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  System having unmodulated flux locked loop for measuring magnetic fieldsThe Patent Description & Claims data below is from USPTO Patent Application 20060164081. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates broadly to systems for measuring magnetic fields using flux licked loops and superconducting quantum interface devices. More particularly, the present invention concerns a system comprising an unmodulated or direct-feedback flux locked loop electrically connected by first and second unbalanced coaxial transmission lines to a superconducting quantum interface device. [0004] 2. Description of the Prior Art [0005] Superconducting quantum interface devices (SQUIDs) are small, cryogenically-cooled magnetic field sensors comprising a ring of superconducting material interrupted by two Josephson junctions. SQUIDs are designed to detect changes in magnetic flux, and, when suitably biased with a small DC current, will exhibit a magnetic flux sensitivity noise floor of approximately 1.times.10.sup.-6 .phi..sub.0/Hz for low temperature devices that operate near 4 degrees Kelvin (typically cooled by liquid Helium), and approximately 7.times.10.sup.-6 .phi..sub.0/Hz for high temperature devices that operate near 77 degrees Kelvin (typically cooled by liquid Nitrogen). SQUIDs exhibit a transfer function that convents magnetic flux into a periodic electrical output signal. [0006] The standard read-out method for SQUID measurements is to inject an alternating current (AC) magnetic field modulation signal into the SQUID and then, using a flux locked loop (FLL) circuit, sense changes in the modulating signal due to external magnetic fields. Without the FLL, the SQUID would have a very limited dynamic range because of its extremely non-linear magnetic field-to-voltage transfer function characteristic. The FLL maintains a stable magnetic flux operating point at the SQUID by introducing a feedback magnetic flux that precisely counteracts the externally applied magnetic field, provided the slew rate and dynamic range of the SQUID and FLL are not exceeded. Measurements of the external magnetic flux can be made by measuring the feedback signal which is an identical image of the external magnetic flux signal within the tracking bandwidth of the FLL. [0007] Also, as the input magnetic signal to the SQUID is varied, the output voltage of the SQUID appears as a distorted sine wave with a period equal to the flux quantum: (.phi..sub.0=h/2e.apprxeq.2femtoWebers), where h is Plank's constant and e is the charge on an electron. Only fields smaller than one-half .phi..sub.0 can be uniquely detected because any change in the magnetic field of greater than one-half .phi..sub.0 results in a nonmonotonic (multivalued) output signal. This small limiting field strength provides little dynamic range and has little practical value. [0008] Systems using SQUIDs for non-destructive testing/evaluation of materials or structures or for making biomagnetic measurements were long impractical for use in field settings (i.e., environments containing high levels of magnetic interference). The prior art had been limited to a flux modulation frequency of approximately 500 kHz with a maximum tracking loop bandwidth of 250 kHz. In magnetically unshielded environments, large amplitude or high slew rate external stray magnetic fields from 50/60 Hz AC power lines, AM broadcast transmitters, small changes in the Earth's magnetic field, and other sources, caused the FLL to lose lock and thereby invalidate any measurement in progress. Furthermore, the prior art employed traditional twisted-pair wires which were highly undesirable for several reasons, including that they had a high degree of linear attenuation versus frequency that severely distorts square waves of even moderate frequencies, they allowed for a large amount of radiated leakage and corresponding susceptibility to radio-frequency interference, and they had a highly variable characteristic impedance that changed with mechanical stress and was difficult to impedance match. The incorporation of digital signal processing (DSP) technology into the FLL had been attempted with limited success due to inherent delays associated with signal acquisition, processing and reconstruction of the feedback signal, and the maximum clock frequency of the DSP. Because of these problems, early attempts to incorporate DSP into the FLL failed to increase the operating frequency above that obtainable with standard analog read-out systems. For these reasons, SQUIDs were restricted to use in controlled environments shielded from magnetic interference, and were typically expensive, bulky, and non-portable. [0009] A great many of these limitations and disadvantages were overcome by the improvements and advances disclosed in U.S. Pat. Nos. 6,420,868; 6,448,767; and 6,356,078 (the '868, '767, and '078 patents, respectively). More specifically, the '868 patent discloses read-out electronics incorporating innovative circuit designs that extend the frequency of operation of the FLL and improve upon the earlier prior art by a factor of at least ten, thereby making operation of the SQUID practical in unshielded environments by alleviating the effects of high levels of magnetic interference on SQUID measurements. The '868 patent also discloses replacing traditional twisted-pair wires with shielded, unbalanced, controlled-impedance transmission lines to overcome many of the problems encountered in the earlier prior art, including reducing the amount of radiated leakage and corresponding susceptibility to radio-frequency interference. The '868 patent also discloses employing DSP algorithms to filter, extract, and measure the weak SQUID output signal. Problems encountered in earlier attempts to incorporate DSP technology into SQUID read-out electronics were overcome by locating the DSP outside of the FLL. [0010] The '767 patent discloses implementing the FLL with analog and radio-frequency (RF) components to improve upon the earlier prior art by a factor of at least ten. The use of RF techniques results in a flux modulation frequency of at least 33 MHz and a maximum tracking loop bandwidth of at least 5 MHz. The FLL is thus able to track, without unlocking, undesired high slew rate magnetic interference, thereby further eliminating the need for expensive and restrictive magnetic shielding for the SQUID. [0011] The '078 patent discloses a system with continuous signals and no time switching devices and therefore none of the associated problems found in the earlier prior art. The '078 patent also discloses operating a plurality of RF FLLs and their associated SQUIDs on different flux modulation frequencies (f.sub.1 through f.sub.N). This allows for a 1.times.N architecture which reduces from 2N to N+1 the number of required cable connections between the cryogenic SQUIDs and their associated room temperature read-out electronics. Thus, for example, a