Patent Publication Number: US-2023142413-A1

Title: Superconducting Quantum Interference Array Receiver and Method for Digitally Controlling Magnetic Flux Bias Thereof

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has ownership rights in this invention. Licensing and technical inquiries may be directed to the Office of Research and Technical Applications, Naval Information Warfare Center Pacific, Code 72120, San Diego, Calif., 92152; voice (619) 553-5118; ssc_pac_t2@navy.mil. Reference Navy Case Number 111214. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention described herein relates to superconducting quantum interference devices (SQUIDs). An array of SQUIDs may be used to detect magnetic fields. An array of identical SQUIDs 15 referred to herein as an “SQA”, while an array of SQUIDs wherein the individual SQUIDs do not necessarily have the same area are referred to herein as a superconducting quantum interference filter or “SQIF”. When an SQA/SQIF is operated in motion, such as on a moving platform, the orientation of the SQA/SQIF sensor with respect to the earth&#39;s magnetic field varies with time. Changing the spatial orientation of the SQA/SQIF with respect to the earth&#39;s magnetic field changes the magnitude of the magnetic field the SQA/SQIF measures. Directionality and variations in the magnitude of earth&#39;s magnetic fields at different geographical locations are sufficiently large to affect the accuracy of SQA/SQIF as it moves between the different geographical locations. There is a need to account for these variations in the earth&#39;s magnetic field while an SQA/SQIF is simultaneously in motion and exposed to incident radio frequency (RF) signals. 
     SUMMARY 
     Disclosed herein is a receiver and method for detecting at least one electromagnetic signal while the receiver is moving with changing orientation relative to the Earth&#39;s magnetic field. In one example embodiment, the receiver comprises an SQUID array, a bias-tee, a memory store, and a logic circuit. The SQUID array is configured to generate an output that is a transfer function of a magnetic flux through the SQUID array. The magnetic flux is supplied from a combination of an oscillating magnetic field of the at least one electromagnetic signal, the Earth&#39;s magnetic field, and a bias magnetic field. The bias-tee is configured to divide the SQUID array output into a direct current (DC) signal and an RF signal. The memory store is configured to store a plurality of voltage and flux bias values. Each voltage value has a corresponding flux bias value that results in maximum SQUID array sensitivity. The logic circuit is configured to find a voltage value in the memory store that most closely matches the DC signal, and to apply to the SQUID array a flux bias corresponding to the most closely matched voltage value. 
     One example embodiment of the method for detecting at least one electromagnetic signal comprises the following steps. One step provides for generating a table of voltages and corresponding flux biases that result in maximum sensitivity of the SQUID array and storing the table in a memory store. Another step provides for moving the SQUID array with changing orientation relative to the Earth&#39;s magnetic field. Another step provides for monitoring an output voltage of the SQUID array. Another step provides for using a processor to incrementally adjust a magnetic flux bias of the SQUID array at timed intervals based on the monitored output voltage so as to maintain the flux bias within a predetermined range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Throughout the several views, like elements are referenced using like references. The elements in the figures are not drawn to scale and some dimensions are exaggerated for clarity. 
         FIG.  1    is an illustration of an example embodiment of an SQUID array receiver. 
         FIG.  2    is a graph of the output of an embodiment of an SQUID array. 
         FIG.  3    is a flowchart of a method for detecting at least one electromagnetic signal comprising the following steps. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The disclosed receiver and method and below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it should be appreciated that any of the underlying principles described are not to be limited to a single embodiment, but may be expanded for use with any of the other methods and receivers described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically. 
     References in the present disclosure to “one embodiment,” “an embodiment,” or any variation thereof, means that a particular element, feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment. The appearances of the phrases “in one embodiment,” “in some embodiments,” and “in other embodiments” in various places in the present disclosure are not necessarily all referring to the same embodiment or the same set of embodiments. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” or any variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. 
     Additionally, use of words such as “the,” “a,” or “an” are employed to describe elements and components of the embodiments herein; this is done merely for grammatical reasons and to conform to idiomatic English. This detailed description should be read to include one or at least one, and the singular also includes the plural unless it is clearly indicated otherwise. 
       FIG.  1    is an illustration of an example embodiment of an SQUID array receiver  10  that comprises, consists of, or consists essentially of an SQUID array  12 , a bias-tee  14 , a memory store  16 , and a logic circuit  18 . The SQUID array  12  is configured to generate an output  20  that is a transfer function of a magnetic flux through the SQUID array  12 . The magnetic flux is supplied from a combination of an oscillating magnetic field of at least one incoming electromagnetic signal  22 , the Earth&#39;s magnetic field  24 , and a bias magnetic field from a bias signal  26 . The bias-tee  14  is configured to divide the SQUID array output  20  into a direct current (DC) signal  28  and an RF signal  30 . The RF signal  30  carries the detected RF electromagnetic (i.e., the incoming electromagnetic signal  22 ) at the SQUID array  12 . It is preferable for the bias-tee  14  to have broadband RF characteristics. The SQUID array  12  may be either a SQA or a SQIF. Although the SQUID array  12  in  FIG.  1    is illustrated as having constituent SQUIDS with the same areas, it is possible that the individual SQUID areas can be varied. 
