Figure 4: New, fixed tuned 2 mm mixer in a cooled optics laboratory receiver gives a DSB receiver noise about 20 K.
As known, the principal frequency limit to SIS mixer low noise performance is given by the gap frequency of the superconductor. In theory SIS junctions may be used up to twice the gap frequency. In practice, mixer operation degrades just at the gap frequency due to the increase of the loss in the superconductor. With the introduction of NbN technology at IRAM, the frequency limit of the devices may reach about 1300 GHz, the gap frequency of NbN, instead of the 700 GHz of Nb. Another reason for introducing the new material is the possibility of simplifying the receiver cryogenics, using the NbN SIS mixer circuit at a temperature higher that 4.2 K.
We demonstrate for the first time low noise receiver operation with a full NbN integrated circuit with SIS junctions in the mixer. Using the NbN-MgO-NbN junction technology developed in our institute, a full NbN circuit was developed and tested at 150 GHz. Special attention was paid to achieve high tolerance to the junction parameters. Even with a junction with of about 50, we arrive at 65 K DSB receiver noise temperature. The mixer with the NbN circuit was at 5.6 K. With improvement of NbN junction technology, we may expect a further reduction of the NbN mixers noise, down to the noise of Nb devices.