-harmonic injection and third-order intermodulation suppression by injection of second-harmonic or/and third-order intermodulation frequency. It is shown that by injecting a small amount (15 dBc or less) of properly phased harmonic or intermod signal at the input, the TWT nonlinearity can be utilized to suppress the distortion signal produced naturally at the output. Experimental