     The memory store  16  is configured to store a plurality of voltage and flux bias values. Each voltage value in the memory store  16  has a corresponding flux bias value that results in maximum SQUID array sensitivity. The logic circuit  18  is configured to find a voltage value in the memory store  16  that most closely matches the DC signal  28 , and to apply to the SQUID array a flux bias signal  26  corresponding to the most closely matched voltage value. The SQUID array receiver  10  may be mounted to a platform  32  that moves with respect to the Earth&#39;s magnetic field  24 . Note,  FIG.  1    is offered as an example illustration only, and it is to be understood that the platform may move in many different directions and/or change orientation with respect to the Earth&#39;s magnetic field  24 . Suitable examples of the platform  32  include, but are not limited to, a ground vehicle, an aircraft, a space-based platform, an underwater vehicle, a surface vessel, and a vehicle designed to move underground. 
     The SQUID array  12  may comprise any desired number of SQUIDs. For example, the SQUID array  12  depicted in  FIG.  1    comprises ten SQUIDs arranged in series, but it is to be understood that any desired number of SQUIDs in any desired configuration may be used for the SQUID array  12 . The SQUID array  12  may be a planar array, but it is not limited to planar embodiments. For example, the SQUID array  12  may be a three-dimensional array of SQAs and/or SQIFs. 
     The bias-tee  14  separates the DC and RF components of the received signal  22 . The DC signal or low frequency source may be used to voltage bias a device such as a SQA/SQIF. The RF signal normally carries data to and from a device. Bias-tees come in a variety of specifications such as insertion loss and cut off frequency and the one used in any given scenario will depend on the application. The memory store  16  may be any non-transitory storage device capable of storing the plurality of voltage and flux bias values in such a way that is searchable and retrievable by the logic circuit  18 . Suitable examples of the memory store  16  include, but are not limited to, magnetic computer disks, optical disks, electronic memories, solid-state memories, and non-transitory computer-readable storage media. The logic circuit  18  may be any physical computing device, such as a hardware device processor, capable of receiving inputs, performing calculations, and outputting a response. 
       FIG.  2    is a graph  200  of a prior art example of an output from a SQUID array, that is representative of the SQUID array output  20 , which is represented by a transfer function  202  between a magnetic flux through the SQUID array  12  along a horizontal axis  204  and an output voltage from the SQUID array  12  along a vertical axis  206  in accordance with an embodiment of the SQUID array receiver  10 . The horizontal axis  202  is representative of the magnetic flux through the SQUID array  12 . The vertical axis  20  is representative of the SQUID array  12 &#39;s output voltage expressed in millivolts. The graph&#39;s origin  208  at the intersection between axes  204  and  206  is where the magnetic flux through the SQUID array  12  has a value of zero flux and the output voltage is zero millivolts. Because the transfer function  202  passes through origin  208 , when no net magnetic flux passes through the SQUID array  12 , the output voltage from the SQUID array is zero millivolts. 
     As the magnetic flux increases from zero at origin  208 , the output voltage from transfer function  202  increases slowly at first, and then enters an extended region of steep slope where the output changes considerably when the magnetic flux changes slightly, especially as compared any other monotonic region of the transfer function  202 . This region is centered on the ideal operating point  210  having a magnetic flux bias  212  and a voltage offset  214 . When the SQUID array  12  is operating at operating point  210 , the transfer function  202  shows that the SQUID array  12  detects and amplifies small changes in the magnetic flux passing through the SQUID array  12  into considerable changes in the output voltage from the SQUID array  12 . This detection and amplification of transfer function  202  is approximately linear across a range of the magnetic flux centered on the flux bias  212  of the operating point  210 , with the linear range extending from nearly zero flux to nearly twice the flux bias  212 . Within this linear range, the output voltage of the SQUID array  12  changes considerably from the voltage offset  214  of the operating point  210  in response to the magnetic flux changing slightly from the flux bias  212  of the operating point  210 —with the linear range extending from an output voltage of nearly zero millivolts to nearly twice the voltage offset  214 . 
     When the SQUID array receiver  10  operates in motion, such as on a ship or an aircraft, the orientation of the SQUID array  12  with respect to the Earth&#39;s magnetic field  24  varies with time. Because the SQUID array  12  responds to the component of the combined magnetic field passing perpendicularly through a plane of the SQUID array  12 , the strength of the perpendicular component of the magnetic field as measured by SQUID array  12  changes with the changing spatial orientation between SQUID array  12  and the Earth&#39;s magnetic field  24 . These changes in relative orientation between SQUID array  12  and the Earth&#39;s magnetic field  24  are sufficiently large to move the operating point from the desired operating point  210  to a non-amplifying operating point  220 . 
     Furthermore, the strength of the Earth&#39;s magnetic field  24  ranges from about 25 μT to 65 μT across the Earth&#39;s surface. This entire range cannot fit within the available linear range of the SQUID array  12  of the example embodiment providing high amplification. Thus, variations in the strength and direction of the Earth&#39;s magnetic field  24  with geographical location are also sufficiently large to move the operating point from the desired operating point  210  to a non-amplifying operating point  220 . 
