Electrically tunable microstrip two-pole band-bass filters for a center frequency near 19 GHz and a 4-percent bandwidth have been designed, fabricated, and demonstrated to be functional. These filters are suitable for use in the front ends of K-band communication receivers that operate at low temperatures.

Figure 1. YBa2Cu3O7– Microstrip Lines are shaped and dimensioned, in conjunction with the underlying layers, to obtain the desired two-pole pass-band frequency response. The pass band is adjusted by varying the dc bias and thus the electric field in the SrTiO3 layer.
Figure 1 depicts the multilayer configuration and microstrip layout of one of these filters. The top layer is a thin film of the high-temperature superconductor (HTS) YBa2Cu3O7δ patterned into microstrip lines. The top layer rests on a layer of the ferroelectric material SrTiO3, which rests on a dielectric layer of LaAlO3. The bottom of the LaAlO3 is coated with a thin film of a normal conductor (Au), which serves as a ground plane.

Tunability is achieved through the nonlinearity (specifically, the variation of permittivity with electric field) of the SrTiO3 layer. Using a commercial electromagnetic-analysis computer-aided-engineering software package, the design of the filter was optimized so that normal operation at the center frequency would occur when the relative permittivity εr of the SrTiO3 was 1,650. This resulted in a requirement to maintain a suitable bias in order to maintains ε at 1,650.

Prototypes of these filters were packaged for swept-frequency measurements of their scattering parameters (Sij) in a helium-gas closed-cycle cryogenic system. In experiments on tunability, a dual-polarity biasing technique was used: Referring again to Figure 1, nodes A and C were biased with a positive voltage, while nodes B and D were biased with a negative voltage of equal magnitude. The dc bias connections at A and D were made via input and output bias tees; the bias connections at B and C were achieved via gold-wire bonds on radial biasing stubs. The dc bias was increased from 0 to ±500 V in steps of ±50 V.

Figure 2. S11 and S12 of a Filter were measured at 77 K. These plots show the desired voltage dependence of the pass band, plus a desirable increase in S12 and a desirable decrease in S11 with bias voltage.
Figure 2 depicts results of some of the measurements; namely, the frequency and voltage dependence of S11 and S12 of one of the filters at a temperature of 77 K and an input power of 10 dBm. With increasing bias voltage, the center frequency of the filter shifted from 17.4 GHz at no bias to 19.1 GHz at a bias of 500 V, giving a tunability factor of 9 percent. The lowest measured pass-band insertion loss of this filter was 1.5 dB at 24 K. Another filter exhibited a tunability factor of 12 percent at a temperature of 30 K. In general, the return losses S11 and S22 of the filters were near or greater than 10 dB in the pass band. The resonance quality factor (Q) of the filters in the absence of loading was estimated to be ≈ 200. Efforts to optimize the HTS and ferroelectric films to obtain lower insertion losses and better tunability near 77 K and at lower bias voltages were underway at the time of reporting the information for this article.

This work was done by F. A. Miranda of Glenn Research Center, F. Van Keuls of the National Research Council and G. Subramanyam of the University of Northern Iowa. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 —8
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Refer to LEW-16751.