Electronic Gradiometer Research Articles

SQUID gradiometry for magnetocardiography using different noise cancellation techniques

Magnetocardiographic (MCG) measurements in unshielded environment require efficient noise cancellation techniques. We have applied two software gradiometry methods to analyze the time series of signal and reference data recorded outside magnetic shielding with high temperature superconducting quantum interference device (HTS SQUID) based gradiometers. One method uses adaptive frequency dependent gradiometer coefficients determined in the Fourier domain to subtract the reference from the signal data. The other method combines recently developed techniques for nonlinear projection with properties of the wavelet transform to extract noise in state space. The analyzed MCG data sets showed improved signal-to-noise ratios for both methods as compared to the data recorded with the electronic gradiometer. In this way, it is possible to increase the bandwidth from 130 Hz for our electronic gradiometer to 250 Hz without using any additional filtering.

Electronic gradiometer using HT/sub c/ SQUIDs with fast feedback electronics

An electronic gradiometer was built using a HTS SQUID magnetometer array. A SQUID magnetometer in the center of the array was used to pick up background noise, and the output signal fed back to other magnetometers to cancel background noise. Fast feedback electronics were built for the background channel with a slew rate about 10/sup 7/ /spl Phi//sub 0//sec and 10 MHz small signal bandwidth. Two other magnetometers of the array were connected to pcSQUID/sup TM/ electronics with 5/spl times/10/sup 4/ /spl Phi//sub 0//sec slew rate using the AC bias mode to decrease 1/f noise. The output signals from these two magnetometers were input to a summing amplifier resulting in a gradiometric output signal without background channel noise. Constructing the gradiometer with different magnetometers on the array enables us to vary the baseline from 0.75 mm to 7.5 mm with 2/spl times/10/sup -12/ T//spl radic/Hz field resolution in an unshielded laboratory environment. This variable-baseline gradiometer can be used for NDE, biomagnetism and other applications.

HTS dc SQUID systems for geophysical prospection

We describe the use of high T/sub c/ flip-chip dc SQUID magnetometers in systems for several geophysical prospection methods. For use in the Transient Electro Magnetics (TEM) method, a one-channel SQUID magnetometer system (vertical field component) was adapted to rough conditions in the field. We report on measurements in South Africa over a geophysically interesting target in real production mode. Transients up to 200 ms could be recorded, which is about a factor of 100 more than with conventional coils. For investigation of the magnetic field gradient a two-channel system, built as a first-order electronic gradiometer with a baseline up to 1 m, was designed and manufactured. It can be used, e.g., for surface exploration or for archeometry. The system can be moved in the Earth's magnetic field. We achieve a common mode rejection of 100. The reasons for the imbalance and the possibility of further improvement of the system are discussed.

A SQUID measurement system for immunoassays

A SQUID measurement system is presented that was developed for analysis of antigen–antibody reactions in fluid samples. The detection method is based on the signal generated by the remanent or relaxing magnetization of nanoparticles which are attached as labels to the antibodies. The measurement system involves two 7.2-mm-diameter multiloop SQUID chip magnetometers (type W7A) with a typical field noise of √ S B =1.5 fT/√ Hz arranged as an electronic gradiometer with a baseline of 160 mm. It is embedded in a liquid helium dewar with a tail bottom allowing 6 mm distance between SQUID and room temperature. A sophisticated direct-coupled read-out-electronics was realized with a bandwidth of 8 MHz. The read-out-electronics can be reset within τ R ⩽0.5 μ s. A 2 channel control unit was developed which enables environmental noise reduction. Data are acquired by a PC-controlled 8 channel system with 16-bit resolution and a maximum sampling frequency of f s =196 kHz. First measurements are presented.

Linearity of sensitive YBa2Cu3O7−x dc superconducting quantum interference device magnetometers

The linearity of sensitive superconducting quantum interference device (SQUID) magnetometers fabricated from high critical temperature (high-Tc) superconductors and operated in a direct-coupled flux-locked loop with bias reversal was investigated. The system nonlinearity was determined by applying a sinusoidal test signal and measuring the output signal harmonics using a setup which ensures the elimination of nonlinearities arising from the test signal source and the spectrum analyzer. The experimental setup enables the simultaneous measurement of the harmonic distortion of a single magnetometer and of a first-order electronic gradiometer formed by two individual magnetometers. Test signal amplitudes and frequencies were chosen to simulate typical laboratory interferences. An analysis of the results was performed using analytical equations in which the SQUID’s inherent nonlinear character is taken into account. At signal frequencies above about 0.1% of the system bandwidth, the system nonlinearity was found to be mainly caused by the nonlinear behavior of the SQUID magnetometers, whereas at low frequencies nonlinearities arising from read-out electronics components predominate. For signal peak-to-peak amplitudes up to 1 μT and frequencies up to 0.5 kHz, total harmonic distortions below about −105 dB for a single sensor channel and −100 dB for an electronic gradiometer were obtained.

