Reflectionless Filter Structures
  This paper expands on the previously described reflectionless filters - that is, filters having, in principle, identically-zero reflection coefficient at all frequencies - by introducing a wide variety of new reflectionless structures that were not part of the original publication. In addition to extending the lumped-element derivation to include transmission line filters, this is achieved by the introduction of a novel method wherein the left- and right-hand side stop-band terminations are coupled to each other using a two-port sub-network. Specific examples of the sub-network are given which increase the stop-band attenuation per filter cell and steepen the cutoff response without disrupting the reflectionless property or increasing the insertion-loss in the pass-band. It is noteworthy that a common feature of all the structures derived by these methods is that most, if not all, of the reactive elements are of equal normalized value. This greatly simplifies the tuning requirements, and has facilitated their implementation as monolithic microwave integrated circuits (MMICs). A number of examples of MMIC reflectionless filters constructed in this way are presented and their results compared to the theory. 
Matthew A. Morgan and Tod A. Boyd
astro-ph.IM physics.optics