CN103389482A - Digitalized simulator for SQUID (Superconducting QUantum Interference Device) - Google Patents

Abstract

The invention relates to a digital simulator of a superconducting quantum interferometer, which is characterized in that the electrical characteristic simulation of the SQUID is realized at normal temperature through an ADC, a microprocessor and a DAC digital circuit; the simulator adopts an embedded system architecture, through The way of analog-to-digital conversion converts the feedback signal of the readout circuit according to the online update SQUID characteristic parameter library established in the microcontroller for magnetic flux conversion, and then performs algebraic calculation with the built-in test magnetic flux signal, and finally according to the characteristic curve based on SQUIDV-Φ The established mathematical model is used for feedback output, so that the hardware-in-the-loop simulation of SQUID with different characteristics in the flux-locked loop readout circuit can be realized on the same platform. The invention greatly improves the integration, flexibility, versatility and range of the SQUID simulator, and effectively simplifies the test of the SQUID readout circuit.

Description

A Digital Simulator of Superconducting Quantum Interferometer

technical field

The invention relates to a simulator of a superconducting quantum interferometer, in particular to a simulation of the electrical characteristics of a superconducting quantum interferometer through a digital circuit, and by modifying the relevant parameters of the simulator through a serial port, different superconducting quantum interferometer hardware can be completed. A device for ring simulation. It belongs to the technical field of superconducting applications.

Background technique

Superconducting Quantum Interference Device (SQUID: Superconducting Quantum Interference Device) is currently the most sensitive magnetic sensor known. It has many applications in the fields of biomagnetism, geophysics and low-field nuclear magnetic resonance. It is mainly used for the detection of extremely weak magnetic fields. During the development and testing of the low-temperature DC SQUID readout circuit, the SQUID itself has problems such as long preparation time, high test cost, and the output signal is susceptible to electromagnetic interference. Relying on imports greatly affects the promotion and application of SQUID.

The development of a superconducting quantum interferometer simulator is currently a very practical method for debugging the readout circuit and testing the SQUID system at room temperature without connecting to the SQUID device. It can analyze the circuit characteristics of the SQUID readout circuit, such as magnetic The slew rate and bandwidth can greatly reduce the dependence on the scarce resource liquid helium, especially in the initial stage of system development. The known SQUID simulators are implemented by using analog circuits based on signal generators and adders but with low integration and poor flexibility.

"Simulation of SQUID Readout Circuit and Design of Debugging Circuit" (vol37, 2008) published in "Rare Metal Materials and Engineering" discloses a SQUID simulator based on analog devices such as inverters and adders. SQUID reads out the mathematical model of the circuit to simulate the circuit behavior, and divides the circuit into modules, establishes the mathematical model of each module, analyzes the key parameters affecting the performance of the circuit through simulation, and then guides the design and implementation of the specific circuit of the SQUID readout circuit. The development of a debugging circuit is introduced, which can realize the debugging of the flux-locked readout circuit based on AC modulation at room temperature. Although the simulator proposed in this paper can simulate the electrical characteristics of the superconducting quantum interferometer, it needs the cooperation of an external signal generator, and the integration level is not high; moreover, the hardware circuit needs to be modified for SQUID with different characteristics, so the flexibility is low. In addition, the simulator is only aimed at the flux-locked readout circuit of AC modulation, which is not suitable for direct-reading, and its versatility is not high; and it can only simulate the change of magnetic flux within a _Φ0 through a simplified model, and the range and accuracy are also limited. not enough.

As another example, the article "Using SQUID Signal Simulator to Measure the Slew Rate and Frequency Response of Superconducting Magnetometer" in Volume 7 of "Data Acquisition and Processing" published a method using an adder, an absolute value circuit and The SQUID simulator realized by analog circuits such as triangular wave/sine wave conversion has the advantage of being equal to the open-loop gain of a real superconducting magnetometer, but its basic working principle is the same as "Simulation of SQUID Readout Circuit and Design of Debugging Circuit" similar, so it will not be repeated here.

To sum up, the existing superconducting quantum interferometer simulator not only has problems such as low integration and low flexibility, but also has problems such as low versatility and insufficient range accuracy, which greatly affects the performance of superconducting quantum interferometers. It is widely used and promoted in the fields of industry, scientific research and medical treatment.

