Compact and broadband tunable hybrid ring coupler using synthetic left- and right-handed transmission lines

In this letter, we present a frequency tunable hybrid ring coupler based on the concepts of a synthetic left- and right-handed transmission line.The proposed ring coupler makes use of lumped inductor-capacitor (LC) elements to greatly reduce the circuit size; meanwhile, the frequency responses of the coupler can be tuned by direct current (DC) bias voltages. The fabricated hybrid ring coupler shows a wideband performance with an insertion loss of less than 2.5 dB from 1.7 to 3.3 GHz. For this frequency band, the isolation is maintained at more than 24 dB. When the circuit is excited to get a pair of differential signal, less than ±5° phase error is observed for the whole frequency range. This hybrid has a 78% size reduction compared to the conventional ones and achieved a 64% of fractional bandwidth at 2.5 GHz. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1379-1382, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25959