Balanced Rat-Race Couplers With Wideband Common-Mode Suppression

In this article, various types of six-port balanced rat-race couplers with wideband common-mode suppression characteristics are presented. In these couplers, the input and output ports can functionalize either as single-ended or differential terminals to realize different operational modes. The power-splitting ratio for these balanced couplers can be flexibly adjusted by means of the values of the characteristic-impedance parameters of their constituent transmission-line segments. Furthermore, the employment of double-sided-parallel-strip-line(DSPSL)-based 180° phase inverters instead of half-wavelength transmission-line-based ones allows for the considerable enlargement of their operation bandwidth in more-compact circuits with lower in-band insertion-loss levels. For experimental-validation purposes, two microstrip prototypes at f0= 0.9 GHz with equal and unequal power division are fabricated and characterized. They exhibit common-mode rejection levels higher than 15 dB from 0 to 1.8 GHz (i.e., 0-2 f0) and differential-to-common-mode-conversion suppression levels above 15 dB within the spectral range 0-1.49 GHz (i.e., 0-1.65 f0).