A 385-500GHz Balanced Mixer with a Waveguide Quadrature Hybrid Coupler

We developed a 385-500GHz balanced mixer with a waveguide quadrature hybrid coupler. The balanced mixer consists of an RF quadrature hybrid coupler, two double sideband (DSB) SIS mixers with noise temperature of — 60K, and an IF 180 degree hybrid coupler covering 4 8 GHz IF band. An RF quadrature hybrid coupler was designed and fabricated whose fabrication error was within 5gm. The noise temperatures of the balanced mixer was similar to those of two DSB mixers in spite of adding an RF quadrature hybrid and an IF coupler. The required LO power for pumping the balanced mixer was reduced by 42dB on average compared with those for the DSB mixers and -15dB coupler. The sideband noise of the local oscillator (a quintupler + a Gunn oscillator) was measured to be 20K at offset frequency of 4 — 8 GHz, which corresponds to 70K4OV. To authors' knowledge, this is the first direct measurement of LO sideband noise at submillimeter range. If a varistor quintupler degrades the signal-to-noise by 10dB (K. Saini 2003 [11), the sideband noise of a Gunn oscillator is 7K4tW at the offset frequency of 0.8-1.6 GHz.