Miniaturized low cost 30 GHz monolithic balanced BPSK and vector modulators: Part II

Part one of this two-article series analyzed single-stage and balanced topo logies in detail. It was illustrated that a common technique for realizing direct biphase-shift keying (BPSK) microwave modulators is to employ reflec tion-type topology with cold pseudomorphic high electron mobility transisto rs (pHEMT) acting as variable resistance elements connected to the coupled and direct pores of a Lange coupler. However, at higher frequencies the par asitics of the cold pHEMTs start to dominate, resulting in large amplitude and phase error characteristics. It was determined that these parasitics ca n be removed by employing the balanced topology which provides near-perfect amplitude and phase balance even at mm-wave frequencies. However, it was a lso noted that the balanced topology would, in theory, require twice the ch ip space of the single-stage topology. A novel microstrip coupler has been proposed that can be employed in the modulator designs to reduce the size o f the MMIC circuits. This second article examines the physical realization of the compact microstrip coupler and its measured performance. The design and performance of the MMIC balanced biphase amplitude and vector modulator chat employ the designed miniaturized coupler then are presented.