Performance results of a 300 degrees linear phase modulator for spaceborne communications applications

A phase modulator capable of large linear phase deviation, low loss, and wideband operation with good thermal stability is described. The phase modulator was developed for deep space spacecraft transponder (DST) applications at X-band (8.415 GHz) and Ka-band (32 GHz) downlinks. The design uses a two-stage circulator-coupled reflection phase shifter with constant gamma hyperabrupt varactors and an efficient modulator driver circuit to obtain a phase deviation of +or-2.5 rad with better than 8% linearity. The measured insertion loss is 6.6 dB+or-0.35 dB at 8415 MHz. It is shown that measured carrier and relative sideband amplitudes resulting from phase modulation by sinewave and square modulating functions agree well with the predicted results.<
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