Computer Simulation of Inter-Modulation Distortion in Broad-Band, Multi-Carrier, Travelling Wave Tube Amplifiers

Travelling-wave tube amplifiers, used in satellite and other broad-band communication systems, must be operated as close as possible to saturation to achieve good d.c. to r.f. conversion efficiency. The non-linearity of a tube close to saturation causes the generation of inter-modulation products whose amplitudes must be small enough to meet the system specification. In designing TWTs for broad-band communication systems the amplitudes of the inter-modulation products generated by uncorrelated signals must be predicted. The inter-modulation performance of a TWT with several correlated carriers can be computed using a large-signal disc model, but this method is too cumbersome to be useful with uncorrelated carriers. Results for both correlated and uncorrelated carriers can be obtained from the single-carrier amplitude and phase transfer characteristics using the program IMAL which models a generalised non-linearity. It is not self-evident that this approach is valid for modelling the non-linear behaviour of a TWT. The inter-modulation performance of a helix TWT was computed using a large-signal disc model with two, three and eight correlated carriers. The results were found to be close to those calculated from the single carrier transfer characteristics using IMAL. The inter-modulation performance of a TWT for both correlated and uncorrelated signals can therefore be predicted from its single-carrier transfer characteristics