Design of distributed elements in ku-band in coplanar-waveguide based MCM-D

Broadband coplanar waveguide quadrature couplers are an ideal component for use in balanced amplifiers and mixers and in the design of QPSK-modulators. In this paper, we report on the feasibilty of integrating 6-finger CPW Lange couplers and couplers using re-entrant sections in a coplanar waveguide-based MCM-D technology. For the 6-finger Lange coupler, we have obtained in the 9.5 GHz - 17 GHz band, an amplitude balance below 0.5 dB, a return loss and isolation better than 16 dB and a phase balance of (90 +/- 2) deg. For the coupler using re-entrant sections, we have obtained in the 6.9 GHz 18.8 GHz band, an amplitude balance lower than 1.1 dB, a return loss and isolation better than 21 dB and a phase-balance of (92.7 +/- 4) deg. The measurement and the design-method are discussed and a trade-off between the two methods is given. The influence of BCB-planarization on the performance of the circuits is discussed. Very low characteristic impedance lines can be easily integrated in multi-layer coplanar waveguide based MCM-D. This is done by putting the strip and ground on different metal-layers.