Construction and measurements of HTS DC SQUID electronic gradiometer to be used in NDE systems

We have designed a non-destructive evaluation system by using an electronic gradiometer constructed with two DC SQUID magnetometers. Typical flux modulation of the SQUIDs used in this work was about 30 /spl mu/V and the noise at 10 Hz was about 14 /spl mu//spl Phi//sub 0///spl radic/Hz. The system includes a non-magnetic stainless steel dewar and a set of coaxial exciting coils, which was used to induce an eddy current in the test piece. The sample motion stage and the SQUIDs were computer controlled and the output data from the electronic gradiometer was obtained by using Labview software.

Highly balanced gradiometer systems based on HTS-SQUIDs for the use in magnetically unshielded environment

Two different concepts for gradiometer formation were tested applying high-temperature rf SQUIDs operated at 77 K in liquid nitrogen. All gradiometer systems are fully based on magnetometers. The first concept applies a compensating magnetometer at different positions to actively cancel the magnetic field at the location of other magnetometers. These magnetometers were arranged in an axial direction. In parts, a system of superconducting plates was used to align the relative magnetic orientation of the magnetometers. The outputs of these sensors were used to form a highly balanced electronic gradiometer. The second concept is based on electronic noise cancellation. A set of three magnetometers arranged in an axial direction was used to form an electronic second-order gradiometer. Different types of reference systems based on HTS-SQUID magnetometers and fluxgate sensors were applied to the gradiometer signal for achieving a high common mode rejection of the environmental disturbances. The performance of the different systems is demonstrated in a magnetically unshielded environment as well as in a shielded environment and the common mode rejection of homogeneous magnetic fields is measured. To demonstrate the performance of the systems, biomagnetic measurements have been performed in shielded and unshielded environments.

Two methods for a first order hardware gradiometer using two HTS SQUIDs

Two types of first order electronic gradiometers have been developed using high temperature superconducting (HTS) SQUIDs. Gradiometry is accomplished in hardware by either 1) subtracting the output of the signal and background SQUIDs in a summing amplifier (parallel technique) or 2) converting the inverted background SQUID output to a magnetic field at the sensor SQUID (series technique). Balance levels achieved are 2000 and 1000 at 20 Hz for the parallel and series methods respectively. Balance level as a function of frequency is presented. Balance level for hardware gradiometry is limited by time delays from the electronics and how well the signal amplitudes are matched. A simple algorithm that allows one to estimate the limit on balance level from these factors is presented and compared with data.

A high-temperature rf SQUID system for magnetocardiography

A first-order axial electronic gradiometer having a baseline of 10 cm was constructed by assembling two rf SQUID magnetometers with coplanar tank resonators, each having a white magnetic field resolution of about at 77 K. The gradiometer's near-field resolution was about , including the Dewar flask's noise. A peak-to-peak noise level of 3 pT was obtained in the bandwidth 0.016-250 Hz. Magnetocardiographic (MCG) measurements were performed using this bandwidth. Measurements on human subjects have been conducted in a magnetically shielded room of moderate shielding factor. Using the signal either of the lower magnetometer or of the gradiometer, high-quality heart signal traces could be collected, which were suitable for diagnostic use. A team of physicians, assisted by two of the authors, used the equipment over 10 months to perform MCG measurements in a medical study of about 80 clinical patients with cardiac arrhythmia problems and healthy persons. The system's performance was stable over that whole period.

