Elliptic low pass filter design using DGS slot for microstrip lines

This technical note describes a compact design of two elliptic low pass filters in which DGS slot with an interdigital shape are introduced. By changing the length of the metal fingers and the distance between the metal fingers, the resonant frequency of the slot can be controlled. The areas assigned to these structures are not changed in these designs. Two microstrip elliptic low pass filters were designed and simulated with a cut-off frequency of 3GHz. In the first design the central aperture was replaced with the interdigital slot structure which improved the stop band response at the resonant frequency around 7.8GHz. But to get better results in the stop band there were introduced two interdigital structures in the bottom layer (ground). Thus by introducing two transmission zeroes in the design the resonance around 7.8GHz was brought down from -12dB (S₂₁) to -20dB (S₂₁) approximately.