Abstract:
A superconducting transversal filter circuit for processing signals in the 2-20 GHz range consisting of a miniature transmission line of niobium or similar material, a series of taps for coupling the input and output, and cryogenic refrigerator.

Description:
TECHNICAL FIELD 
     This invention relates to signal processing devices and, in particular, to transversal filter circuits operating in the 2 to 20 gigahertz range. 
     BACKGROUND OF THE INVENTION 
     The government has rights in this invention pursuant to Air Force and Army Contract No. AF19628-80-C-0002. 
     Attention is directed to an article by the inventors and a colleague entitled &#34;Passive Superconducting Microwave Circuits for 2-20 GHz Bandwidth Analog Signal Processing&#34; Proc. of the IEEE Int&#39;l. Microwave Symposium (June 15, 1982), hereby incorporated by reference. 
     The use of signals in 2-20 GHz range has become increasingly important in communications and radar. Consequently, construction of signal processing devices such as matched filters and linear phase filters, in particular, has become a goal for researchers. 
     At lower bandwidths various signal processing devices have been constructed that yield high time-bandwidth (TW) products. For example, analog discrete-time devices with bandwidths up to 20 MHz have been made with charge-coupled devices (CCDs); analog continuous-time devices with bandwidths up to 1000 MHz (1 GHz) have been made using surface-acoustic-wave (SAW) devices; and recent research effort has explored acoustooptic (A/O) devices and magnetostatic wave (MSW) devices, both with bandwidths of about 1 GHz. The propagating wave velocities in these devices are substantially below the speed of light; thus one can achieve large interaction times in relatively compact forms. However, a host of physical limitations such as propagation loss, dispersion, and transducer inefficiency prevents the practical utilization of these techniques at bandwidths above 2 GHz. 
     Elctromagnetic delay lines offer bandwidths of tens of gigahertz, well beyond those realizable with acoustic delay lines or sampled data structures such as CCDs. However, the high electromagnetic velocity requires the use of long lines to achieve significant delay. For example, a 100 ns device would require about 30 meters of free space delay or about 10 meters if the medium had a dielectric constant of 10. This length of coaxial cable or waveguide would be physically cumbersome. Such a delay also could be achieved with a copper microstrip delay line on low-loss 0.4-mm thick alumina substrate and would require an area of about 500 cm 2 . However, for 5-GHz bandwidth operation centered at 10 GHz, it would have a loss of about 40 dB at room temperature. Thicker substrates would give lower losses but would require larger area for a given delay. Because of this trade-off of large area or high loss, conventional electromagnetic delay technology has been unsuitable for microwave signal processing to date. 
     Nonetheless, there exists a need for signal processing devices in the 2-20 GHz range and electromagnetic delay lines constructed using microfabrication processing techniques would be particularly valuable as components in large scale integrated circuits. Specifically there exists a need for matched filters, in particular, &#34;upchirp&#34; and &#34;downchirp&#34; filters, and linear phase filters. 
     SUMMARY OF THE INVENTION 
     We have discovered that processing devices for signals in the 2-20 GHz range can be constructed using the principles of electromagnetic delay lines and microfabriciation techniques and can be effective as signal processors by operating the devices at low temperatures in a superconducting mode. Extremely long lines can be formed into a small package without prohibitive insertion losses by using materials such as niobium at 4.2° K. Such conductors can be used to fashion transversal filter structures of high signal processing capacity. The transversal filters consist of transmission lines and taps. The presently preferred tapping method employs an array of backward-wave couplers, each of which couples energy propagating in the forward direction on the input line to a backward-propagating wave on the output line. 
     We have demonstrated that by using a rugged refractory superconductor such as niobium, which at 4.2° K. and 3 GHz has a surface resistivity of about 0.01% that of copper at room temperature, one can make very narrow microstrip or stripline microwave transmission lines and hence pack extraordinarily long delay on easy-to-handle 5-cm-diameter substrates. We have made lines which are 2.5 meters long, and lines which are 100 meters long appear feasible. Because stripline has insignificant dispersion and the conductor and dielectric loss are approximately independent of bandwidth, for fixed time-bandwidth product, the major bandwidth constraint is imposed by the coaxial cable and coax-to-stripline transitions at the input and output of the device. 
     Recent advances in refrigeration apparatus make our microstrip devices commercially viable. Low cost, efficient refrigerators operating at 4.2° K. and requiring only a few kilowatts of power can be employed as the means for cooling the transmission lines of our devices. 
