Novel S-Band direct phase modulator using hybrid coupler

This paper describes the design, simulation and practical results of an S-Band direct phase modulator. A modulation index of 1.1 radian with a very small phase distortion was obtained at 3.785 GHz. The modulator consists of 3 cascaded stages of Branch line coupler, each stage employing pair of transistors in place of varactor diodes. AM/PM ratio was minimized in the design. Simulation is performed using ADS 2005A. The third order intermodulation products are less than -45 dBc in the modulator. Direct modulator uses minimum number of IC´s with zero dc power consumption and modulates frequency from 16 KHz to 3.785 GHz. In the simulation, excellent isolation is achieved for the 1^st and 4^th port of Branch line coupler.