This paper presents the design of a 3.8 ~ 8.0 GHz wide-band quadrature coupler on a multi-layer package substrate. The asymmetric coupled-line 3-dB quadrature coupler has been designed on a four-layer microwave substrate, with a 10-mil thick top layer of Roger's RO4350B substrate press-joined to a 20-mil thick bottom layer of RO4350B, through 4-mil thick bond-ply material RO4450B. In the proposed design, the second and third metal layers are used as coupling layers, while the fourth (bottom) layer provides four signal pads and one large ground pad for connection with the test circuit. The mutual coupling is achieved through the overlay of coupled lines. Four VIA holes are used for signal transition from coupling layers to the bottom-layer pads. The SMD package quadrature coupler provides the ease of integration with other microwave circuits. The quadrature coupler chip size is 4.0 mm x 8.0 mm x 0.9 mm. The measurement results show a close resemblance to the EM-simulation results. The measured results depict reasonably flat 3-dB coupling and quadrature phase difference. The amplitude imbalance remains within 1.0 dB, while the phase imbalance always remains much less than 3.0 degrees. The return loss and isolation are better than 13 dB, throughout the whole frequency band. The proposed design is quick and simple. The manufacturing process is also cost-effective. To the best of the author's knowledge, these measured performance parameters in 71% fractional bandwidth associated with the compact size of the self-packaged device are better than those of the earlier published 4-layer design schemes of wideband quadrature couplers.
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