A Compact Negative-Group-Delay Branch-line Coupler with Equal Power Division

A compact negative group delay (NGD) branch-line coupler with equal power division is proposed. The NGD characteristic is achieved by coupling the resistor-loaded lambda/4 resonators to the branch lines. For obtaining an equal power division, the horizontal- and verticalbranches are simultaneously coupled with the resistor-loaded lambda/4 resonators. To improve the isolation, two high-impedance short-circuited stubs are loaded at appropriate positions from the through- and coupling-ports. To verify the proposed method, an NGD coupler with a size of 0.75 lambda(g) x 0.96 lambda(g) is designed and fabricated. From the measured results, identical insertion losses of about 6.075 dB for the through- and coupling-ports are obtained for equal power division. The group delays for the through- and coupled-ports are -1.415 ns and -1.042 ns at the center frequency of 1.999 GHz, respectively. The NGD bandwidth is 38 MHz from 1.980 to 2.018 GHz, in which the return loss is greater than 11 dB, and the isolation is greater than 18 dB.