     The range of compensation needed is as much as the 130 μT change occurring when an aircraft carrying the SQUID array receiver  10  reverses direction from parallel to anti-parallel to the Earth&#39;s magnetic field  24  at a geographical location where the Earth&#39;s magnetic field  24  is maximal at 65 μT. Significant changes in the Earth&#39;s magnetic field  24  (e.g. on the order of 1 μT) due solely to changes in geographical location can require hours of movement of the platform  32 . Minutes are required for significant changes in orientation between a large embodiment of the platform  32  such as a ship and the Earth&#39;s magnetic field  24 . About one second is required for significant changes in orientation between a small embodiment of the platform  32  such as an aircraft and the Earth&#39;s magnetic field  24  during a banking maneuver. Less than a second is required for significant changes in orientation between a small aircraft and the Earth&#39;s magnetic field  24  during a roll maneuver. Thus, the cut-off frequency may be selected as 200 Hz for an example embodiment of the SQUID array receiver  10  to compensate for changes in the Earth&#39;s magnetic field  24  within an order of magnitude of the expected changes. 
     In one example embodiment of the SQUID array receiver  10 , the movement that changes the orientation of the SQUID array  12  relative to the Earth&#39;s magnetic field  24  induces some or all of a low-frequency portion of the external magnetic field. In such a scenario, the bias signal  26  provides nullifying feedback to the SQUID array  12  that counterbalances at least the changing orientation between SQUID array  12  and the Earth&#39;s magnetic field  24 . Further, in this scenario, the SQUID array output  20  detects the high-frequency portion of the oscillating magnetic field of the at least one incoming electromagnetic signal  22 . For example, one embodiment of the SQUID array receiver  10  may be used to concurrently detect multiple radio-frequency electromagnetic signals  22  with frequencies up to 100 GHz. 
     In one embodiment, the SQUID array  12  may be a planar array of bi-SQUIDs, each including a loop of superconducting material broken by three Josephson junctions. The SQUIDs in this planar array embodiment are connected in series for conducting a shared current from a current source. The SQUIDs in the planar array are spatially arranged with respective sizes distributed to provide the transfer function  202  for a particular value of the shared current from the current source. To provide the transfer function  202  providing a range of high amplification of the magnetic flux through the SQUID array  12 , the number of SQUIDs in the SQUID array  12  may be hundreds or thousands of SQUIDs, with more SQUIDs in the SQUID array  12  generally providing higher amplification because the SQUID array output  20  accumulates the voltage across each individual SQUID in the planar array. 
     Referring briefly back to  FIG.  2   , if the magnetic flux through the SQUID array  12  gets too high, such as more than twice the flux bias  212 , the transfer function  202  exits the linear range with high amplification. There are several possible causes for the magnetic flux through the SQUID array  12  getting too high. An electromagnetic signal strongly driven from a nearby antenna can produce an incoming signal  22  having a range of magnetic flux spanning more than twice the flux bias  212 . Then, even if the DC operating point is operating point  210 , the oscillating magnetic field of the incoming signal  22  produces an amplified output voltage with distortion that truncates the peaks of the oscillating magnetic field of the incoming signal  22 , and this non-linearity generally inhibits concurrently detecting other weaker electromagnetic signals of different frequencies. Another possible cause for the magnetic flux getting too high is an external magnetic field, such as the Earth&#39;s magnetic field  24 , providing a nominally constant flux that moves the operating point from the desired operating point  210  to a new operating point  220 , which produces little or no amplification of oscillating magnetic field of the incoming signal  22 . 
     Typically, the characteristic magnetic-flux-to-voltage transfer curve of an SQUID array is not periodic. In spite of the non-periodicity of the flux to voltage transfer curve, the SQUID array receiver  10  may be biased in any desired time frame to appropriately compensate for changes in orientation of the SQUID array receiver  10  with respect to the Earth&#39;s magnetic field  24 . For example, the SQUID array receiver  10  may be biased in near real time (i.e., &lt;10 milliseconds). A specific, initial flux bias may be set for the SQUID array  12  resting at any position. This initial flux bias value may be found by searching through a database of realistic flux bias values. The logic circuit  18  is configured to incrementally adjust the flux bias of the SQUID array  12  while the SQUID array receiver  10  is in motion with respect the Earth&#39;s magnetic field  24 . 
     The bias-tee  14  may be used to set a current bias point of the SQUID array  12  and a second independent bias for the flux bias. One example embodiment of the bias-tee  14  is configured to divide the SQUID array output signal  20  into DC and RF signals where the cutoff between the two frequency ranges is about 12 kHz. The bias-tee  14  also separates the lower and higher frequency components of the SQUID array output  20 . The voltage on the DC terminal of the bias-tee  14  represents the measured bias point of the SQUID array  12  and is the input to the logic circuit  18 . The logic circuit  18  may be configured to periodically find the voltage value in the memory store  16  that most closely matches the DC signal  28 , and to apply to the SQUID array  12  the flux bias corresponding to the most closely matched voltage value. Every measurement period, the logic circuit  18  may find the voltage value by performing the following control calculation: 
     