system comprising an array of ten SQUIDs, which previously would have required at least twenty cable connections, with their associated heat transfer and added complexity, now requires only eleven connections. The '078 patent also discloses reducing redundancy of FLL componentry by sharing certain components among the SQUID subsystems, thereby reducing complexity, cost, and size of the system as a whole. Thus, while each SQUID still requires some dedicated, frequency-specific FLL componentry, other non-frequency-specific FLL component functions are performed by shared or common components. [0012] Thus, the '868, '767, and '078 patents greatly improved upon and enhanced the usability of prior art FLLs and SQUIDs. These prior art patents, however, refer to and make use of only modulated FLLs. Unfortunately, modulation is associated with a greater number of electronic components, a greater number of more difficult adjustments, and distortion-producing non-linear RF components such as, for example, modulation oscillators, that emit RF interference. Modulation of the SQUID transfer function can also create unwanted distortion and signal sidebands with high level magnetic field signals applied to the SQUID. Modulated FLLs also require substantial bandwidth to process signal information. Modulated FLLs are also non-linear and therefore require band-limiting RF filters, which results in lower slew rates and narrower tracking bandwidths [0013] Due to the above-identified and other problems and disadvantages in the prior art, a need exists for an improved FLL for use with a SQUID in a system for measuring magnetic fields. SUMMARY OF THE INVENTION [0014] The present invention overcomes the above-described and other problems and disadvantages in the prior art with a system for measuring magnetic fields, wherein the system comprises an unmodulated or direct-feedback FLL connected by first and second unbalanced RF coaxial transmission lines to a SQUID. The FLL operates for the most part in a room-temperature or non-cryogenic environment, while the SQUID operates in a cryogenic environment, with the first and second lines extending between these two operating environments. [0015] The FLL maintains a stable magnetic flux operating point at the SQUID by introducing a feedback magnetic flux that precisely counteracts an externally applied magnetic field. Measurements of this external magnetic flux can then be made by measuring the feedback signal which is an identical image of the external magnetic flux signal within the tracking bandwidth of the FLL. [0016] The FLL broadly includes a bias tee; an impedance match; a low noise amplifier; a loop gain adjustment; a first DC amplifier; a first integrator network; a second DC amplifier with a DC offset adjustment; a second integrator network; an output amplifier; and a matching combiner. The bias tee is a controlled-impedance bias tee that allows both for injecting an operating bias current into the SQUID and for extracting the output signal generated by the SQUID via the second line. The impedance match terminates the second line in its characteristic impedance at the input of the low noise amplifier to prevent signal reflections and re-reflections from occurring. The low noise amplifier operates down to DC and amplifies the weak SQUID output signal from DC to the bandwidth limit of the low noise amplifier. The loop gain adjustment is used to optimize the gain of the FLL for different SQUIDs, thereby allowing for optimizing performance both by preventing the FLL from oscillating and by maintaining the slew rate and bandwidth of the FLL at a desired level. The first DC amplifier is wideband and similar to the low noise amplifier. The first integrator network is a passive lead-lag network that functions in conjunction with the second integrator network to provide the poles and zeros required for stable phase locked feedback of the SQUID output signal. [0017] The second DC amplifier performs four basic functions: providing wideband signal gain, providing a low output driving point impedance for the second integrator network, providing a place for controlling the DC offset of the loop using the DC offset adjustment, and providing a high input impedance for the first integrator network. The DC offset adjustment is required with all FLLs, whether modulated or unmodulated, and the amount of DC offset voltage is approximately the same for either system. In the present invention, however, changing the length of the first or second lines does not require re-adjustment of the DC offset. [0018] The second integrator network is a lead-lag passive network having an additional zero and operating in conjunction with the first integrator network to provide the overall performance of a two-pole integrator. This maximizes the signal tracking frequency range and slew rate and creates an unconditionally stable feedback loop. The overall loop performance depends on the combined effect of both the first and second integrator networks working together. The output amplifier must meet several requirements for FLL operation, including being a wideband DC amplifier, presenting a high impedance to the second integrator network, and driving undistorted feedback current into the low impedance first line and the feedback coil of the SQUID. The matching combiner matches the low characteristic impedance of the first line and combines any external input signals used. [0019] The SQUID is adapted and operable in a conventional manner to detect changes in magnetic flux. The SQUID is the only non-linear device in the system. The first unbalanced RF coaxial transmission line extends between the SQUID and the matching combiner of the FLL. The second unbalanced RF coaxial transmission line extends between the SQUID and the bias tee of the FLL. [0020] Thus, it will be appreciated that the system and, more particularly, the FLL of the present invention provides a number of substantial advantages over the prior art, including, for example, that the direct-feedback FLL is the simplest way to linearize the SQUID. The direct-feedback FLL also uses fewer, smaller, and less expensive electronic components; requires fewer adjustments which are easier to make; and eliminates distortion-producing, non-linear, bulky, and expensive RF components used in prior art modulated FLLs. [0021] These and other important features of the present invention are more fully described in the section titled DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT, below. BRIEF DESCRIPTION OF THE DRAWINGS [0022] A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein: Continue reading... Full patent description for System having unmodulated flux locked loop for measuring magnetic fields Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this System having unmodulated flux locked loop for measuring magnetic fields patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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