Two methods for a first order hardware gradiometer using two high temperature superconducting quantum interference devices

Two different systems for noise cancellation (first order gradiometers) have been developed using two similar high temperature superconducting quantum interference devices (SQUIDs). “Analog” gradiometry is accomplished in hardware by either (1) subtracting the signals from the sensor and background SQUIDs at a summing amplifier (parallel technique) or (2) converting the inverted background SQUID signal to a magnetic field at the sensor SQUID (series technique). Balance levels (ability to reject a uniform background magnetic field) achieved are 2×103 and 1×103 at 20 Hz for the parallel and series methods, respectively. The balance level as a function of frequency is also presented. The effects which time delays (phase differences) in the two sets of SQUID electronics have on these balance levels are presented and discussed. It is shown that these delays, along with geometrical considerations, are the limiting factor for balance level for any electronic gradiometer system using two (or more) SQUIDs, a very different situation from the case with wire-wound gradiometers. Results using a dipole field to study the performance of both the parallel and series devices functioning as gradiometers in an unshielded laboratory are presented and compared with theory.

Compensation techniques for high-temperature superconducting quantum interference device gradiometers operating in unshielded environment

We have tested two methods of compensating environmental disturbances applicable to high-temperature superconducting quantum interference device (SQUID) systems operating in magnetically unshielded environments. For testing, we used first- and second-order axial electronic gradiometer setups with rf SQUID magnetometers operating at 77 K and base lines between 7 and 8 cm. The magnetometers were single-layer washer rf SQUIDs with bulk or thin-film magnetic flux concentrators in flip-chip geometry. The tested methods resulted in disturbance compensation levels comparable to those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in 13.5 fT/cm √Hz for first-order and 22 fT/cm2 √Hz for second-order compensation down to a few Hz. Common mode rejection was balanced to better than 10 000 for homogeneous fields and better than 200 for gradient fields with second-order compensation.

Compensation techniques for HTS-rf-SQUID magnetometers operating in unshielded environments

Abstract We have developed two methods of compensating environmental disturbances applicable to high- and low-temperature superconducting quantum interference device (SQUID) systems, operating in a magnetically unshielded environment. For testing, we used first- and second-order axial electronic gradiometer setups with rf SQUID magnetometers operating at 77 K, with different baselines between 1.8 and 8 cm. The magnetometers were single-layer washer rf SQUIDs, with bulk or thin-film magnetic flux concentrators and transformers in flip-chip geometry. The tested methods resulted in disturbance compensation levels comparable with those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in maximum gradiometric sensitivities of 13.5 fT/cm √Hz for first-order and 22 fT/cm2 √Hz for second-order compensation, with baselines between 7 and 8 cm down to 10 Hz. Common mode rejection was balanced to, better than 104 for homogeneous fields, and better than 200 for gradient fields, with second-order compensation. Magnetically unshielded measurements of biomagnetic signals with different baselines were demonstrated.

Electromagnetic nondestructive evaluation: moving HTS SQUIDs, inducing field nulling and dual frequency measurements

We have previously shown that simple, single layer HTS SQUIDs can be used effectively in electromagnetic nondestructive evaluation (NDE) using eddy current techniques in a magnetically unshielded environment. HTS SQUID systems for NDE applications are expected to be small and portable allowing non-stationary measurements to be carried out in the Earth's field above a stationary sample. Here we present application-oriented results showing the ability of our HTS electronic gradiometer to cope with the movement of the sensors above a series of simulated flaws in aircraft grade aluminum samples. To permit the detection of fine surface and subsurface structures we have applied active field nulling to the two SQUIDs to increase the effective signal to noise ratio. The excitation signal is applied via a non-superconducting coil to provide a lower field environment for each device. We also present results using a dual frequency eddy current technique to allow depth profiling of flaws in multilayer structures.

Electronic gradiometry for NDE in an unshielded environment with stationary and moving HTS SQUIDs

Difficulties in the fabrication of multilayer high-temperature superconductor (HTS) devices have led to recent interest in the use of simpler HTS SQUID magnetometers in electronic gradiometers. One application of such systems is electromagnetic non-destructive evaluation. We have developed a prototype two-SQUID system and we present recent results in this paper. We first demonstrate the level of interference suppression by comparing magnetometer and gradiometer signals. Then we present several results taken conventionally with the HTS SQUIDs stationary above moving specimens and, for the first time, with the SQUIDs unshielded in motion above stationary specimens. The specimens comprise a pair of wires in a return current loop as a calibration source, and an aircraft-grade aluminium plate with fine slits mimicking fatigue cracks, first exposed and then covered with an additional aluminium sheet to simulate internal flaws. These results are an important, though by no means final, step towards practical non-destructive evaluation of real test subjects with HTS SQUIDs.

