Varactor S-Band Direct Phase Modulator

This paper describes the design technique and the test results for an S-band direct phase modulator with extremely linear phase characteristics. A modulation index of 2.4 radians (300/spl deg/ phase variation) with a phase distortion of less than 0.5 percent was obtained at 2.2 GHz. The modulator consisted of two cascaded stages, each stage employing a pair of low-loss varactor diodes. Amplitude modulation was minimized by operating the diodes in a reflection mode. The phase characteristic of an ideal reflection type modulator is a nonlinear function of a signal voltage (2 tan/sup -1//spl omega/C(v)/Y/sub0c/, where C(v) is the diode capacitance andY/sub 0c/ is the circulator characteristic admittance). A new technique was developed to linearize the phase characteristics. Consideration was also given to improvement of the S-band frequency response. The effect of the series resistance of the diode on the amplitude modulation is discussed. The modulator will handle over 20 mW S-band power with no appreciable change in its operational characteristics. The modulation bandwidth is from dc to 20 MHz.