Sensitivity of harmonic injection and its spatial evolution for nonlinear distortion suppression in a TWT

This paper presents an experimental investigation of the spatial evolution and sensitivity of third-order nonlinear distortion suppression obtained by harmonic injection in a TWT to fundamental frequency shifts as well as injected second harmonic amplitude and phase shifts. Harmonic injection schemes offer a promising solution to reduce the nonlinear distortion effects in a TWT. Experiments were carried out on a TWT with two fundamental input tones of f_a and f_b at 1.90 and 1.95 GHz at 15 dBm/tone drive. Second-harmonics of these frequencies at 3.80 (2f_a) and 3.90 (2f_b) GHz were then injected and iteratively adjusted in amplitude and phase to achieve maximum suppression of -31.4 and -28.4 dB respectively for the upper (2f_b-f_a) and lower (2f_a-f_b) third-order intermodulation levels (3IMs) at the output. Results show that second harmonic injection suppression is fairly insensitive to the input parameters.