Jülicher S UID GmbH
Contents
1. 20 Applications for HTS rf SQUIDs by JSQ Products How to contact JSQ nel 23 224 J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com By J licher SQUID GmbH 3 a Sensortechnik 1 SQUID Fundamentals A SQUID Superconducting QUantum Interference Device is a superconducting interferometer serving as an extremely sensitive magnetometer It is formed by a superconducting loop incorporating a weak link a so called Josephson junction The laws of physics demand that the magnetic flux enclosed by a superconducting ring is quantized n Do n 0 1 42 Changes of the external magnetic field are compensated for by a variation of the superconducting current around the ring In the case of a weak link within the ring this shielding current however is limited The critical current of the Josephson junction le its Ohmic resistance in the normalconducting state Rn and the inductance of the ring L are the fundamental parameters governing the behavior of the SQUID rf Osc Lsa Li Ry Q G Flux Tank circuit Fig 1 rf SQUID principle The SQUID is read out inductively by means of a tank circuit a LC resonance circuit operated close to its rf resonance By means of a VCO voltage controlled oscillator the operating frequency of the tank circuit is adjus2. volatile memory of the Tiger Controller Slight re adjustments may become necessary in order to achieve optimum performance or to compensate for changes of the ambient temperature of the electronics The VCO voltage controlled oscillator slide control is used to adjust the frequency of the tank circuit inductively coupled to the SQUID The VCA voltage controlled attenuator slide control regulates the amplitude of the bias current applied to the tank circuit switch electronics to Test switch the oscilloscope to x y mode and the vertical y channel to AC coupling Adjust VCA slide control to a value of about 1000 Adjust VCO slide control to achieve maximum peak to peak signal If several maxima are seen then choose the one with the largest amplitude Adjust VCA slide control to maximum peak to peak signal Fig 17 Fig 17 Typical Oscillogram of the Magnetometer Test Signal during operation in unshielded environment Due to the ambient 50 Hz power line noise one does not obtain a standing picture The triangular shaped transfer function is continuously shifted horizontally modulated by the ambient 50 Hz disturbances J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com is f J licher SQUID GmbH a Sensortechnik Fig 18 Typical Oscillogram of the Test Signal inside magnetic
3. applications 10 10 magnetic field spectrum fT Hz1 Lu spny dwe pjay oyau ew 10 vl i 10 a 10 10 10 10 10 10 10 10 10 frequency Hz The white area shows the principle operation regime of sensors and electronics by JSQ The regimes of operation of our sensors and electronics are plotted in the figure above The figure shows that for our new sensors even a moving operation directly exposed to the magnetic field of the earth becomes possible This is a true breakthrough The new HTS rf SQUIDs by JSQ are especially qualified for the following tasks Non Destructive Evaluation NDE of materials i e detection of flaws and cracks in airplane parts or checking steel reinforcement buried inside concrete structures These applications use frequencies between 1 Hz and 1 MHz Mobile operation in the earth magnetic field is essential and therefore gradiometry is the best choice for stable operation Sensitivity is not the critical issue but stability and slew rate are J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 9 J licher SQUID GmbH IA gt Sensortechnik Geological surveying needs a very high bandwidth from 0 001 Hz up to 10 Hz and a high sensitivity while directly exposed to the full earth magnetic field The problem of mo
4. fy Jiilicher SQUID GmbH Sensortechnik User Manual for JSQ Magnetometer SQUID and Electronics The J lich SQUID Company JSQ Manufacturer of the world s finest HTS rf SQUIDs licensed by Research Center J lich Forschungszentrum J lich GmbH 2 V J licher SQUID mbH p Sensortechnik Contents 1 SQUID Fundamentals 2 Components of the JSQ Magnetometer SQUID Sensor System 2 1 JSQ Magnetometer Capsule and SQUID Holder ses 6 2 2 JSG HTS rf SQUID Electronics VAO 7 aoro oerte ranae en 7 2 3 JSQ Tiger Controller for JSQ SQUID Electronics V4 0 s eese 9 2 4 Power Supply 2 10 3 SQUID Operation salt 3 1 Electrical CoOnnectiori mu 11 crues DO 13 3 3 Installing the JSQ SQUID Sensor Software on your PC ssessseeee 13 3 4 Starting the JSQ SQUID Sensor Software on your PC a 19 3 5 Adjustment of the SQUID operating point 14 3 6 Locking the SQUID Magnetometer for Measurement nmeeennennne 16 3 7 Adjusting the Feedback Loop Parameters ccscccsssesesessessssseseseseseeensneneseasees 17 3 8 Saving the Pararmelers ooo rotorns EENAA XL RE Sau sees 18 3 9 Quitting and Warming up 2 18 4 Troubleshooting during SQUID Operation 20 4 4 Radio Frequency Interfererice coercet o t o eee v tar e ee tto 20 Lol agno
