Design & Analysis of an X-Band QPSK Modulator using Direct Carrier Modulation Technique

This paper presents the design and simulation of a QPSK modulator (quadrature phase shift keying) based on direct carrier modulation technique with a very simple circuitry. In this design, two double balanced mixers; branch-line coupler and Wilkinson power combiner are used. The selection of a double balanced mixer, being the main component of the design, is a critical issue. Specifications of mixer such as conversion loss, interport isolation etc are discussed. Branch-line coupler is used to produce in phase and quadrature signals and Wilkinson power combiner is used to combine the two modulated orthogonal signals at the output. These passive components are easy to fabricate, occupy reasonable area and provide satisfactory performance as compared to their counterparts. The simulated spectrum and static constellation diagrams of the modulator are presented. The circuit has been designed at 8.3 GHz X Band for 150Mbps bit rate.