Contents of the invention

In order to overcome the existing SQUID simulator parameter adjustment is cumbersome and requires the cooperation of external signal generators, etc., the purpose of the present invention is to provide a superconducting quantum interferometer simulator realized by digital circuits, modify the simulator through the serial port The relevant parameters can complete the hardware-in-the-loop simulation of different superconducting quantum interferometers. The device not only has high integration and flexibility, but also can be read out in two different flux-locked loops of the flux modulation type and the direct-reading type through software. In the circuit, the simulation of multiple Φ ₀ magnetic flux changes is realized strictly according to the V-Ф characteristic of SQUID.

The technical solution adopted by the present invention to solve its technical problems is: the V-Ф characteristic of SQUID is nonlinear and periodic (period is one magnetic flux quantum Ф ₀ ), the voltage change is only dozens of uV, and it is necessary to adopt a flux-locked loop based on FLL (Flux-Locked Loop) readout circuit to achieve linear, high-precision flux-voltage conversion. There are two types of flux locked loop readout circuits: flux modulation and direct reading. The difference is that the flux modulation adds a modulation and demodulation circuit and a transformer for noise matching on the basis of the direct reading. The present invention is mainly aimed at the direct-reading readout circuit, but it is also applicable to the magnetic flux modulation type, except that the simulator needs to demodulate after collecting the feedback signal of the magnetic flux modulation in the FLL, and then output it according to the V-Ф characteristic of the SQUID The output signal needs to be modulated.

The flux-locked loop is mainly composed of a SQUID, a front-end amplifier, a bias regulator, an analog integrator, a feedback resistor and a feedback coil, wherein the SQUID is connected to the front-end amplifier through a low-temperature cable, and then the output of the front-end amplifier is sequentially connected in series to bias regulator, analog integrator, and feedback resistor, and finally connect the feedback resistor to the feedback coil of the SQUID.

The SQUID digital simulator proposed by the present invention adopts an embedded system architecture, and is mainly composed of three parts: an analog-to-digital converter (ADC), a microprocessor, and a digital-to-analog converter (DAC). The feedback signal of the circuit is converted into magnetic flux according to the SQUID characteristic parameter library that can be updated online in the microcontroller, and then algebraically calculated with the built-in test magnetic flux signal, and finally based on the mathematical model established based on the SQUID V-Φ characteristic curve. Feedback output, so as to realize the hardware-in-the-loop simulation of SQUID with different characteristics in the flux-locked loop readout circuit on the same platform.

The ADC and its drive circuit in the described digital simulator complete the digital conversion of the flux-locked loop output analog signal; the microprocessor then needs to complete three tasks in addition to realizing the control of the ADC, DAC and their auxiliary devices. 1. Collect the SQUID flux-locked loop and read out the signal output after the circuit is locked. According to the ratio of the mutual inductance coefficient of the SQUID feedback coil in the circuit to the resistance value of the feedback resistor, calculate the magnetic communication between the flux-locked loop and the SQUID superconducting loop. Second, carry out algebraic operation on the test magnetic flux signal built in the microprocessor and the magnetic flux signal obtained in task one, and the result is based on the V-Φ curve model established by the actual working characteristics of SQUID or the approximate SQUID V-Φ curve The transfer function obtains the corresponding voltage value; thirdly, the slew rate of the DAC output signal is controlled at the initial stage of flux locking to achieve the purpose of soft start and prevent the flux locked loop circuit from losing lock due to insufficient slew rate; the DAC is completed by Analog conversion of digital negative feedback signal obtained by SQUID V-Φ characteristic curve. In order to obtain a better signal-to-noise ratio of the magnetic compensation signal and reduce the influence of the direct output signal of the DAC on the SQUID, a low-pass filter for signal smoothing and a follower for impedance matching can be connected in series at the output of the DAC as required device.

In this process, the mutual inductance coefficient of the SQUID feedback coil solidified in the microprocessor and the V-Φ curve model established according to the actual working characteristics of the SQUID or the transfer function of the approximate SQUID V-Φ characteristic curve can be completed by downloading through the serial port. The electrical characteristic simulation of the type SQUID; similarly modifying the feedback resistor resistance value solidified in the microprocessor can complete the electrical characteristic simulation of the flux-locked loop at different feedback resistors without changing the actual circuit.

