A Planar Balanced Crossover

The mixed-mode S-parameters of a generic four-port balanced crossover have been analyzed. According to the S-parameters, a four-port balanced crossover has been presented for the first time. The crossover consists of two microstrip ring-shaped circuits connected by four transmission-line sections and exhibits bisymmetry. Employing even-odd-mode analysis, design equations have been proposed and theoretically verified. For a special case where quarter-wavelength transmission-line sections are utilized, all of the impedances of transmission-line sections in the reduced circuits could be arbitrarily selected. Bandwidth analysis has been conducted to provide a guideline of determining these impedances. For verification, a 2.45-GHz balanced crossover which occupies an area of 0.95x0.95.2 has been implemented and measured. The measured differential-mode return loss, isolation, and insertion loss at 2.45 GHz are 25.0, 20.8, and 0.86 dB, respectively. The measured bandwidths in the differential mode are 22.0%, 4.1%, and 5.6% for the 17-dB return loss, 20-dB isolation, and 1-dB insertion loss, respectively. Cross-mode S-parameters are less than -31 dB at 2.45 GHz with -20-dB bandwidths of 13%. Bandwidths of 20-dB common-mode rejection are no less than 45.0%. The measured data agree with the full-wave simulated results and validate the proposed design.