MMA Memo #151

Design of Planar Image Separating and Balanced SIS Mixers

A.R. Kerr, S.-K. Pan

March 1, 1996

Keywords:

With noise temperatures of SIS receivers now in the range of 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 separation 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 wuth 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.

We describe the design of single-chip balanced and image separating mixers for 200-300 GHz. The circuits are designed using a modified form of coplanar transmission line which has a convenient range of characteristic impedences while minimizing coupling to adjacent circuit elements. It is hoped ultimately to combine the image separating and balanced designs to make a balanced image separating mixer on a single chip.

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Last modified: 09 December, 1999