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 3.785GHz. In the simulation, excellent isolation is achieved for the 1(st) and 4(th) port of Branch line coupler.