Low-noise submillimeter receivers using single-diode harmonic mixers

Submillimeter Schottky diode mixers operating on the second LO harmonic have been built, achieving noise temperatures only 1.3-1.5 times higher than those of fundamental mixers at the same frequency. These mixers use only a single diode junction for ease of fabrication, and allow construction of a complete receiver of greatly reduced complexity relative to those using fundamental mixers. Several such mixers have been built for frequencies from 490 to 670 GHz. These mixers are pumped by frequency multipliers driven by InP Gunn oscillators, with a net LO input which is typically 2 mW. The best results are receiver noise temperatures of 4600 K SSB at 550 GHz and 5100 K at 665 GHz. The theory of mixer noise and conversion loss agrees fairly well with the experimental results, and this same theory predicts that a third-harmonic mixer near 1 THz would also achieve excellent performance. >