Design of high-performance microstrip and coplanar low-pass filters based on electromagnetic bandgap (EBG) structures

In this paper, a rigorous method for the design of high-performance n-order Chebyshev low-pass filters based on electromagnetic bandgap (EBG) stepped impedances in microstrip and coplanar (CPW) technologies is proposed. Fifth-order low-pass filters implemented by means of circular patterns etched in the ground plane of microstrip and CPW lines have been designed, simulated and measured on an RO3010 substrate (∊r= 10.2, tanδ = 0.0023 at 10 GHz, substrate thickness h = 0.635 mm and copper thickness t = 0.018 mm). The designed filters have shown a good agreement between EM simulation and measurement results. The proposed EBG-based low pass filters, with six etched circles and a size of 42 × 20 mm2, achieve a passband with a cut-off frequency fc = 6 GHz measured at |S11| = −20 dB, return loss RL = 20 dB, 5 reflection zeros in the frequency axis, a ripple lower than 1.1 dB, and a stopband greater than 5 GHz with a rejection level higher than 30 dB. Therefore, the designed low-pass filters have an ultra-wide passband with high return loss, low insertion losses and a low ripple. In addition, they exhibit a large stopband with high rejection level compared to other reported wide stop-band low-pass filters based on EBG, defected ground structures (DGS), or modified complementary split ring resonators (CSRR). This technique can be used to design high-performance Chebyshev EBG-based low-pass filters with a high degree of control over the characteristics of their frequency responses. This allows to replace the EBG-, DGS- and CSRR-based wide stop-band low-pass filters in microwave circuit and antenna applications, with filter structures exhibiting high return loss, equiripple responses in the passband, and high rejection level in the stopband.