To sum up, the present invention aims at the problems that SQUID itself has long preparation time, high test cost, and the output signal is susceptible to electromagnetic interference during the development and testing process of the superconducting quantum interference device (SQUID: Superconducting QUantumInterference Device) readout circuit. The above-mentioned digital simulator realizes the electrical characteristic simulation of SQUID at room temperature through ADC, microprocessor, DAC and other digital circuits. The digital simulator adopts an embedded system architecture, and converts the feedback signal of the readout circuit according to the online updateable SQUID characteristic parameter library established inside the microcontroller by means of analog-to-digital conversion, and then converts the magnetic flux with the built-in test magnetic The algebraic operation is performed on the signal, and finally the feedback output is performed according to the mathematical model established based on the SQUID V-Φ characteristic curve, so as to realize the hardware-in-the-loop simulation of SQUID with different characteristics in the flux-locked loop readout circuit on the same platform. The present invention can realize the digital simulation of SQUID electrical characteristics, and can conveniently realize the hardware-in-loop simulation of SQUIDs with different characteristics and the feedback resistance of the readout circuit through the serial port download mode without an external signal generator, and can accurately Simulating multiple Φ ₀ magnetic flux changes greatly improves the integration, flexibility, versatility and range of the SQUID simulator, and effectively simplifies the development and testing of the SQUID readout circuit.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Figure 1 is a flux locked loop readout circuit.

Figure 2 is a schematic diagram of the SQUID digital simulator.

Fig. 3 is a schematic diagram of SQUID hardware-in-the-loop simulation.

Figure 4 is the SQUID V-Ф characteristic curve.

In the figure 1. SQUID, 2. Front-end amplifier, 3. Bias regulator, 4. Analog integrator, 5. Flux locked loop working state control switch, 6. Flux locked loop feedback resistor, 7. SQUID Feedback coil, 8. ADC, 9. Microprocessor, 10. DAC, 11. Smoothing filter and follower, 12. Computer.

Detailed ways

In order to make the purpose, specific solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

SQUID1 in the flux-locked loop readout circuit in Fig. 1 is the object to be simulated in the present invention. The device noise of SQUID1 is more than an order of magnitude lower than that of the front-end amplifier 2. To give full play to the advantages of SQUID1 in magnetic field measurement accuracy and resolution, it is necessary to match the noise of the two. For the flux-locked loop readout circuit shown in Figure 1, there are currently two main methods: flux modulation and direct-reading, in which the flux modulation mainly increases a turns ratio between SQUID1 and front-end amplifier 2 It is realized by the transformer of 25~30, and the direct reading type is realized by means of Noise Cancellation. If the flux-locked loop readout circuit in Figure 1 is divided into two parts: the low-temperature superconducting magnetic sensor that needs to be simulated and the normal-temperature readout circuit that needs to be developed, then the difference between the above two is that the flux modulation method is increased on the basis of the direct-reading method. Modulation and demodulation circuits and transformers for noise matching. The present invention is mainly described for the direct-reading readout circuit, but it is also applicable to the magnetic flux modulation type, except that the ADC8 of the simulator needs to demodulate after collecting the feedback signal of the magnetic flux modulation in the FLL, and then according to the V of SQUID1 The output signal needs to be modulated before the output of -Ф characteristic.

