Electrical Solitons for Microwave Systems

123 1527-3342/19©2019IEEE N onlinearity and dispersion are often nuisances that microwave engineers seek to curb. However, there is a system that can harmonize these two effects to produce useful—and fascinating—dynamics with unique microwave engineering opportunities: the nonlinear transmission line (NLTL) [1]–[11]. The NLTL is a ladder network of lumped inductors and varactors [Figure 1(a)] that can be alternatively built by periodically loading a smooth transmission line with lumped varactors [Figure 1(b)]. The NLTL is a nonlinear dispersive medium. On one hand, it exhibits nonlinearity due to the varactors, which are typically realized as reverse-biased p-n junctions or Schottky diodes; their voltage-dependent capacitance is responsible for the nonlinearity. On the other hand, the NLTL is dispersive due to its structural periodicity. This periodicity makes the NLTL a low-pass filter with a cutoff frequency near which the NLTL becomes increasingly dispersive. The star of the harmony between nonlinearity and dispersion is the soliton [12]–[14], a bell-shaped traveling voltage pulse [Figure 1(c)], where nonlinearity and dispersion exactly counteract each other. Due to this balancing act, the soliton neither spreads (no dispersion) nor steepens to form a shock wavefront (no nonlinearity). It propagates down the line, preserving its shape, at least in the absence of loss.