5. order to linearize the transfer function Fig 3 the SQUID is operated in a so called flux locked loop The SQUID is kept at a well defined external flux state locked at a working point A see Fig 3 by generating a magnetic feedback field compensating all measured external flux variations 500 cable coupling coil tank L circuit SQUID gt chip Fig 4 Tank circuit concept J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com If J licher SQUID GmbH a Sensortechnik 2 Components of the JSQ Magnetometer SQUID Sensor System 2 1 JSQ Magnetometer Capsule and SQUID Holder The JSQ Magnetometer SQUID Sensor consists of a capsule containing the Magnetometer SQUID chip the tank circuit and the coupling coil A 50 Q transmission line cable is used for connection with the readout electronics channels see Fig 5 5001 T cable Magnetometer F CE Fig 5 Principle of the JSQ Magnetometer SQUID Sensor Fig 6 shows the layout of the Magnetometer SQUID The sensor was developed at Forschungszentrum J lich It is manufactured by JSQ On a LaAlO single crystal substrate a ditch is prepared by ion beam etching An epitaxial layer of YBazCu307 x is grown by laser ablation Then the sensor layout is structured using a photolithographical process in a clean room environment Fig 6 Layout of th
6. voltage proportional to the feedback current gives the measured magnetic field relatively to the arbitrary working point This operating principle ensures a linear transfer function It allows a dynamic range from a few 10 to about 10 magnetic flux quanta with excellent linearity Fig 9 depicts the functional block diagram of the JSQ SQUID Electronics V4 0 J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 8 vuticher squiD cmbn a Sensortechnik Fig 8 Photograph of the JSQ HTS rf SQUID Electronics V4 0 Fig 9 Schematic of the JSQ V4 0 electronics J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com Ly J licher SQUID GmbH Sensortechnik Fig 10 and Table denote the parameter adjustment range of the JSQ rf SQUID electronics V4 0 Table Specifications of the JSQ HTS rf SQUID Electronics V4 0 RF power output 75 dBm to 115 dBm adjustable frequency 630 MHz to 970 MHz adjustable signal output ampl 10 V test outputampl 1 Vp mod output ampl lt 1 Vpp 1000 950 900 850 800 750 Frequency MHz 700 650 600 1000 2000 3000 4000 VCO va
7. Fig 2 The amplitude l of the bias current is adjusted by means of the VCA control Fig 16 From the U I characteristics of an rf SQUID Fig 2 the U characteristics the so called transfer function Fig 3 is derived The transfer function is a periodic function if the magnetic flux threading the SQUID loop with the magnetic flux quantum p denoting the periodicity constant J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com f J licher SQUID GmbH 5 a Sensortechnik U AU 0 o 20 Du Fig 3 U characteristics transfer function of an rf SQUID The transfer function is periodic in the magnetic flux quantum co A denotes the working point see text The JSQ SQUIDs use an inductively coupled tank circuit Fig 4 With this design the tank circuit noise is minimized and the quality factor Q of the tank circuit is maximized An important advantage of this concept is that no galvanic connection from the electronics to the SQUID therefore static discharges pose no threat to the SQUID s life A 50 Q transmission line connecting the readout electronics with the SQUID is terminated by a coupling coil adjacent to the SQUID and its tank circuit The coupling coil serves two purposes applying the pumping radio frequency to the SQUID s tank circuit and generating the feedback flux In
8. d by a modern lithographical process Their unique design is protected by patents JSQ offers these sensors complete with room temperature drive and read out electronics The JSQ electronic control units run at 600 to 1000 MHz Adjustment of the SQUID s working points is conducted by a microcontroller integrated with the electronics The HTS rf SQUIDs by JSQ are reliable with fully guaranteed sensitivity and have demonstrated very stable performance J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 24 f J licher SQUID GmbH Sensortechnik How to contact JSQ telephone 49 2401 60 5487 or 49 2461 61 4431 fax 49 24 61 61 46 73 e mail JSQ jsquid com mail JSQ GmbH Lankenstr 29 D 52428 J lich Germany The JSQ team Management Willi Zander Simin Krause Chris Buchal Design Yi Zhang Thin Film Technology Fabrication Electronics amp Computing Systems Machine Shop Shipping Please visit our website at J rgen Schubert Marko Banzet Ralph Otto Norbert Wolters Dieter Lomparski Hans Joachim Krause Hans Wingens Manfred Plum http www jsquid com J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com y J licher SQUID GmbH Sensortec