The flux-locked loop readout circuit shown in Figure 1 is used to complete the measurement of the magnetic signal to be measured, and its main structure is as follows: SQUID1 placed in Dewar liquid helium is connected to the front-end amplifier 2 through a low-temperature cable, wherein SQUID1 has Two working modes: current bias and voltage bias, the present invention adopts the voltage bias mode, and the front-end amplifier 2 selects an inverse amplifier with a gain of 80-100dB, and its output will have a DC voltage offset due to the electrical characteristics of SQUID1 Therefore, the output of the front-end amplifier 2 is connected to the bias regulator 3 based on the adder to eliminate the DC voltage offset; the output of the bias regulator 3 is connected to the key circuit analog integrator 4 that forms the PID negative feedback , the analog integrator 4 can be used to adjust the PID negative feedback through its time constant, and also includes a reset function and a bypass function for SQUID1 to work in the Tune state; the output of the analog integrator 4 is connected to the magnetic flux locked loop working state control switch 5 connection, which controls the traditional flux-locked loop readout circuit shown in Figure 1 to work in Tune or Lock state. In the Lock state, the flux-locked loop working state control switch 5 is connected to the SQUID Feedback coil 7 through the flux-locked loop feedback resistor 6 in series, and transmits the flux feedback signal of the flux-locked loop to SQUID1 in a coupled manner. In the Tune state, by adjusting the operating parameters of SQUID1 (bias voltage, amplifier gain and other parameters), the output signal amplitude of the analog integrator 4 is maximized in the case of bypass, so as to achieve the best operating point.

As shown in Figure 2, the SQUID digital simulator is mainly composed of ADC8, microprocessor 9, DAC10, smoothing filter and follower 11 in series. In order to better illustrate the functions and specific connections of its main components, the present invention intends to replace SQUID1 and its affiliated SQUID Feedback of the flux-locked loop readout circuit shown in Figure 1 with the SQUID digital simulator shown in Figure 2 Coil 7 is described in detail, that is, the SQUID hardware-in-the-loop simulation shown in FIG. 3 .

As shown in Figure 3, the flux-locked loop feedback resistor 6 in the flux-locked loop readout circuit in Figure 1 is no longer connected to the SQUID Feedback coil 7, but is connected to the ADC8 and its drive circuit in the SQUID digital simulator in Figure 2. ADC8 mainly realizes the digital conversion of the analog signal output by the flux-locked loop. It adopts a single-ended to differential mode to suppress common-mode interference, and sets different input ranges to meet different test requirements. Here, ADC8 chooses resolution and sampling rate. All moderate 16-bit successive approximation types, but when the signal bandwidth and slew rate are small, you can consider choosing a Delta-Sigma type ADC with high resolution but low sampling rate.

Microprocessor 9 (DSP, FPGA and other processors) is connected to ADC8 through a standard serial bus or parallel bus such as SPI, and reads the collected data Data read from ADC8 through digital communication according to the magnetic flux locked loop in Figure 1. Calculate the ratio of the mutual inductance coefficient Mf of the SQUID Feedback coil 7 in the circuit to the resistance value Rf of the flux-locked loop feedback resistor 6, and calculate the flux signal Φf fed back from the flux-locked loop to the SQUID superconducting loop, where Φf=Data*Mf /Rf, then sample the test magnetic flux signal Φt built in the microprocessor according to the sampling rate of ADC8, and then perform algebraic operation with the feedback magnetic flux signal Φf, the result Φr=Φt–Φf, and finally according to the actual working characteristics of SQUID The established V-Φ curve model or the transfer function that approximates the SQUID V-Φ curve obtains the corresponding output voltage value Ve, where the former uses the periodicity of the SQUID V-Φ curve to establish a periodic parameter model, and then uses the look-up table method Perform fast output, and then the present invention selects the latter for illustration; Fig. 4 is a kind of SQUIDV-Ф characteristic curve adopting the Noise Cancellation direct reading mode, and is also the most common SQUIDV-Ф characteristic curve, and its approximate transfer function is a cosine function: Vo =-Acos(2∏*Φ _a /Φ ₀ ), where Vo is the output voltage, A is half of the signal peak-to-peak value in the SQUID V-Ф characteristic curve, Φ _a is the input magnetic flux signal, and Φ ₀ is a magnetic flux quantum. It can be seen that in the SQUID hardware-in-the-loop simulation in Figure 3, the corresponding output voltage Ve=-Acos(2∏*Φ _r /Φ ₀ ) is obtained from the transfer function of the approximate SQUIDV-Φ curve.

For online modification of the characteristic parameters of the SQUID digital simulator in Figure 2, the microprocessor 9 is connected to the computer 12 through an isolated serial port, so the mutual inductance coefficient Mf and the approximate SQUID V of the SQUID feedback coil 7 solidified in the microprocessor 9 can be modified by downloading through the serial port - The transfer function parameters of the Φ characteristic curve can complete the electrical characteristic simulation of different types of SQUIDs, and the resistance value Rf of the flux-locked loop feedback resistor 6 solidified in the microprocessor 9 can also be modified without replacing the actual circuit. Complete the simulation of the electrical characteristics of the flux-locked loop at different feedback resistors 6 .

