Slit-Slot Loaded Parallel Coupled-Line Filter

The miniaturisation of microstrip circuits is very important for systems that must be light weight and/or compact. Examples of such systems include high temperature superconducting (HTS) filters as well as those within satellite, aerospace, and portable devices. Inter-digital and parallel-coupled resonators are amongst the most commonly used forms of coupled line circuit, for filtering applications. The parallel-coupled line filter was proposed by Cohn in 1958. It has remained popular ever since, due in part to its: (1) ability to realise a wide range of bandwidths, (2) insensitivity to fabrication tolerances, and (3) simplicity of design. Unfortunately half-wavelength resonators are excessively long, at low microwave frequencies. Furthermore filters based on half-wavelength resonators exhibit a spurious pass-band at twice the fundamental frequency. This is caused by dispersion resulting from the unequal even and odd mode phase velocities. For these reasons there have been sustained research efforts to address both size and spurious pass-band limitations. One of the techniques for suppressing spurious harmonics involves cutting grooves along the edges of each coupled line. The new filter, presented in this paper, utilises a periodic loading comprising a combination of a slot and groove. This technique also gives the ability to tune the centre frequency of the filter.