9. e SQUID Magnetometer In order to protect the superconducting YBCO thin film from moisture and other harmful substances the JSQ SQUID is encapsulated in a sealed package The sensor package consists of a magnetometer the corresponding tank circuit the coupling coil and the coaxial connecting cable Fig 7 shows the SQUID holder with the SQUID Capsule mounted at the end J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com d J licher SQUID GmbH 7 A Sensortechnik Fig 7 Photograph of the JSQ SQUID Holder 2 2 JSQ HTS rf SQUID Electronics V4 0 The JSQ SQUID Electronics V4 0 see Photograph Fig 8 is used to generate the pumping radio frequency for SQUID operation with the required adjustments of rf frequency and amplitude for working point adjustment Furthermore the electronics serves as preamplifier and demodulator of the rf response of the SQUID After demodulation of the SQUID signal from the rf carrier the signal is integrated and fed back to the coupling coil in order to operate the SQUID in a so called Flux locked loop FLL Thus the SQUID is kept at a well defined flux state e g working point A see Fig 3 All external flux variations detected from the moment of locking onward are compensated by a magnetic feedback flux of opposite sign The SQUID serves as a null detector The feedback
10. flux quanta that can be measured in other words a larger dynamic range of the sensor The Gain factor determines the internal amplification of the loop Fig 20 shows typical transfer functions for different values of the feedback resistor of the loop J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 18 d J licher SQUID GmbH Sensortechnik 100 gt Parameters 10 C4 G1 o o a E lt o a c E n 100 1000 10000 Frequency Hz Fig 20 Typical transfer function of a SQUID for different feedback resistor settings Intuitively one would then choose a minimum capacitance and a minimum resistance in order to achieve maximum slew rate bandwidth and dynamic range However the larger the integrator s capacitance and the larger the feedback resistance the more stable the flux locked loop is Therefore the capacitance and the resistance should be chosen only so low as the application requires One should try to achieve a compromise between performance and stability The JSQ default factory settings for the capacitance and resistance are 4700 pF and 10 kQ 3 8 Saving the Parameters Pressing the Save button results in saving the currently active working point and feedback loop parameters of the Magnetometer SQUID to the non volatile memory of the Tiger Controller The sa
11. hnik Magnetometer Data Sheet Date 19 09 07 SQUID Standard Washer 83 5 mm Loop 100 x 100 um Tank circuit Lumped element Resonance frequency 731 MHz Coupling rf absorption 26 dB Quality factor 340 SQUID signal amplitude 0 6 Vpp Transfer coefficient at R 10 kQ 0 195 V o Calibration 9 3 nT do Feedback Resistor R1 R2 R3 R4 R5 R6 R7 R8 R O 1 3 6 10 15 20 50 100 Transfer coefficient 21 60 120 195 290 380 950 1900 mV do Integ C C1 C2 c3 c4 c5 ce c7 gt N E o N N ES C nF 0 33 1 22 Open loop SQUID signal in shielding Steuerbefehle f r SQUID Elektronik ber die RS232 Schnittstelle Beschreibung Setze Attenuation value 20 4095 Kanaln 1 Setze Frequenz Setze Offset Setze Feedback Widerstand Setze Integrator C Setze Kanal Freigabe Setze Autoreset Setze Modulation ON alle Kan le Setze Modulation OFF alle Kan le Save Data Load Data Restart Program Setze Baudrate Setze ECHO ON Setze ECHO OFF Setze Messmodus Reset Kanal m 1 O ALL Setze Testmodus Setze Switch Output 0 OFF Suche Resonanz Adjust Resonanz Attenuation Frequency Offset Fast Adjust Resonanz Get Attenuation Kanal 1 4 Get Frequenz Kanal 1 4 Get Frequenz 100 Kanal 1 4 Get Offset Kanal 1 4 Get Widerstand Kanal 1 4 Get Integrator C Kanal 1 4 Get Freigabe Kanal 1 4 Ge
