Elliptic low pass filter design using DGS slot for microstrip lines

Abstract

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 (S 21 ) to -20dB (S 21 ) approximately.Index Terms--Defected ground structure (DGS), low pass filters, microstrip filter, transmission zero.

Microstrip Low Pass Filter Designs Using Defected Ground Structure

“…The transmission line length l can be calculated from the equations as follows Here no transmission zeroes were introduced in the design. The transmission zeroes are introduced by replacing the apertures in the ground layer [4]. This leads to the designs introducing transmission zeroes.…”

“…The metal finger has a width of 0.4mm and length of 3.2mm. The cut off can be shifted back by reducing the inductance of the narrow line [4], [8]. By increasing the width of the strip the inductance can be reduced.…”

“…In recent years there is increasing demand for broader bandwidth in wireless communication systems and many researches are going on the wideband filters [1][2][3][4]. In this work the Defected Ground Structures (DGS) [5][6][7][8][9][10] presented here is to improve the performance of a traditional microstrip low pass filters.…”

“…For improving the pass band responses of the microstrip filters useful design equations are given. Here two elliptic low pass filter designs were designed and simulated than fabricated on an FR-4 substrate with a relative dielectric constant of 4.4 and thickness of 1.2mm [3], [4]. One transmission zero is introduced in the first low pass filter design by replacing the central aperture with interdigital structure which consists of metal fingers.…”

“…The resonant frequency of the slot can be varied without changing the slot size by changing the number of metal fingers [13][14][15][16][17][18][19][20]. In the figure 1 the interdigital DGS slot [4] introduced here is shown. In this technical note all dimensions are in mm.…”