Injection schemes for TWT Linearization

Traveling Wave Tube (TWT) amplifiers, used in communication and ECM applications for their high power output, are nonlinear devices. This leads to harmonic and intermodulation distortion in the output spectrum which compromises the device performance. In communication systems, intermodulation products (IMPs) lead to adjacent channel interference while in ECM applications, harmonic generation adversely affects the efficiency at the fundamental. In this paper we investigate various linearization schemes based on signal injection to condition the output spectra. We focus primarily on IM3 (third-order intermodulation) suppression for two-tone drive since the IM3s, being closest to the fundamentals, are the main concern in a communication system. Two primary techniques investigated are second harmonic injection ([1], [2], [3], [4]) and IM3 injection. These involve injecting amplitude and phase optimized harmonic and IM3 signal respectively, to suppress the inherently generated IM3 at the output of the TWT. We also propose a scheme based on simultaneous injection of both second harmonic and IM3 that offers the prospect of eliminating the need for precise phase control of the former two schemes.