12. l connections 11 Prepare all electrical connections according to Figs 13 and 14 The power supply must not be connected to 230 V until all other cables are in place and checked carefully Oscilloscope Power supply 5 V g Controller c for V4 0 230 V 50 Hz JSQ rf SQUID Electronics 4 V4 0 Magnetometer SQUID J Fig 13 Sketch of the electrical connections for SQUID operation Fig 14 Photograph of the electrical connections for SQUID operation J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 12 fy J licher SQUID GmbH Sensortechnik The electrical connections should be made as follows 1 For SQUID operation the coaxial cable of the JSQ Magnetometer SQUID Sensor has to be connected to the SMA socket of the JSQ SQUID Electronics V4 0 Connect the JSQ rf SQUID Electronics V4 0 with the JSQ Tiger Controller using the serial cable with DSUB9 connectors Connect the DSUB9 female connector of the Data cable with the JSQ Tiger Controller Connect the other side male DSUB9 connector with the serial COM port of your PC Connect the 3 pin power connector of the JSQ power supply with the corresponding power jack of the JSQ Tiger Controller Connect the Signal out connector coaxial LEMO jack of the JSQ Tiger controller with the vertical y channe
13. l of a standard oscilloscope Connect the Modulation out connector coaxial LEMO jack of the JSQ Tiger controller with the horizontal x channel of a standard oscilloscope see Fig 15 The oscilloscope should be Switched to the x y mode for SQUID working point adjustment Make sure that all connections are made according to Fig 13 Plug the power cable into a 230 V 50 Hz outlet Allow for at least 5 min better 10 15 min of warm up time for the electronics and cool down time for the SQUID see Chapter 3 2 before adjusting the SQUID s working point see Chapter 3 5 Otherwise the working point of the SQUID will drift during the warm up phase and readjustments of VCO and OFFset will be necessary Fig 15 LEMO connectors for visualization of the SQUID test signal with an oscilloscope J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com if J licher SQUID GmbH 13 L Sensortechnik It is possible to operate up to three SQUID channels simultaneously by the Tiger controller if more JSQ rf SQUID Electronics V4 0 front ends are connected to the controller 3 2 Cooling The SQUID holder has to be operated immersed in liquid nitrogen Allow for at least 5 min better 10 15 min of cooling time for the SQUID before adjusting the SQUID s working point see Chapter 3 5 Ensure that the cryostat and the c
14. lue Fig 10 Typical calibration curve of a JSQ SQUID Electronics V4 0 2 3 JSQ Tiger Controller for JSQ SQUID Electronics V4 0 The JSQ Electronics V4 0 is operated and remote controlled by means of a micro controller the JSQ Tiger Controller The Tiger Controller is integrated with the electronics see Fig 11 It is based on the Tiger micro controller chip by Wilke Electronics Aachen J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com ks y J licher sau n cmon Sensortechnik The working point of the Magnetometer SQUID sensor is stored in non volatile memory In addition the Tiger micro controller handles the communication to a user front end personal computer via its RS 232 port serial port COM1 or COM2 V 24 protocol Fig 11 JSQ Tiger Controller for SQUID Electronics V4 0 2 4 Power Supply Fig 12 displays the power supply for the JSQ Tiger controller to be plugged into a 230 V 50 Hz outlet after completing the electronic wiring according to chapter 3 1 Fig 12 Power Supply for the JSQ Tiger Controller J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com ij J licher SQUID GmbH Sensortechnik 3 SQUID Operation 3 1 Electrica
15. n case of SQUID operation in unshielded environment is the degradation of performance due to environmental Radio Frequency Interference RF Interference results in a reduction or even suppression of the transfer function of the SQUID which can be observed in the test mode JSQ suggests to operate the SQUID package in a well grounded metal container e g a cylinder of thin aluminium or lead foil if the environmental high frequency noise is unacceptably high 4 2 Flux trapping Another well known problem in case of SQUID operation in unshielded environment is flux trapping due to large external field amplitude changes Flux trapping results in spontaneous signal jumps due to flux vortex hopping within the superconducting thin film the so called shot noise An increased 1 f noise can be observed in extreme cases SQUID operation can be jeopardized Warming up the SQUID and releasing the trapped flux solves this problem J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com d J licher SQUID GmbH 21 Sensortechnik Applications for HTS rf SQUIDs by JSQ With our HTS rf SQUIDs it is not only possible to measure slowly varying magnetic fields but also rapidly changing fields with extremely high sensitivity This performance yields additional information for known applications and opens the road towards new
