Linearizability of TWTAs using predistortion techniques

Predistortion linearization is a very effective technique for improving the linearity and efficiency of traveling wave tube (TWT) amplifiers. In this paper, we will study the effectiveness of predistortion techniques for TWT linearization using single-tone, two-tone, and quadrature-amplitude-modulation signals. The results from a series of predistortion linearization experiments for five TWTs covering L-, C-, Ku-, and Ka-bands and including both helix- and coupled-cavity TWTs will be presented. We will demonstrate the additional improvement of fifth-order predistortion linearization over the more commonly used third-order predistortion linearization. To circumvent the complexity and limited availability of a pure fifth-order linearizer, a technique for realizing nonlinear functions of order greater than or equal to five using cascaded third-order nonlinear functions is described. The technique can be used to efficiently generate higher order nonlinearities for predistortion linearization applications. We will demonstrate experimentally that the use of two cascaded third-order functions is comparable to a pure fifth-order implementation in performance.