Integrated LTS gradiometer SQUID systems for unshielded measurements in a disturbed environment

Various types of thin-film dc gradiometer SQUIDs with integrated pickup loops were investigated. The SQUIDs were prepared on a chip in the well developed technology. It was shown that the SQUIDs work in an unshielded environment. Our best SQUIDs have a field gradient sensitivity better then . All gradiometers have some response to a homogeneous field, i.e. a parasitic area. The effective pickup area, the field gradient sensitivity, and the components of the parasitic area for the field perpendicular and parallel to the gradiometer plane were measured for all types of gradiometer. The calculations of flux coupling efficiency are compared with experimental results. The origins of parasitic area are discussed. A further reduction of external disturbances was achieved by using an electronic second-order gradiometer. Examples of magnetocardiographic measurement in a physical laboratory without any shielding with first and second-order gradiometers are shown.

Vitamin receptors: Vitamins as ligands in cell communication

Difficulties in the fabrication of multilayer high-temperature superconductor (HTS) devices have led to recent interest in the use of simpler HTS SQUID magnetometers in electronic gradiometers. One application of such systems is electromagnetic non-destructive evaluation. We have developed a prototype two-SQUID system and we present recent results in this paper. We first demonstrate the level of interference suppression by comparing magnetometer and gradiometer signals. Then we present several results taken conventionally with the HTS SQUIDs stationary above moving specimens and, for the first time, with the SQUIDs unshielded in motion above stationary specimens. The specimens comprise a pair of wires in a return current loop as a calibration source, and an aircraft-grade aluminium plate with fine slits mimicking fatigue cracks, first exposed and then covered with an additional aluminium sheet to simulate internal flaws. These results are an important, though by no means final, step towards practical non-destructive evaluation of real test subjects with HTS SQUIDs.

A second-order SQUID gradiometer operating at 77 K

We describe a portable second-order electronic SQUID gradiometer cooled with liquid nitrogen and capable of operating in unshielded space. The measured magnetic-field resolution is <300 fT Hz-1/2 and <170 fT Hz-1/2 When operating in the first-order mode. The gradiometer was successfully used for the recording of screening magnetocardiograms of over 200 human subjects at four different geographic locations.

Using a 77 K SQUID to measure magnetic fields for NDE

A bare HTS SQUID of commercial design was used in 77 K experiments concerning NDE. The SQUID was operated with flux-locked instrumentation to provide a noise floor of 80 pT//spl radic/Hz. The effective sensor area was measured to be approximately 70 /spl mu/m/sup 2/ equivalent to an ideal point detector for NDE. The SQUID was used unshielded in a normal laboratory environment in a special purpose LN/sub 2/ cryostat positioned above a motorized computer-controlled scanning system. We measured magnetic fields associated with current flowing in wires and compared them with calculations. We also detected a simulated flaw in an aluminum plate using an eddy current technique and made a preliminary depth assessment by frequency sweeping. Although developments in electronic gradiometers and gradiometric SQUID's should make the use of single bare magnetometer SQUID's unnecessary, we show that these already have sufficient sensitivity for NDE research, even without flux-focusing washers or pick-up coils.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two-hole bulk high Tc squid electronic gradiometer for magnetocardiography in unshielded space

A first order electronic gradiometer has been developed to operate in liquid nitrogen. The use of two-hole bulk YBa 2Cu 3O 7 RF SQUIDs permitted optimization of the characteristic SQUID parameter, β L = 1, without compromising the noise performance. Additional bulk flux concentrators made it possible to attain a field/flux transformation coefficient of 8 × 10 −10 T Φ o −1. A mechanical fixture for electronic balancing provided a common mode suppression of 10 3. In an unshielded environment, the balanced system had a peak-to-peak noise amplitude of < 10 pT within a signal bandwidth of 2–30 Hz. Human heart beat signals (the QRS peak) could be recorded in these conditions.

YBa2Cu3O7 thin film SQUID gradiometer for biomagnetic measurements

Low-noise rf SQUID washers fabricated from YBa2Cu3O7 epitaxial thin films have been used to construct a first-order electronic gradiometer operating at 77 K and suitable for biomagnetic measurements. Mechanical adjustment of the two-SQUID gradiometric setup made it possible to attenuate signals due to far magnetic field sources by three orders of magnitude. A magnetic field resolution of ≤280 fT/Hz1/2 above 2 Hz was attained through the use of large flux focusers. The fine structure of human heart magnetocardiograms was recorded in unshielded space. In a shielded room, magnetoencephalograms were obtained. The system was used to obtain new data on auditory evoked cortical response.