A Novel Compact Isolation Circuit Suitable For Ultracompact And Wideband Marchand Baluns

A novel compact isolation circuit (IC) is proposed for the ultracompact and wideband UHF Marchand balun topology, and exact design formulas are presented for arbitrary coupling coefficients and impedance-transforming termination impedances. The wideband Marchand balun mainly consists of two identical coupled transmission-line sections which are far shorter than 90 degrees long, connecting segment, two or three lumped elements and the proposed IC. The novel IC features -180 degrees phase delay between two balanced ports, and makes the ultracompact implementation possible, which is quite different from the conventional ones with 180 degrees phase delays between two balanced ports. In addition, due to the -180 degrees phase delays, excellent isolation performance and excellent phase responses can be performed. For the proof-of-concept demonstrations, one prototype with each coupled-line section being only 20 degrees long and the half IC being only 20 degrees long is tested at 700 MHz. The measured frequency responses are in good agreement with the predicted ones with the performance of the 10-dB return loss bandwidth of 40% and -15 dB isolation bandwidth of 121.4%. In terms of sizes versus bandwidths, the fabricated Marchand balun can be regarded as the best and the smallest ever recorded, based on the low-cost microstrip technology.