Analysis of the parasitic effects in double-Y baluns

Effects limiting the bandwidth of a double-Y balun that transforms a finite-ground-plane coplanar waveguide (CPWFGP) to a coplanar strip line (CPS) are investigated both theoretically and experimentally. Analysis shows two kinds of parasitic effects: parasitic effects that depend on the electrical length of the input transmission lines for the even and balanced modes, and those that depend on the length of the open- and short-circuited stubs, which are observed in both the regular odd mode and balanced mode. The developed theory allows the parasitic resonances in CPWFGP-CPS baluns to be predicted and shifted out of the operating frequency bandwidth by changing the corresponding physical dimensions of the balun. Theoretical results have been experimentally verified.