Comparison of noise reduction techniques in RF SQUID magnetic detection systems Navid M. S. Jahed, Farrokh Sarreshtedari, Farshad Forooghi, Mehdi Fardmanesh
Jurgen Schubert, Marko Banzet Institute of Bio and Nano-systems Forschungszentrum Jülich 52425 Juelich, Germany
Department of Electrical Engineering Sharif University of Technology Tehran, Iran
[email protected]
Abstract—The noise level of the magnetometer and gradiometer RF SQUIDs were investigated using different shielding methods. The used methods include different active and passive shielding, such as Helmholtz configurations, locally compensation coils, superconducting bulks and μ-metal shields. For the passive shielding approach, using FEM simulation we have investigated the shielding effectiveness of superconducting bulks versus the use of μ-metal shielding. The superconducting shield is a YBCO circular bulk, which was made using melt-texture method and located in a distance in front of the SQUID. In this work the results of these shielding methods are presented and compared, while their effectiveness is discussed by measuring the noise spectrum of the SQUIDs in each configuration. Keywords-component; SQUID, shielding, superconductor bulk.
I.
Noise
reduction,
Active
INTRODUCTION
Superconducting Quantum Interference Devices (SQUIDs) are one of the most sensitive magnetic sensors which have ever been known. They can be considered as flux to voltage transformers with extremely high sensitivity, which makes them suitable for widespread applications in different field of science and engineering such as geology, bioengineering, aerospace, nuclear research [1]. In all applications the reduction of the noise is a major challenge to enhance the performance and sensitivity of the system. There have been many efforts and studies to discover the noise origins and its generation mechanism in SQUID devices and systems [2],[3],[4]. Noise is mainly divided to two major categories according to its nation and origin: Intrinsic noise and extrinsic noise [1]. The flicker noise ( 1/ f ) is considered a major type of intrinsic noise and is a severe problem in High TC SQUID sensors. There are two major sources for ( 1/ f ) noise, One is due to fluctuation of the critical current and the other is due to the motion of vortices trapped in the Superconductor [2]. Reduction of the flicker noise is performed by using different techniques. Vortex pinning is one of the techniques used in fabrication process of the SQUID. The vortex hopping rate increases exponentially as the ratio of pinning energy to thermal energy is reduced. As a result, the films quality and Proceedings of ICEE 2010, May 11-13, 2010 978-1-4244-6760-0/10/$26.00 ©2010 IEEE
the related pinning energies play an important role in determining the low frequency noise level of the SQUIDs [3]. While utilizing a SQUID sensor, some special techniques can be used to reduce the flicker noise. The ZFC (Zero Field Cooling) technique in comparison to FC is a successful method for reducing the noise of the SQUID. In ZFC method the generations of the vortices are abolished by removal of the ambient field [4]. Extrinsic noise spans a wide frequency range, from RF to low frequency range. The major source for RF noise in residential area is the signal of cell phones. This noise mainly affects the associated readout electronics of the RF SQUID by unlocking the flux locked loop of the system. On the other hand in the low frequency range, the main noise source is the interference of power electric networks. While the high frequency noises are cancelled using passive shielding, in low frequency active shielding is preferred [5]. This is because of the fact that in many applications, SQUIDs are used for the detection of slowly varying fields. In this work we have compared the effect of different shielding methods on the noise level of the RF SQUID magnetometer and gradiometer. This comparison includes the effect of low frequency passive shielding with μ-metal, active shielding using a Helmholtz structure while the system was stabilized using a high frequency shielding which was an appropriately designed copper shield. We have also examined the effect of a configured Superconducting bulk on the low frequency noise suppression and the effect of ZFC technique versus FC in reduction of flicker noise. II.
