An SIS Mixer With 2hf/k DSB Noise Temperature at 163 - 211 GHz Band

We present the design and the performance of a fixed-tuned, all niobium (Nb) Superconductor-Insulator-Superconductor (SIS) mixer covering RF frequencies from 163 to 211 GHz with 4-8 GHz IF bandwidth. The mixer uses two Nb/Al-AlOx/Nb junctions of size 3 μm² each with the R_nA product of 30 Ω·μm ², in a twin junction configuration. An local oscillator (LO) directional coupler made of superconducting lines with slots in the ground plane is integrated on the SIS mixer chip. The isolated port of the LO coupler is terminated using a wide-band resistive load with a sheet resistance of 12 Ω/□. The SIS tuning circuitry is optimized to achieve best power match between the twin junctions and the embedding circuitry. The measurement of the double sideband mixer noise at 171 GHz shows that the receiver noise temperature is approximately 17 K, which is just 2 hf/k. The estimated noise contribution from the RF loss and the IF chain at 176 GHz LO frequency is 8 K and 7.5 K respectively, resulting in a SIS mixer noise of hf/2k . The mixer noise performance across the entire RF band varies between 19 to 25 K. This is to our knowledge the best reported noise temperature at these frequencies till now.