S.A. Reible

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.

Superconductive delay-line technology and applications

Microwave analog signal-processing filters have been realized in the form of coupled niobium striplines on silicon dielectric substrates. Device responses with ± 2-dB amplitude accuracy and 9°-rms phase error have been achieved in amplitude-weighted filters with 37.5 ns of dispersion and 2.3-GHz bandwidths. Relative side-lobe levels of -26 dB and less are currently obtained. The achievable dispersion for stripline circuits on a single pair of 5-cm-diameter, 125-μm-thick wafers is limited to about 40 ns by the electro-magnetic coupling between neighboring lines. To achieve greater dispersion two approaches are under development: (1) Stripline circuits are being fabricated on multiple wafer pairs which are physically stacked and electrically concatenated to produce dispersive delay lines with 4-GHz bandwidth and 75-ns dispersion time. Phenolic resin is used as an adhesive to ensure the mechanical integrity of the stacked structure. (2) A technique to fabricate dense stripline circuits on very thin (15-μm) single-crystal silicon superstrates supported by thicker substrates has been demonstrated and preliminary results will be described. A chirp-transform system capable of real-time spectral analysis has been constructed using a pair of the superconductive delay-line filters. A resolution of 43 MHz over an unprecedented 2400-MHz bandwidth with amplitude uniformity of ±1 dB and side-lobe levels of -18 dB was demonstrated.

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.

Wide Bandwidth Acoustoelectric Convolvers

The use of convolvers as programmable matched filters for spread-spectrum comnunication provides the ability to change the coding waveform from bit-to-bit, thus offering improved security and protection against r epeat jamming. In many applications, maximum signal bandwidth is desired because increased bandwidth provides for improved tolerance to effects of multipath and fading, increased c overtness, and larger p otential processing gains when maximum bit lengths a re limited, e.g. by Doppler shifts. In order to better meet these needs, the bandwidth of acoustoelectric convolvers has been extended by a factor of two over previously demonstrated convolver bandwidths. A gap-coupled convolver for decoding differential phase-shi ft-keyed (DPSK) signals having a 200-MHz bandwidth and a time-bandwidth product of 2400 has been developed and evaluated. range of 46 dB and a conversion efficiency (F-factor) of -77 dBm have been obtained. formance details are presented and compared with theoretical Predictions. A device dynamic Design and per

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.

Gap‐coupled InSb/LiNbO3 acoustoelectric convolver operating at 77 K

A gap‐coupled InSb/LiNbO3 acoustoelectric convolver has been fabricated and tested at 77 K. The results suggest the possibility of using a similar structure with a high‐density InSb diode array as an acoustically scanned infrared imaging device. Measurements indicate that near optimum bias, the convolution efficiency was −63 dBm, the convolution output was uniform along the device length to within 1.5 dB, the insertion loss was 26 dB, the 1‐dB compression point occurred at 18 dBm power input, and the efficiency was constant to ⩽ 3 dB over the range 66–70 MHz.

Transverse modes in gap‐coupled surface‐wave devices

The support structure used to maintain a uniform air gap in separated‐media acoustoelectric devices can cause considerable distortion of the acoustic‐beam profiles. These distortions, which include the decomposition of the incident acoustic beam into multiple waveguide modes, have a detrimental effect on the efficiency and frequency response of the devices. Experimental observations have led to a simple rail array which minimizes the undesirable effects of the support structure.