Ultra-linear traveling wave tube development

Summary form only given. Traveling wave tubes (TWT) intended for telecommunications applications are increasingly required to have high linearity and good intermodulation performance, especially for use as modern multi-carrier power amplifiers (MCPA) and in systems that use digital signal encryption. The linearity of a TWT can be improved by optimization of the taper of the helix circuit to reduce the gain compression and phase shift of the output signal as the drive level is increased, and by operation of the tube significantly backed off from saturation in the linear region of the AM/AM and AM/PM transfer curves. However, backed off operation at a given output power level requires increased beam current and power handling capabilities compared to a design operating at saturation, and also results in a greatly reduced efficiency of the TWT. At Hughes, we have developed L-band and S-band TWTs for telecommunication applications that provide good intermodulation performance and high efficiency compared to solid-state amplifiers of similar power levels. To date, the TWTs have produced up to 150 W of continuous output power with an 8-tone continuous-random-phase signal intermodulation level of about -30 dBc and an efficiency of over 20%. At similar power levels, a comparable solid state MCPA has an efficiency well below 10% and nearly twice the cost. These low-frequency communications tubes are normally operated 9 to 10 dB backed off from saturation, and they utilize an optimized helix taper and multi-stage depressed collector design to increase the overall TWT performance and efficiency. The characteristics and intermodulation performance of several TWTs that operate in the 1.5 to 2.4 GHz frequency range will be presented.