DESIGN OF PLANAR IMAGE SEPARATING AND BALANCED SIS MIXERS

With noise temperatures of SIS receivers now in the range 2-4 times the photon temperature (hf/k), the overall sensitivity of radio astronomy measurements can be seriously degraded by atmospheric noise, and sometimes by noise from the local oscillator source. In spectral line measurements, atmospheric noise in the unwanted (image) sideband can be eliminated by using an image separating scheme. To reduce local oscillator noise, balanced mixers can be used. It is possible to realize image separating and balanced mixers using quasioptical or waveguide RF circuits, but they are difficult to fabricate and bulky. We believe it is now practical to include the necessary signal and LO power dividers, couplers, and cold loads with the SIS mixer on the same quartz substrate. The complete image separating or balanced mixer can be fabricated using a standard niobium SIS mixer fabrication process with one or two additional layers.