LAUSR

In this paper, a novel broadband lumped-element quadrature hybrid MMIC for interferometric correlation radiometer is proposed. First, the cause of the bandwidth limitation in the lumped-element branch-line coupler is analyzed, which is the two identical parallel branches have the same frequency response leading to reinforced resonance and narrow band. In order to reduce such unfavorable superposition effect, parallel branches with opposite frequency responses are used, which exhibits the characteristic similar to rat-race coupler. To realize a quadrature hybrid, a phase-shifting network is cascaded to the rat-race coupler, and the bandwidth is further extended by compensating the passband phase trend and optimizing the inter-stage matching of the two parts. Finally, to facilitate the application, the quadrature hybrid is combined with a variable gain amplifier to implement an integrated MMIC based upon 0.15 $\mu {\mathrm{ m}}$ GaAs pHEMT process, and the size of the hybrid part is $1.2\times 1.4$ mm². Within the frequency range of 3.8~8.5 GHz (75% fractional bandwidth), the amplitude and phase imbalance are less than 0.5 dB and 2.6°. All reflections and isolations are better than 16.4 dB. The measured results verify the design theory and illustrate that the lumped-element quadrature hybrid is suitable for compact broadband circuit applications.