There are many distributed line topology that can be used to design a filter. One of these is the Interdigital topology. The interdigital utilizes multiple cascading coupled lines arranged in such a way that the termination of each stub (or called resonator) is alternate between short and open circuit. As one of the planar transmission line topology, it can be fabricated using technologies such as microstrip, stripline and suspended stripline.
In comparison to other topologies such as the waveguide, interdigital is relatively simple to design, relying on mechanical arrangement of the resonators to achieve the filtering effect. The simple design would mean the cost would be lower too. Its design is also consider relatively robust and less sensitive to losses. Although its Q-factor (quality) is not as good as more sophisticated topology such as waveguide, it offers good Q-factor nonetheless for designs that does not require a very high Q-factor.
In this video, we demonstrate how to design an interdigital filter using EDS software. The response of the filter is generally affected by the width and length of each individual resonator and also the gap between them. As expected of every real life design, losses will be present, mostly contributed by the gap between the resonators (the gap consist of air as medium, which would cause losses to the signal).
To help compensate for the lossy response, the tuning function of EDS will be utilized. In this video, the gap is set to a variable of “a”, which would then be enabled for tuning to find the gap distance that would produce the best desired performance.