| A low cost, broadband QPSK modulator by CPW (Coplanar Wave Guide) structure is introduced in this thesis.; This QPSK modulator consists of a quadrature MB coupler that creates two orthogonal channels (I and Q, which are individually BPSK modulated and then power-combined by an in-phase 3dB power combiner. The technique for realizing the bi-phase modulators is to employ reflection topology using a quadrature coupler with HBTs acting as switches on the direct and coupled ports. A balancing topology for the BPSK modulator is employed to cancel the phase and amplitude errors caused by the parasitics of HBTs.; On thin GaAs substrates used for MMIC's, chip area is the main concern based on economic fabrication, especially in this QPSK modulator in which nine couplers are included.; A two-stage power combiner was realized by CPW and microstrip on two conductor layers respectively and the microstrip sits on top of CPW ground so that the entire chip size is same as one-stage case.; The design of an overlay structure coupler and a two-stage power combiner was successfully fabricated and the EM simulation predicted their performance well. (Abstract shortened by UMI.)... |
