A BROADBAND QUADRATURE HYBRID USING IMPROVED WIDEBAND SCHIFFMAN PHASE SHIFTER

In this paper, a broadband quadrature hybrid is presented. The hybrid comprises a Wilkinson divider, followed by an improved Schifiman phase shifter. An improved wideband Schifiman phase shifter on a single layer printed circuit board is accompanied by a lumped capacitor between two coupled lines. Lumped capacitor is in parallel with the odd-mode capacitance of the two coupled lines, hence, the odd-mode capacitance is increased and consequently the odd-mode impedance is decreased. Therefore, by this method we can control the ratio of the even mode impedance to the odd mode impedance which is critical in Schifiman phase shifter design. Compared with the cascading Microstrip multisection coupled lines, our proposed single layer phase shifter is smaller in size. Also, the proposed phase shifter has the greater bandwidth compared to the patterned ground plane Schifiman phase shifter and its realization is simpler because of its single layer structure. As an example, a Schifiman phase shifter at frequency f0 = 650MHz is designed and simulated. With the proposed phase shifter a 90 - hybrid is designed, simulated, fabricated and measured. Exprimental results show that the amplitude and phase imbalance between two paths at worth case are within 0.75dB and 5 - , respectively, over the frequency band from 362MHz to 891MHz, or around 84% bandwidth. The measured return loss is better than ¡10.8dB over the operating frequency band.