EXPERIMENTAL SETUP
Fig. 1 shows the schematic of the used experimental setup. The utilized sensor of the system is a High-Tc YBCO RF SQUID Magnetometer with washer area of 3mm diameter which is coupled to a LC resonator operating at 860 MHz [6]. The RF SQUID gets cooled in a vacuumed liquid nitrogen dewar to be held in superconductivity state and its signal is detected using a low noise readout electronics which is also implemented in SERL. The system also incorporates a 1-D square shape Helmholtz coil, which is designed for low frequency noise cancelation. This coil is a 130cm×130cm
square structure capable of producing a uniformity region of about 30cm. For the noise spectrum measurement we have used the Agilent 35670A, Dynamic signal analyzer.
condition for the surface of the outer sphere is set to be Zero magnetic flux ( n × A = 0 ), this is done by setting the predefined boundary status to be Magnetic Insulation. For the inner boundaries or the boundaries between the copper coils and the air we set the boundary condition to be continuity ( n × (H 1 − H 2 ) = 0 ). The Meisner effect in a superconducting bulk can be a means for magnetic shielding. We have examined the shielding effectiveness of circular bulks when it is placed at different distances from the SQUID sensor. We have also simulated and verified its shielding process by means of numerical analysis using FEM. Fig.2 shows the simulation result of the superconductor bulk magnetic shielding.
Figure 1. Block diagram of the experimental setup
Because of the extremely high sensitivity of the sensor, there are different important EMC (Electromagnetic Compatibility) issues which are considered in this setup. These include the suitable grounding and shielding of different blocks of the system as long as the cables, and also issues like minimization of the area of the inevitable wiring loops. And also special consideration in designing of the Helmholtz structure to avoid noises due to locally generated current loops and mechanical vibrations of the structure.
III.
FINITE ELEMENT SIMULATIONS
Our simulations were performed in the COMSOL's multiphysics environment. The model of the Helmholtz structure was constructed in 3-D space using COMSOL's predefined quasi static electromagnetic module. The problem of electromagnetic analysis on a macroscopic level is that of solving Maxwell’s equations subject to certain boundary conditions. Assuming a static current and field the magnetic potential A must satisfies the following governing equation:
∇ × ( μ −1∇ × A ) = J
(1)
where μ is the permeability of the ambient. The relation between magnetic potential and magnetic field is as follows: B = ∇×A
(2)
For the simulation to be performed appropriate boundary conditions must be identified. The boundaries for 3D and 2D axial symmetry environments usually are set to be sphere and cylinder with sufficient radiuses respectively. The boundary
Figure 2. The simulation of circular bulk diamagnetic ( =0.00001) instead of a superconductor
In this case the model was constructed with 2D axial symmetry using COMSOL's predefined azimuthally induction current quasi static magnetic module. The governing equations are identical to Helmholtz's except that we defined the suitable boundary conditions for 2D axial symmetry simulation. The condition for the surfaces of the outer cylinder were defined to be magnetic field ( H 0 ) where H 0 is supposed to be the applied magnetic field. Meshing was done using the Delaunay algorithm, which is the default meshing technique by COMSOL Multi physics. The Superconducting bulk was defined to be much smaller than the ambient so the meshes are set to be finer around it and in other places they are coarser to avoid computational costs.
IV.
RESULTS AND DISCUSSIONS
The effectiveness of different shielding methods has been investigated by measuring and comparing the noise spectrum of the SQUID signal and also the magnitude of its especial components including 50Hz. The measurements are done using a magnetometer RF SQUID in an unshielded environment unless it is mentioned.
Fig. 3 shows the noise spectrum of the device without applying any noise cancellation or shielding technique. As it is evident, the spectrum is filled with the 50Hz component and its harmonics. Noise floor of the device is emphasized in the figure at about 500Hz. In Fig.4 the noise spectrum is measured in the μ-metal and in Fig.5 the effect of active noise cancellation on the spectrum is presented. It can be inferred from Fig. 4 that by applying a proper low pass filter in the deriving circuit for active shielding with the cut-off frequency of about 30Hz, all the high frequency harmonics of the electricity network are reduced in magnitude by the order of ten, while these harmonics are completely cancelled using μ-metal shield. The noise components about 400Hz in the Fig. 3 are due to the frequency signal generator which is used in addition the RF SQUID electronics for measuring the SQUID signal.
