Reflection-Type Topologies With Arbitrary Wideband Flat Group Delays Using Coupled Lines

This paper presents the analytical design techniques of several reflection-type group delay (GD) circuits (types-I-III) with arbitrarily prescribed wideband flat GD responses using <inline-formula> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula>/4-coupled lines. The type-I GD circuit consists of the <inline-formula> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula>/4 transmission lines and an open-circuited coupled line with a short-circuited load, whereas type-II consists of a short-circuited coupled line with an open-circuited load. For a compact circuit size, the type-III circuit is realized by a parallel combination of the open-circuited and short-circuited coupled lines. To obtain the arbitrarily prescribed response of wideband flat GD, closed-form analytical design equations are provided. An analytical analysis shows that the wideband flat GD response can be obtained by controlling the appropriate even- and odd-mode impedances of the coupled lines. For an experimental validation of the proposed structures, prototypes of the three GD circuits were designed and fabricated at the center frequency of 2.5 GHz. To realize a higher GD response over a wideband bandwidth, a number of reflection-type GD circuit units are cascaded and measured. The measurement results agree soundly with the simulation and theoretical prediction results.