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Abstract:

In this paper, we describe a simple method for widening the instantaneous frequency (IF) bandwidth of a high frequency superconductor-insulator-superconductor (SIS) mixer. This method requires only a standard PCB technology, without incurring any difficulties or complication in altering the tunnel junction fabrication. It involves the use of a simple multi-stage IF transformer to match the dynamic output impedance of the SIS mixer to a 50 Ω load over a wide IF bandwidth. We use an existing 650 GHz SIS mixer chip as an example to illustrate the methodology of the IF transformer design. In this example, we show how we widen the IF bandwidth of the mixer to operate over 4–12 GHz bandwidth using this simple impedance matching method.

Abstract:

We present the design of a broadband balanced superconductor-insulator- superconductor (SIS) mixer, which uses two back-to-back unilateral finline tapers. A key feature of our design is the use of a thin silicon-on-insulator (SOI) substrate, which enables the employment of a finline mixer at high frequencies. We integrated all of the required superconducting RF circuits onto a single chip using planar circuit technology, and retained the simplest fabrication processes without any lumped elements. This approach results in a very simple mixer-block design, which is desirable for constructing medium-format focal-plane imaging arrays. In the paper, we present in detail the electromagnetic modelling of each of the planar components making up the entire balanced mixer. The electromagnetic simulations were carried out using HFSS, and the heterodyne performance of the complete balanced chip was accessed using SuperMix.