Signal Integrity Analysis of Modal Filters Formed by Modification of Microstrip Lines and Coplanar Waveguides

Modal filters (MFs) have been extensively studied for their effectiveness in protecting against dangerous ultra-wideband interferences. The simplest types of MFs are formed by modifying microstrip lines (MSLs) and coplanar waveguides (CWs). This paper presents the results of a comprehensive analysis of signal integrity for three MF prototypes created by modifying the reference conductor. The first two prototypes are based on modified MSLs, differing in the coupling of the active and passive conductors. The third prototype is formed by modifying the CW. To assess the integrity of the useful signal, S-parameter were measured for all three prototypes. Additionally, pseudo-random bit sequences with different data rates of 10,000 bytes were applied to the input of the MFs using the ADS program. It was observed that the eye remains open within the bandwidth for all MFs. However, the first two MFs exhibited the bandwidths of 69 MHz and 24 MHz, respectively. To enhance the bandwidth, simulations were performed by reducing the length of these MFs. As a result, the first MF achieved a bandwidth of 145 MHz, while the second MF achieved a bandwidth of 50 MHz. In all variations considered, the eye remains open within the band-width, and the transition between logical levels is easily discernible. The results imply that the proposed MFs can be employed in signal circuits, including those operating at data rates of 2.5 Gbit/s.