Session chairperson and summary author: Philippe Thouvenin
For the first time this topic covered two sessions , thus outlining the increasing importance of linearity requirements in modern Digital Communications. The new challenges for telecommunication VEDs designers had been well introduced by Dan Goebel during the opening session , through a review of different modulation schemes used in high data rate communications and corresponding RF amplifier linearity specifications. The purpose of this session was to review different techniques allowing to reduce the intermodulation products at microwave tube or RF amplifier level.
The first talk , given by Dr Yehuda Goren of Teledyne Electronic Technologies , presented a new method to linearize TWTAs based on phase and amplitude compensations through adequate variations of helix and grid voltages versus RF input level. A simple circuitry has been developed and tested on a 800W S band TWT , showing a two tone IM3 improvement up to 23 dB at 6 dB O.B.O..
The following paper by John G. Wöhlbier -PhD Student at University of Wisconsin - reviewed different simulation studies performed with a 1-D non linear multifrequency TWT model S-MUSE to analyze the Physics of Harmonic Injection in a TWT. The influence of injected harmonic amplitude and phase has been studied on a broadband ECM TWT , showing that the optimum phases for 2 tone IM3 or H2 level reduction differ completely from the value allowing to maximize the fundamental RF output power. This topic was further explored by John Scharer – professor at University of Wisconsin – who presented the corresponding experiments performed on the XWING TWT , a research version of a Northrop Grumman 2-6 GHz TWT. The tests were performed with 1.95 and 2.00 GHz fundamental drive tones and 4.00 GHz harmonic tone injection , yielding an upper IM3 reduction by 24 dB in optimum conditions.
Craig Wilsen of Northrop Grumman reported on the IM3 simulations and experiments performed to validate a new code allowing to compute intermodulation products and harmonics in multi-cavity klystrons. Preliminary 3 tone simulations show the possibility to substantially reduce an IM3 level by injecting a weak signal at the same frequency with appropriate amplitude and phase.
The last paper of the session , presented by Richard Jenkings PhD student at Lancaster University , was devoted to a parametric study of efficiency and linearity of an helix TWT with respect to frequency and beam voltage. One conclusion was that the optimum performance trade-off is obtained for a lower beam voltage in Back-off operation , as compared with saturation.