Port Tuning a Microstrip-Folded Hairpin Filter [Application Notes]

When the first commercial electromagnetic(EM) simulators appeared in the late 1980s and early 1990s, we were thrilled to be able to accurately simulate a single microstrip discontinuity, even if the simulation ran overnight. Today, computer hardware is cheaper and much faster, and EM simulation software is more efficient and sophisticated. Even so, putting an EM simulator inside an optimization loop is still not practical in most cases. Even our simple N = 5 hairpin filter example does not lend itself to directly driven EM optimization. The desire to efficiently optimize microwave filters using accurate EM simulation data led us to the port-tuning concept. This article replicates a design example that has been used by others to illustrate the port-tuning method for planar microwave filters. The aim is to present a simpler, more accessible approach to the port-tuning method. A simple analytical method to compute physical corrections to the EM-based filter model from the circuit simulator tuning results is also outlined. When teaching the port-tuning method, this step has been a consistent stumbling block for students. The article also points out important differences in closed-box and laterally open method of moments (MoM) EM simulators when analyzing planar filters. This discussion includes microstrip combline and edge-coupled filter examples. In the process of writing this article, I discovered that the folded hairpin topology appears to be unique when compared to other planar topologies such as edge-coupled, interdigital, and combline. Unlike these other distributed planar topologies, the folded hairpin filter appears to be insensitive to the cutoff waveguide channel normally used to enclose a planar filter.