Improvement of low pass and band pass filter characteristics using Cross-DGS-resonators, logarithmic series and coupling matrix methods

A new method to improve the filter features and to control the results of the LPF using Cross defected ground structure (CDGS) and logarithmic method is proposed in this work. The etched DGS topologies on the ground plane as well as its coupling provide the slow-wave characteristics [1, 2. In order to control the operate frequency range of the filter and its results, the Chebyshev's rule, suspended Layer's technique first order and logarithmic method are employed. The proposed filters are partially measured, simulated, optimized and compared. The lowpass filters designed at cutoff frequency of 2.0GHz/3GHz/4.5GHz, which are suitable for GSM900, radar and wireless local area network applications. The proposed filters have low insertion loss, high return loss in the passband and wide stopband as well as very sharp roll-off characteristics for near pass-band. The proposed topologies are designed, optimized and simulated on RO4003 substrate using AWR software. The measurements show good consistency with the simulations. The minor discrepancy between both results can be attributed to the unexpected tolerance of the manufacturing process and reflections from the SMA connectors.

I. Introduction

Recently, research on the defected ground structure (DGS) such as a photonic bandgap (PBG) transmission line [3], [4], which has periodic arrays of defects, has been reported with various configurations in microwave and millimetre frequency-band applications. The DGS with periodic or nonperiodic arrays provides a rejection band in some frequency range due to the increase of the effective inductance of a transmission line. Defected ground structure (DGS) has been reported in the recent publications in various applications such as filters [5]–[27], power dividers, and amplifiers due to its high-Q frequency characteristics, area effectiveness, precise equivalent circuit models and simple applications to any microstrip circuits. The conventional DGS consists of narrow and wide etched areas in backside metallic ground plane, which give rise to increasing the effective capacitance and inductance of a transmission line, respectively. It is composed of two square defected areas and a narrow connecting slot which are the sources of the equivalent LC elements. Actually, it is well known that an attenuation pole can be generated by combination of the inductance and capacitance elements. Fig. 1.

Three-dimensional view of the cross-head-DGS cell.