Small-sized quasi-elliptic function microstrip low pass filter based on defected ground structures and open stubs

In this article, a microstrip structure for a small-sized elliptic function low pass filter (LPF) is developed from a modified dumbbell-shaped defected ground structure (DGS) and additional open stubs. The steps taken in developing the structure are presented. Cross-junction open stubs are placed right above rectangular- and I-shaped slots in the ground plane of the microstrip line. A comparison with previous work 1 1 shows that this approach provides a steeper attenuation slope and a smaller circuit size. Since the study did not use a periodic or array configuration of DGS cells, considerable circuit size reduction, less than one fifth, is possible, compared with a conventional stepped impedance LPF having an identical cutoff frequency (2.665 GHz) to that of this LPF. Furthermore, the fine tuning capability of the stopband can provide precise manufacturing conditions for applications such as IF filters for downconverters and the bias circuits of oscillators and amplifiers to reject harmonic and spurious signals, since the stopband tuning rate with respect to frequency variation is small compared to that of a conventional DGS.

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