Microwave Transmission Technique for Accurate Impedance Characterization of Superconductive Bolometric Mixers

The impedance of the superconducting hot-electron bolometer mixer was measured in the range 0.2-4 GHz. A special technique relying on determination of the Sz, transmission parameter of the device was used. Many advantages of the technique (wide frequency range, in situ calibration, low test power, accuracy) are demonstrated. The estimate of the mixer bandwidth from the impedance data using the mixer theory is in good agreement with the results of the direct bandwidth measurements. Superconductive hot-electron bolometric (HEB) mixers are rapidly becoming a very attractive alternative sensor for low-noise heterodyne spectroscopy applications at terahertz frequencies. It has been dernonstratedlthat for this type of mixer a noise temperature below 1000 K is possible up to 2.5 THz . The mixer sensitivity is not expected to degrade through the entire far-infrared spectral range. One can expect that HEB mixers will be used in practical heterodyne receivers very soon. One of the important issues for designers of bolornetric mixers is a careful evaluation of the mixer bandwidth. This parameter is usually set by the intrinsic relaxation time which depends on the material and size of the device. There is an extensive literature on the bandwidth measurements in two major types of HEB mixers (diffusion-cooled and phonon-cooled) 2’3’4+5 and, generally, bandwidths between 1 GHz and 6 GHz has been demonstrated. Lack of widely tunable local oscillators (LO) at terahertz frequencies often requires large mixer bandwidths; up to 10 GHz or 20 GHz when fixed-tuned lasers are used as the LO source. Thus, further improvement of the HEB bandwidth may be necessary. In this paper we describe an accurate microwave impedance measurement technique which provides a good bandwidth estimate in an HEB device, thus eliminating the need for tunable submiilitneter radiation sources which are expensive ancVor not always available. These impedance measurements can be performed using a conventional microwave network analyzer. A special calibration routine for S ~, measurements allows the device impedance to be de-imbedded from the surrounding circuit (and its parasitic) without using any calibration standards. The results presented show the implementation of this technique for a Nb diffusion-cooled mixer in a frequency range up to 4 GHz. The data obtained with the novel technique are in good agreement with the direct measurements of the bandwidth using two tunable 600 GHz sources. During past decade there has been detailed studies of the mixing mechanism in hotelectron bolometers made from superconductive f~l;ls. The theoretical basis describing the mixer operation is well established at present ‘ . According to the theory, the mixer bandwidth is given by