     Our invention will next be described in connection with certain preferred embodiments; however, it should be clear that those skilled in the art can make various modifications and changes to our invention without departing from the spirit or the scope of the claims. For example, our invention may be used to process frequency-modulated signals, phase-modulated signals or amplitude-modulated signals or a combination of these types of modulation. Our devices may be used as matched filters or vice versa as the generator of a particular wave form. While linear upchirp and downchirp filters are described in detail, non-linear chirp filters and circuits designed for unique wave forms are also contemplated. 
     Moreover, while niobium has been chosen as a preferred material for the transmission line, other superconducting materials such as lead, niobium alloys (such as niobium-tin) and the vanadium alloys (such as vandium-silicon) as well as other superconductors may also be used. The line widths for the transmission lines may vary from 50 to 5 microns and could be made even thinner with ongoing advances in the field of microfabrication. The length-to-width aspect ratio may vary from 10 5  to 10 7  in typical devices and may be even greater if desired for a particular application. 
     Additionally, our transmission lines may take various geometric shapes. In addition to the double spiral shown in our preferred embodiment, quadruple spirals may also be employed. Single spirals with one terminal at the center could also be used for particular applications. For other applications, a meandering line might be preferred. While we have described a device in which separate input and output channels are connected by backward coupling proximity taps, another embodiment could employ a single line with backward coupling achieved by expansion of the line width at predetermined locations to achieve the same energy tapping function. Additionally, various other substrates may be used besides sapphire, for example, silicon and quartz; and less powerful refrigerators (for example, refrigerators operating at about 10° K.) could be employed with other superconducting materials and substrates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a schematic diagram of a double-spiral embodiment of our invention. 
     FIG. 1b is a schematic diagram of the tap spacing of our invention. 
     FIG. 2a is a photograph of a upchirp filter built according to our invention. 
     FIG. 2b is an expanded view of a portion of FIG. 2a. 
     FIG. 3a is a photograph showing the output of an upchirp filter. 
     FIG. 3b is a photograph showing the output of a downchirp filter. 
     FIG. 3c is an enlarged view of the compressed pulse of FIG. 3a. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1a a schematic view of one embodiment of our invention performing a downchirp filtering function is shown. Microwave energy entering the device through input port 1 is selectively coupled to output port 2 by taps 5. The inner ends of the double spiral 3 and 4 are preferably terminated in the characteristic impedance of the device (i.e., 50 ohms). In FIG. 1b the frequency selectivity of the downchirp filter can be seen; tap 5a closest to the input and output permits high frequencies (typically generated at the end of an upchirp signal or present in an exciting impulse) to &#34;jump tracks&#34; first while lower frequencies must pass further down the line before they reach a compatible coupling point (i.e., 5b, 5c, etc.). If the input is the matching upchirp the net result at output 2 is a substantially compressed signal. 
     As can be seen from FIG. 1b, the same filter can be run backward (by using terminals 3 and 4 as input and output, respectively) to produce an upchirp filter. Alternatively, a separable device with the reversed order of tap lengths and spacing may be fabricated to produce an upchirp filter. Where both an upchirp and downchirp circuit are to be used in tandem a quadruple spiral design with all the terminals located at the outer edge of the wafer may be employed. 
     In FIG. 2a an actual upchirp device is shown. This linear-FM dispersive delay line gave 27 ns of dispersion over a 2 GHz bandwidth centered on 4 GHz. The stripline structure comprised a 2000-Å-thick patterned niobium film sandwiched between two 2&#34;-diameter, 5-mil-thick sapphire wafers with surrounding niobium ground planes. The pattern consisted of two parallel lines wound in a spiral pattern. The input lines was coupled to the output line at prescribed points by bringing the two lines into and out of closer proximity, thereby forming quarter-wavelength-long backward-wave couplers (see FIG. 2b). The resonant frequency of the couplers was designed to be a linear function of distance along the line pair, producing the desired linear group delay-vs-frequency relation, in this case an upchirp. The couplers in this device were not amplitude-weighted, so that the magnitude of the frequency response increased linearly with frequency. A matching device, identical except for the sign of the delay-vs-frequency slope, was also fabricated. 
     A 200-mV dc step with a 25-ps risetime was applied to the input of the expander, in this case the downchirp device. The resulting 27-ns long linear-FM pulse is amplified and time-gated, producing the pulse shown in FIG. 3b. This is applied to the input of the compressor, the upchirp device. The resulting compressed pulse is displayed in FIG. 3a. Expanded in time, this same pulse is also shown in FIG. 3c.