16. onnecting cable to the electronics are completely enclosed by a metallized rf shield which is connected to the electronics ground chassis If this precaution is not being observed SQUID operation may be strongly disturbed or even prevented by rf interferences from the environment 3 3 Installing the JSQ SQUID Sensor Software on your PC In case of first operation or in case the PC is changed the JSQ SQUID Sensor Software has to be installed on the PC Insert the JSQ SQUID Sensor CD and copy the folder JSQ SQUID Program to your hard disk It is advisable to create a shortcut on the desktop and or in the list of programs The software configures the serial RS232 COM port to a data transfer rate of 19200 Baud 8 data bits parity N 1 stop bit flow control Hardware In case the Tiger controller is not connected to the serial port COM 1 please open the file DuoSensor ini with any editor and enter the port number you are using in line three e g PortNum 2 for serial port COM 2 3 4 Starting the JSQ SQUID Sensor Software on your PC Execute the file DuoSensor exe on your PC Upon starting the data of the working point of the Magnetometer SQUID is read from the non volatile memory of the Tiger Controller via the RS232 connection This reading is equivalent to a loading operation when pressing the Load key In case of successful transfer the values appear on the Software panel Fig 16 within a few seconds and the slide cont
17. rols for VCA VCO and Offset and the settings for the Flux locked loop parameters are set accordingly If this is not the case one should check all cable connections and ensure that the power supply is connected to the JSQ SQUID Electronics J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com I J licher SQUID GmbH Sensortechnik Check the active check box of those electronic channels with a SQUID connected Make sure that those channels that do not have a SQUID connected are not checked Otherwise the Auto Reset function will not work properly because the electronics controller will try to lock an open loop leading to continuous resets JSQ Duo Sensor Control Integr C 4700pF X mex Test Fig 16 Panel of the JSQ SQUID Sensor Software 3 5 Adjustment of the SQUID operating point The controls of the JSQ SQUID Electronics V4 0 with Tiger Controller have been factory adjusted to the operating point of the SQUID Magnetometer Upon starting of the software J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 15 If J licher SQUID GmbH a Sensortechnik or upon pressing the Load button the working point parameters are read from the non
18. shielding If the ambient 50 Hz power line magnetic fields are small enough a standing picture is obtained Manual adjustment of the Offset Oscilloscope Check zero position of the y channel Switch y channel to DC coupling Using the OFF slide control the level of the SQUID signal is adjusted exactly symmetric to the zero level For a subsequent stable SQUID operation a precise offset adjustment is important If the offset is adjusted too low or too high the SQUID signal may drift to the limits of this range of voltage supply By using the OFFset controller a fine tuning of the offset is feasible in order to adjust the SQUID signal exactly in the central region between the limits If the signal reaches the limits the Reset button should be pushed and the OFFset slide control must be fine tuned In most cases this can be avoided by adjusting the offset of the modulation signal very precisely to the central position before switching from test signal to operation signal 3 6 Locking the SQUID Magnetometer for Measurement e Switch the oscilloscope to the time diagram mode displaying the y channel as a function of time Set channel y to DC coupling Set the y range to allow for voltages up to 10 V e Switch electronics to Measure Reset In case one does not obtain a stable locked signal especially in case the signal runs off and remains either at 10 V or at 10 V one should press Measure Reset again The precise adjustmen
19. t Autoreset Kanal 1 4 Normaler Prompt Erscheint nach Ausf hrung eines Befehls Fehler Prompt Erscheint nach fehlerhaften Eingabe Zuordnung der Baudrateneinstellungen 300 Baud 1200 Baud 2400 Baud 4800 Baud 9600 Baud 19200 Baud 38400 Baud 76800 Baud
