Pulsed, high power, RF generation from nonlinear dielectric ladder networks - Performance limits

An extensive study of the generation of bursts of soliton-like pulse trains on nonlinear dielectric/ capacitance ladder networks has shown that there are some fundamental physical problems that limit this method of generating high power, pulsed RF signals. The purpose of this paper is to review these issues and suggest areas where research may lead to significant performance improvements. High power operation has been confined to nonlinear ladder networks or lumped element transmission lines (NLETLs) that use ferroelectric ceramic dielectrics such as barium or strontium titanate ceramics as the nonlinear dielectric/capacitance elements. These dielectrics are commonly used in high voltage ceramic capacitors. An obvious radio frequency power limitation results from the dielectric breakdown strength of these materials. The difficulty of coupling the RF pulse from the nonlinear ladders or lines efficiently into a linear resistive load is well known and is caused, primarily, by the voltage dependency of the ladder/line impedance. A closer examination of the way that the voltage and current waveforms on the line are propagated and also the reflection process at a linear resistive load explains the structure of the voltage waveforms, seen at various stages on the line, and also possible solutions to the coupling problem. Results from experimental lines and new computer modelling techniques are presented. Finally attempts to generate radio frequency bursts at frequencies into the microwave bands have also yielded disappointing results. In order to do this, lumped element lines have been built which usually comprise a parallel plate transmission line that is periodically loaded with nonlinear dielectric tiles or slabs. This approach has resulted in the generation of poorly modulated HF radio frequency bursts and consequently this technology is limited to operating frequencies below a few hundred MHz. An explanation for this phenomenon is given which shows that loss in the nonlinear line elements is the most likely cause of the problem. This is backed up with results from experiments and modelling of ladders with simulated variable loss.