In-Line Microstrip Filters with Ease-of-Control Transmission Zeros Based on Frequency-Variant Parallel Coupled Structures

This paper presents a reasonable solution, based on a direct synthesis approach, for high selectivity microstrip filters by using frequency variant parallel-coupled structures. The approach relies on the analysis and modelling of two elementary structures gathering parallel-coupled microstrips with grounded or open stubs. Note that the resultant circuit models require only a few circuit parameters and are found equivalent to a pair of resonators & a frequency-variant coupling (FVC) in between. The FVC produces a transmission zero (TZ) natively, whose position can be easily assigned (either above or below the passband) and controlled depending on the shunt stub situation. In consequence, high selectivity microstrip filters with multiple TZs can thus be realized by properly arrowing the proposed structures. For the validation, we carry out an illustrative example along with simulation and measurement results.