This 345 GHz sideband-separating (2SB) prototype receiver achieves instantaneous ultra-wide bandwidth by using superconductor-insulator-superconductor (SIS) mixer designs and a multioctave IF hybrid outside the cryostat. A newly designed one-junction SIS was fabricated, and performance was compared to a previously proven three-junction SIS in the receiver. Measurements included a usable IF range or 4-13 GHz and 4-17 GHz, 96 ± 8 K from 312 to 363 GHz and 109 ± 14 K from 314 to 367 GHz single sideband (SSB) noise temperature and 12.3 and 11.0 dB image rejection for 90% of the data points for the one-junction and three-junction SIS designs respectively.