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

Summary form only given. This paper presents an experimental investigation of the sensitivity of suppression obtained by harmonic injection in a Traveling Wave Tube to injected frequency, amplitude and phase drifts. Harmonic injection schemes offer a promising solution to reduce the nonlinear effects in a TWT. Experimental results for two-carrier drive show that high suppression of up to 55 dBc can be obtained for simultaneous suppression of both the upper and lower third-order intermodulation (3IM) products by injecting second-harmonics of both the fundamentals. This is close to the desired value (60 dBc) for many communication applications. In this paper we examine the frequency, amplitude and phase tolerance permitted for the injected frequencies to maintain a desired degree of suppression and the spatial evolution of different components of the wave spectrum. The experimental observations reported in this paper indicate that suppression by harmonic injection is quite insensitive to drifts in the input frequency. Experiments were done on the XWING (eXperimental WIsconsin Northrup Grumman) TWT with two tones of 1.90 and 1.95 GHz. A second-harmonic frequency at 3.90 GHz was injected to obtain 30 dB (55 dBc) suppression at the output. It was observed that suppression of 19 dB (44 dBc) is maintained as the frequency of the other fundamental (1.90 GHz) was varied over 0.9 GHz.