The microprocessor 9 is connected to the DAC10 through a standard serial bus or parallel bus such as SPI, wherein the resolution and sampling rate of the DAC10 should be consistent with that of the ADC8. In Figure 2, the SQUID digital simulator also has a resolution of 16 bits, mainly completing The analog conversion of the output voltage value Ve obtained by the transfer function of the approximate SQUID V-Φ curve, in addition, the microprocessor 9 also needs to control the slew rate of the DAC10 output signal at the initial stage of the magnetic flux lock, so as to realize the purpose of soft start and prevent the occurrence of The flux locked loop circuit loses lock due to insufficient slew rate. In order to obtain better signal-to-noise ratio of the magnetic compensation signal and reduce the influence of the DAC10 output signal pair, a smoothing filter and a follower 11 for signal smoothing and impedance matching are connected in series at the output end of the DAC10;

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (6)

1. a digital simulator of a superconducting quantum interferometer is characterized in that the electrical characteristic simulation of SQUID is realized at normal temperatures by ADC, microprocessor and DAC digital circuits; In the way of digital conversion, the feedback signal of the readout circuit is converted into magnetic flux according to the online update SQUID characteristic parameter library established inside the microcontroller, and then algebraically calculated with the built-in test magnetic flux signal, and finally based on the SQUID V-Φ characteristic curve The established mathematical model is used for feedback output, so that the hardware-in-the-loop simulation of SQUID with different characteristics in the flux-locked loop readout circuit can be realized on the same platform. 2. digital simulator according to claim 1, is characterized in that: gather the output signal of magnetic flux locked ring by ADC, under the condition of input built-in test magnetic flux signal, carry out relevant algebraic operation to both by microprocessor , and then convert the magnetic flux to the voltage according to the V-Ф characteristic curve of the SQUID, and finally output the converted voltage by the DAC to complete the simulation of the SQUID. 3. digital simulator according to claim 1, is characterized in that: revise the mathematical model parameter of the Feedback coil mutual inductance coefficient involved in the simulation SQUID, flux-locked loop feedback resistance and V-Ф characteristic curve by serial port download mode, realize Load the SQUID to be simulated online. 4. The digital simulator according to claim 1, characterized in that said SQUID digital simulator is formed in series successively by ADC, microprocessor, DAC, smoothing filter and follower. 5. digitization simulator according to claim 4, it is characterized in that microprocessor is connected with ADC by the serial bus of SPI standard or parallel bus, and with the acquisition data Data that reads from ADC by digital communication, magnetic The ratio of the mutual inductance coefficient Mf of the SQUID Feedback coil in the readout circuit of the flux-locked loop to the resistance value Rf of the feedback resistor of the flux-locked loop is used to calculate the flux signal Φf fed back from the flux-locked loop to the SQUID superconducting loop, where Φf=Data *Mf/Rf, and then sample the test magnetic flux signal Φt built in the microprocessor according to the sampling rate of the ADC, and then perform algebraic operations with the feedback magnetic flux signal Φf, the result Φr=Φt–Φf, and finally according to the actual SQUID The V-Φ curve model established by the working characteristics or the transfer function of the approximate SQUID V-Φ curve can obtain the corresponding output voltage value Ve. The former uses the periodicity of the SQUID V-Φ curve to establish a periodic parameter model, and then uses the look-up table to output quickly. 6. digital simulator according to claim 5, it is characterized in that the characteristic parameter of revising SQUID digital simulator on-line is that microprocessor is connected with computer through isolation serial port, revises the SQUID feedback solidified in microprocessor by serial port download The mutual inductance coefficient Mf of the coil and the transfer function parameters of the approximate SQUID V-Φ characteristic curve can complete the electrical characteristic simulation of different types of SQUID; similarly modify the resistance value Rf of the feedback resistor of the flux-locked loop solidified in the microprocessor. The simulation of the electrical characteristics of the flux-locked loop at different feedback resistors is completed without replacing the actual circuit.

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