R.S. Withers

High-T/sub c/ superconductive microwave filters

Four-pole microstrip bandpass filters have been fabricated using both postannealed and in-situ-grown thin films of Y-Ba-Cu-O. The 4-GHz, 3% bandwidth filters exhibit, at 77 K, a passband insertion loss as low as 0.3 dB, compared with the 2.8-dB loss of similar gold metallic filters at the same temperature. These results demonstrate that complex passive microwave devices can be designed and implemented using high-temperature superconducting (HTS) material. More aggressive filter designs utilizing many more poles will result in much greater performance advantages for an HTS filter technology over a normal metal technology. Ultimately, ultrasharp-skirt filters can be produced, which would allow for more closely spaced communication channels and more efficient use of microwave communication bands than is currently possible with conventional filters.

Superconductive tapped delay lines for microwave analog signal processing

Passive superconducting tapped delay lines have been fabricated for use as matched filters for multigigahertz bandwidth analog signal processing. Specifically, linear frequency-modulated dispersive delay lines, also known as chirp filters, having a bandwidth of 2.6 GHz centered at 4 GHz and a dispersion time of 35 ns have been constructed. The stripline structure consists of a 4000-A-thick patterned niobium film sandwiched between 5-cm-diameter, 125-μm-thick sapphire wafers. Two parallel striplines, each 1.6-m long, are wound in a spiral pattern. The taps are backward-wave couplers formed by bringing the two lines into and out of proximity at specified locations. Pulse expansion and compression have been demonstrated with these devices and are in close agreement with a new theoretical model for this class of signal processors.

High-temperature superconductive passive microwave devices

Two important classes of passive superconductive microwave devices are described. These devices have been demonstrated using thin films of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7-x/. The devices include tapped-delay-line transversal filters with multigigahertz bandwidths and time-bandwidth products as large as 30, and narrowband (less than 3% bandwidth) microstrip filters. These devices illustrate the potential performance advantages superconductive thin films offer to the designer of a high-frequency, wide-bandwidth analog signal processing system.<<ETX>>

High-T/sub c/ superconductive delay line structures, and signal conditioning networks

The design and fabrication of high-T/sub c/ chirp-response tapped delay line filters are discussed. The key components in this development were long delay lines with more than 10 ns of delay, impedance transformers, and backward-wave couplers. All of the typical transmission line geometries were examined, including microstrip, coplanar and stripline. Designs were developed using microwave CAD routines and superconducting niobium delay line prototypes. Tapped delay line chirp filters with up to 12 ns of total delay have been successfully fabricated in YBaCuO.

Substrates for superconductive analog signal processing devices

Superconductive analog devices are being developed which utilize long electromagnetic delay lines, typically 2 to 50 m long on a single planar substrate Criteria for the choice of substrates for these devices are established. Desirable characteristics are low dielectric loss, low dispersion, high dielectric constant, dielectric isotropy and compatibility with fine-line lithography of robust superconductive films such as niobium. A number of different substrates have been examined including crystalline and vitreous quartz, sapphire of various orientations, alumina, calcium fluoride, and silicon. Silicon is shown to be an excellent substrate for most envisioned devices. Experimental results indicate that a 100-m long line with a total loss of 8 dB at 5 GHz is feasible. A technique to allow the use of a 25-μm-thick, 5-cm-diameter silicon substrate to realize an 18-m-long niobibm line is discussed.

Nonvolatile analog memory in MNOS capacitors

Long-term storage of analog signals of wide dynamic range has been successfully demonstrated for the first time in single MNOS capacitors. After a reset state is established by majority carrier tunneling, measured pulses of light are used to generate minority carriers which tunnel to nitride traps and in turn induce shifts in the flat-band voltage proportional to the minority carrier charge. Linear voltage windows of 12 volts are observed, and logarithmic decay rates are as low as 30 mV per decade of storage time per volt of initial flat-band shift. Analog signals can be stored linearly over a dynamic range of 40 dB for 30 hours.

Superconductive delay line with integral MOSFET taps

A superconducting tapped delay line with programmable MOS transistors for tap weights has been designed, fabricated, and tested. The device operates between 2 and 5 GHz at 4.2 K. The silicon substrate of the integrated semiconductor-superconductor device is used both as the semiconductor material for MOS processing and as the dielectric for the microstrip delay line. The superconducting material is niobium, which is processed after the semiconductor fabrication because the superconducting properties of niobium can degrade if exposed to high temperatures. Both aluminum and niobium are used for transistor gates and interconnects. The novel niobium-gate transistors worked as well as the aluminum-gate transistors at 4.2 K with channel mobilities 3 to 5 times higher than at room temperature. Test results on the weighted tapped delay line show that the amplitude of the tapped outputs may be varied linearly over an 18-dB range between 2 and 2.4 GHz by gate-voltage modulation of the MOSFET channel conductance.

High‐temperature superconductive delay lines and filters

Passive microwave devices are expected to be among the first devices made from high‐temperature superconductors to be inserted into system applications outside of a laboratory environment. The deposition and patterning of YBa2Cu3O7−x (YBCO) thin films has progressed sufficiently that a variety of high‐quality passive microwave devices can now be fabricated on LaAlO3 substrates. Demonstrated microwave devices include tapped‐delay‐line transversal filters with multigigahertz bandwidths, untapped delay lines, and narrowband (2% or less) microstrip filters. Potential system applications at operating temperatures of 77 K and below are discussed. The YBCO microwave devices described here demonstrate the potential performance advantages that superconducting thin‐film devices offer for high‐frequency, wide‐bandwidth, high‐performance analog signal processing systems.

Analog signal correlator using superconductive integrated components

Analog superconductive components have been integrated to form a device capable of cross-correlation between wideband analog input signals. The device contains a tapped niobium delay line, tunnel-junction mixers, a lumped-element L-C resonator, and a tunnel-junction comparator. The tapped delay line is realized by a niobium stripline folded in a meander pattern on a rectangular silicon substrate. An array of Nb/Nb 2 O 5 /Pb tunnel junctions acts as a mixer to form the product of delayed samples of two carrier-offset analog signals counterpropagating along the transmission line. The resultant mixer products from the junction arrays are integrated and stored in a high-Q (≈ 600) resonator consisting of a lumped-element L-C network, tuned to the offset frequency. The low-leakage capacitor dielectric is formed from electrolytically anodized niobium. A superconductive tunnel junction imbedded in the resonator circuit is operated as a variable-threshold comparator to detect the time-integrated current stored in the resonator. Performance results from such a time-integrating correlator are presented, along with a discussion of the important design issues as they relate to analog signal processing.

Superconductive chirp-transform spectrum analyzer

Spectral analysis over an instantaneous bandwidth of 2.4 GHz has been demonstrated utilizing superconductive dispersive delay lines in a chirp-transform configuration. Two-tone resolution of 43 MHz and ±1.2-dB amplitude uniformity were achieved.