20. t of the OFFset is of great importance for obtaining a stable locked state It might be necessary to readjust the OFFset manually according to the procedure described in chapter 3 5 3 J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com s J licher SQUID GmbH 17 Sensortechnik Fig 19 Typical Oscillogram of the Magnetometer signal in the Measurement mode flux locked loop during operation in unshielded environment Usually the ambient 50 Hz power line noise strongly dominates the observed signal in unshielded environment Switching the oscilloscope to Line Trigger one obtains a quasi standing picture 3 7 Adjusting the Feedback Loop Parameters The controls for the Feedback Loop Parameters are also found on the software control panel The value of the integrators capacitor and of the feedback resistor may be adjusted separately for each SQUID sensor In short the integrator s capacitor determines the velocity of the feedback loop A larger capacitance leads to a slower feedback velocity and thus to a smaller slew rate and bandwidth of the feedback loop The feedback resistor determines how the feedback voltage 10 V translates into the feedback current and thus determines the dynamic range of the feedback loop A smaller feedback resistor gives a smaller voltage to flux coefficient and thus a larger number of
21. ted The tank circuit is inductively coupled to the SQUID The amplitude lI of the bias current applied to the tank circuit is regulated by a VCA voltage controlled attenuator If the bias current of the tank circuit is increased the voltage drop U across the tank circuit is also increased If the current I is sufficiently high J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com if J licher SQUID GmbH a Sensortechnik to cause the first flux jump then U does not rise further A plateau in the Url characteristics is formed because the energy needed to change the magnetic flux is delivered by the tank circuit AU OFFset adjustment ext n VCA adjustment I lee Fig 2 U 1 characteristics of an rf SQUID Depending on the externally applied flux this point is reached at different values of the current I point A and B in Fig 2 and the plateau is formed at different values of U In order to be able to further process this voltage electronically it is necessary to choose the amplitude of the bias current such that the emerging voltage difference AU according to the external applied flux becomes maximum This can be achieved if the amplitude I of the bias current is adjusted so that the U l characteristic line takes the shape of a plateau for any external flux e g in
22. ved values will be automatically activated upon the next power up 3 9 Quitting and Warming up The execution of the JSQ SQUID sensor Software is ended by simply pressing the Quit button or by closing the window There is no Message Box reminding to save changes in J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com y J licher SQUID GmbH 19 Sensortechnik the settings of the SQUID parameters Unsaved working point and feedback loop parameters are lost However the factory settings in conjunction with the Fast Adjust upon power up and warm up usually suffice to find the working point again The JSQ SQUID capsule is not sealed Therefore special care has to be taken when taking the SQUID out of the liquid nitrogen The SQUID should be carefully warmed in the warm air of a blowdryer Check with your hand that the air at the SQUID location is not too hot otherwise the SQUID may be damaged Condensing water on the outside of the SQUID capsule should be wiped off when the SQUID is warming up J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com 20 i J licher SQUID GmbH Sensortechnik 4 Troubleshooting during SQUID Operation 4 1 Radio Frequency Interference A well known problem i
23. ving within the earth field is less important because normally the sensors are not moving during operation Instead they are securely positioned and fixed in a small hole dug into the ground Biomagnetic diagnostics especially of the heart functions and the brain need the highest sensitivity but use a small frequency range 0 01 Hz up to 300 Hz With a sensitivity limit below 30 fT VHz our new HTS rf SQUIDs are exceptionally well suited for cardiac measurements Even Fetal Magnetocardiography has been shown feasible with our SQUIDs J licher SQUID GmbH JSQ Lankenstr 29 D 52428 J lich Germany Phone 49 24 01 60 54 87 Fax 49 24 61 61 46 73 E mail JSQ jsquid com www jsquid com fy J licher SQUID GmbH 23 Sensortechnik Products SQUIDs by JSQ are very easy to handle radiofrequency rf driven sensors for extremely low magnetic fields present limit 20 fT VHz and magnetic field gradients lt 1 pT cm Hz These sensors are very fast Therefore a realtime measurement of rapidly changing magnetic fields up to 1 MHz becomes possible The sensor material is superconducting YBaCuO grown in thin films by a patented process During operation the sensors have to be cooled by liquid nitrogen or cryocoolers Due to the relatively high operation temperature 198 C compared to Low Temperature Superconductors LTS this material is called a High Temperature Superconductor HTS The sensors are fabricate
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