       
         
           
             
               Φ 
               n 
             
             = 
             
               
                 Φ 
                 
                   n 
                   - 
                   1 
                 
               
               + 
               
                 
                   1 
                   B 
                 
                 ⁢ 
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     n 
                   
                   
                     ( 
                     
                       
                         v 
                         i 
                       
                       - 
                       
                         v 
                         
                           i 
                           - 
                           1 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     where B represents the sensitivity of the SQUID array  12 , v n  represents the voltage measured at time n, Φ n  represents the magnetic flux at a time n represents the number of periods, and i is an index. The time n may be measured in number of measurement periods. The logic circuit  18  finds B, v 0 , and Φ 0  by storing the relationship between flux bias and output voltage in an array of values in the memory store  16 . The logic circuit  18  then processes those values to obtain the flux bias that results in the maximum sensitivity of the SQUID array  12  at a given time n and the corresponding output voltage. The sensitivity at the point where v=v 0  may change with time. The system remains stable as long as the actual sensitivity preferably stays above 3 dB below the initial measured sensitivity. 
     In one embodiment of the SQUID array receiver  10 , the logic circuit  18  is configured to find the voltage value periodically at a frequency less than or equal to 12 kilohertz. This allows for a slow biasing system operating below 12 Hz, which allows for the system to be implemented with a slower computational device than an analog circuit, such as a processor. The lack of periodicity requires the bias point to be measured and set before maintenance of it can begin. 
       FIG.  3    is a flowchart of a method  40  for detecting at least one electromagnetic signal comprising the following steps. A step  42  provides for generating a table of voltages and corresponding flux biases that result in maximum sensitivity of a superconducting quantum interference device array (SQUID array) and storing the table in a memory store. Another step  44  provides for moving the SQUID array relative to the Earth&#39;s magnetic field. Another step  46  provides for monitoring an output voltage of the SQUID array. Another step  48  provides for finding, at timed intervals, a voltage value in the memory store that most closely matches the output voltage. Another step  50  provides for applying to the SQUID array a flux bias from the table that corresponds to the most closely matched voltage value. The magnetic flux bias can be actively updated to maintain a minimum sensitivity threshold in response to changes in SQUID array sensitivity. 
     From the above description of the SQUID array receiver  10  and the method  40 , it is manifest that various techniques may be used for implementing the concepts of the SQUID array receiver  10  and the method  40  without departing from the scope of the claims. The described embodiments are to be considered in all respects as illustrative and not restrictive. The receiver/method disclosed herein may be practiced in the absence of any element that is not specifically claimed and/or disclosed herein. It should also be understood that the SQUID array receiver  10  and the method  40  are not limited to the particular embodiments described herein, but are capable of many embodiments without departing from the scope of the claims.