An improved mathematical model enables the somewhat more accurate prediction of the spectral response of a mixer circuit (see figure) that comprises a twin-slot antenna coupled via coplanar waveguides to a hot-electron bolometer (HEB). The development of the improved model is part of a continuing effort to understand and overcome the limitations of circuit models in order to enhance capabilities for designing and analyzing heterodyne mixers to operate at frequencies in the terahertz range.

This HEB Mixer was designed to operate at a frequency of 2 THz. The HEB is a submicron-size device embodied in the microbridge indicated in the middle.
The improved model was developed in conjunction with, and tested in, experiments that involved measurements of the direct-detection and impedance spectra of HEB mixers that had been designed and predicted, by use of the prior model, for resonance at various nominal frequencies from 0.6 to 2.5 THz. The point of departure for the development of the improved model was a prior model implemented in a method-of-moments computer code. The prior model is a simplified one in that it accounts for the slot antennas and the coplanar-waveguide embedding circuit, but not for the parasitic effects associated with the couplings of these circuit elements and with the geometry of the HEB. In both a previous experimental study and in the experiments reported here, the measured frequencies of peak direct detection response were found to be of the order of 20 percent lower than the resonance frequencies predicted by use of the prior model.

The improved model is an extension of the prior model, incorporating two major additions: The first addition is that of a submodel of the junctions between the coplanar waveguides and the slot antennas. The fringing fields at these junctions add parasitic reactance to the circuit. The second addition is that of a submodel of the reactance (predominantly inductive) of the very narrow HEB microbridge and of the tapered transition pieces, if any, with which it is connected to the center conductors of the coplanar waveguides. These additions exert a small effect on the real part of the embedding impedance, but a large effect on the imaginary part. The predictions obtained by use of the improved model show that in a typical case, the inductance of the narrow HEB microbridge dominates the estimated shift in the resonance frequency below that of the prior model. However, because the results of the experiments showed that the improved model does not account for all of the observed frequency shift, it is apparent that further refinements of the model are still necessary.

This work was done by Andrea Neto and Rolf Wyss of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line  under the Electronics & Computers category.


This Brief includes a Technical Support Package (TSP).
Improved Model of Spectral Respomse of an HEB Mixer

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This article first appeared in the November, 2001 issue of NASA Tech Briefs Magazine.

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