Design and Analysis of Passive-Integrated Absorptive Flat-Group-Delay RF Bandpass Filters in GaAs Technology for Digital Communications

A family of on-chip passive-integrated absorptive flat-group-delay RF bandpass filters (BPFs) in gallium arsenide (GaAs) technology is presented. These wideband BPFs feature broadband quasi-reflectionless behavior along with quasi-constant group-delay responses beyond their associated 3-dB-bandwidth (BW) ranges. Firstly. by means of a $\pi $ -shape network composed of a reflective first-order BPF and two shunt identical lossy bandstop filters (BSFs), a two-port-absorptive RF BPF is engineered. To further increase the stopband attenuation levels, the extension of this filter concept to higher-rejection BPFs using n cascaded reflective single-pole BPF units and ( ${n} +1$ ) replicas of a shunt lossy BSF is then approached. Subsequently, in order to equip such BPFs with higher-selectivity filtering responses, the development of input- and two-port-reflectionless BPFs with multiple transmission zeros (TZs) is addressed. Moreover, a multi-TZ flat-group-delay BPF with input-absorptive behavior is devised. It exploits a reflective BPF channel, which is shaped by a high-selectivity BPF unit with two close-to-passband TZs and a shunt series-LC resonator that produces an additional TZ, along with a shunt absorptive BSF in a complementary-diplexer-based topology. Finally, by cascading two duplicated high-selectivity BPFs and the associated absorptive BSFs with a modified shunt series-LC resonator in a back-to-back connection, a type of two-port-reflectionless BPF with multiple TZs is further engineered. Following this approach, a modified shunt lossy BSF instead of the previous shunt series-LC resonator is employed in the overall reflectionless BPF to obtain a sharper-rejection passband and flatter group delay versus the corresponding beyond-3-dB BW. The RF operational foundations of these absorptive BPFs are detailed with analyses of their relevant lumped-element-based equivalent circuits. Furthermore, proof-of-concept prototypes for the five suggested RF BPFs are simulated, built, and measured to experimentally validate their design concepts for application in power-efficient high-data-rate digital-communication systems.