Design and analysis of low-loss linear analog phase modulator for deep space spacecraft X-band transponder (DST) application

The authors summarize the design concepts, analyses, and development of an X-band transponder low-loss linear phase modulator for deep space spacecraft applications. A single-section breadboard circulator-coupled reflection phase modulator has been analyzed, fabricated, and evaluated. A linear phase deviation of 92 degrees with a linearity tolerance of +or-8% was measured for this modulator from 8257 MHz to 8634 MHz over the temperature range -20 degrees C to +75 degrees C. The measured insertion loss and the static delay variation with temperature were 2+or-0.3 dB and 0.16 ps/ degrees C, respectively. Based on this design, two and three cascaded sections have been modeled and simulations performed to provide an X-band DST phase modulator with +or-2.5 rad (+or-143 degrees ) of peak phase deviation to accommodate downlink signal modulation with composite telemetry data and ranging with a deviation linearity tolerance of +or-8% and insertion loss of less than 10+or-0.5 dB. A two-section phase modulator using constant gamma hyperabrupt varactors and an efficient modulator driver circuit was breadboarded. The measured results satisfy the DST phase modulator requirements, and show excellent agreement with the predicted results.<>