Figure 5. Noise spectrum using active shielding
As mentioned in the last section, because of meisner effect in the superconductors, the superconductor bulks can be used as magnetic shields. Fig.6 shows the comparison between the experimental measurements and simulation results of bulk shielding effectiveness.
Figure 3. Noise spectrum in unshielded environment
Figure 6. The normalized magnetic field versus distance from the bulk for both experimental and numerical results
Figure 4. Noise spectrum in μ-metal shield
As previously mentioned, one of the effective techniques in the reduction of flicker noise of SQUID devices is Zero Field Cooling (ZFC). In this method the transition to superconducting state takes place while the SQUID is placed in a μ-metal shield. Due to the removal of ambient field the vortices formation is abolished which lead to the decrease in the 1/ f noise. The noise spectrums of SQUID for each of these cooling methods are measured and illustrated in Fig.7 and Fig.8. As it is evident in the figures by incorporating the
ZFC approach the low frequency noise spectrum has been reduced for about 5 times at 20Hz.
region which makes it capable to be used for configured gradiometer systems.
Figure 7. The noise spectrum of the SQUID with Field cooling Figure 9. Shielding effectiveness of different methods on the reduction of 50Hz noise due to electrical network
V.
Figure 8. The noise spectrum of the SQUID while cooled in μ-metal shield (ZFC)
CONCLUSION
We examined different shielding methods and their effectiveness on noise cancellation. As it was discussed 50Hz background power line noise is the largest component that usually affects the SQUID based imaging systems operation. It was shown that this component can be reduced impressively with the use of active shielding. The use of passive shielding declines the noise level more efficiently but on the other hand we benefiting active shielding techniques features such as its selectivity over frequency which makes them attractive choice for shielding. REFERENCES
The reduction in the magnitude of the 50Hz noise due to electrical power network is an important issue in shielding systems. Using different shielding methods and measuring the 50Hz component of the noise spectrum, a quantitave comparison for their effectiveness in cancellation of this noise source is illustrated in Fig.9. For this comparison we have also utilized another active shielding approach where in that a local coil placed in the cryostat just above the SQUID sensor out of the RF shield. A similar driving circuit and identical control approach to the Helmholtz coil system’s has been used for the local coil derive and control. As it is apparent the 50Hz noise can be reduced drastically by the use of local coil shielding. The other most effective shielding methods rather than μ-metal are the use of superconducting bulks and Helmholtz coils respectively. It should be noted that the most advantage of active shielding is its flexibility and agility over passive shielding by filtering specific frequency band. And also the use of Helmholtz coil has the advantage of producing a uniformity
[1] A. l. Braginski, J. Clarke, “SQUIDs Handbook”, New York: Wiley Publications, 2004. [2] R. H. Koch,W. J. Gallagher, B. Bumble, W. Y. Lee, “Low-noise thinfilm TlBaCaCuO dc SQUIDs operated at 77 K”, Appl. Phys. Lett. 54, 951–953, 1989. [3] M.J. Ferrari, M. Johnson, F. C. Well stood, J.J. Kingston, T.J. Shaw, J. Clarke, “Magnetic flux noise in copper oxide superconductors”, J. Low Temp. Phys. 94, 15–61,1994. [4] Nilesh Tralshawala, James R. Claymcomb, and J. H. Miller, “Reduction of Ambient Magnetic field noise for High-Tc SQUID applications”, IEEE Transaction on applied superconductivity,vol. 7, No.2, 1997. [5] D.F. He, M. Daibo, and M.Yoshizawa, "Mobile HTS rf SQUID Magnetometer", IEEE Transacrion on Applied superconductivity, vol. 13, No.2, June 2003. [6] J. Schubert, M. Siegert, M. Fardmanesh, W. Zander, M. Prompers, Ch. Buvhal, Judit Lisoni, C. H. Lei, "Superconducting and Electro-optical Thin Films Prepared By Pulsed Laser Deposition Technique", Appl. Surface Science, Vol. 168, pp. 208-214, Dec 2000.
