WO2017128706A1

WO2017128706A1 - Readout circuit and method for direct-reading flux modulation

Info

Publication number: WO2017128706A1
Application number: PCT/CN2016/096327
Authority: WO
Inventors: 荣亮亮; 张懿; 蒋坤; 王永良; 伍俊; 谢晓明
Original assignee: 中国科学院上海微系统与信息技术研究所
Priority date: 2016-01-29
Filing date: 2016-08-23
Publication date: 2017-08-03
Also published as: CN105676152A

Abstract

A readout circuit (2) for direct-reading flux modulation comprises a SQUID (21); a pre-amplifier (22) configured to amplify an output signal of the SQUID (21); a high-pass filter (23) configured to filter out a direct current component and low-frequency noise; a modulation/demodulation signal generator (24); a demodulator (25) configured to demodulate an output signal of the high-pass filter (23); an integrator (26) configured to integrate and generate a response voltage signal; and a feedback module (27) configured to feed back the response voltage signal to the SQUID (21). Also provided is a readout method for direct-reading flux modulation. In the method, modulation is used to enable an operating point of the SQUID (21) to jump between two operating points having opposite variation trends, and a tested signal is amplified, high-pass filtered, demodulated, integrated, and then fed back to the SQUID (21), so as to lock the operating point. The circuit (2) of the present invention realizes separation of low-frequency noise and output direct current bias of the pre-amplifier (22) by using the high-pass filter (23); avoids decrease in the SQUID flux-voltage conversion coefficient due to load effect; does not generate higher harmonic distortion on a SQUID flux-voltage curve; avoids thermal noise issues of transformers; and boasts a simplified circuit structure and higher practicability.

Description

Direct reading magnetic flux modulation readout circuit and method

Technical field

The present invention relates to the field of magnetic sensor technology, and in particular, to a direct reading flux modulation readout circuit and method.

Background technique

Superconducting Quantum Interference Device (SQUID) is a highly sensitive magnetic sensor that can be used to construct superconducting magnetic sensors. It is widely used in the field of weak magnetic field detection such as biological magnetic field, earth magnetic field anomaly, very low field nuclear magnetic resonance and geophysical exploration. Its detection sensitivity has reached the level of Feite (10 ^-15 Tesla). SQUID magnetic sensor is an important magnetic sensor device for extreme detection and scientific research, and has high scientific research and application value.

In practical applications, SQUID works in the low frequency band (less than 10Hz), and the low frequency noise performance is an important performance indicator that determines the application effect. The low frequency noise of the preamplifier and the critical current fluctuation of the SQUID chip are the main sources of noise for the low frequency noise of the SQUID magnetic sensor, which seriously restricts the application of SQUID in the field of low frequency magnetic measurement (especially for biomagnetic detection and geophysical exploration applications). In response to this problem, there are currently international patents for the low frequency noise suppression of SQUID magnetic sensors:

In the prior art, a method for reducing low frequency noise of a magnetic sensor based on a magnetic flux modulation technique is disclosed. The method modulates a low frequency signal to be measured at a high frequency by adding a higher frequency modulation magnetic flux to the sensor (generally about 100 kHz). Therefore, the low frequency noise segment of the preamplifier is avoided to achieve low frequency noise suppression; at the same time, the method can also effectively suppress the in-phase low frequency noise introduced by the critical current fluctuation (especially for the high temperature SQUID). In order to achieve good impedance matching to reduce preamplifier noise, a transformer is introduced here between the SQUID and the preamplifier. As shown in FIG. 1, the magnetic flux modulation circuit 1 for implementing the method includes a SQUID device 11, a transformer 12, a preamplifier 13, a demodulator 14, an integrator 15, a modem signal generator 16, a feedback resistor Rf, Feedback coil Lf and resistance. The SQUID device 11 is connected to the primary side of the transformer 12 via a resistor, and the secondary output of the transformer 12 is connected to the preamplifier 13. The output of the preamplifier 13 is connected to the demodulator 14 for demodulation, and the demodulated output is output. After the integration integrator 15 is integrated, the magnetic flux is fed back to the SQUID device 11 through the feedback resistor Rf and the feedback coil Lf, thereby ensuring that the SQUID flux operating point does not change, and the signal modulating the magnetic flux is generated by the modem signal generator 16. After being attenuated by the resistor, the feedback coil Lf is also sent to modulate the magnetic flux operating point of the SQUID, and the modem signal generator 16 also generates a synchronous demodulated signal to be sent to the demodulator 14. The introduction of transformer 12 has three negative effects: (1) the SQUID output voltage signal amplitude is reduced (transformer source side load); (2) the high frequency harmonic components of the SQUID output signal are filtered out, resulting in the operating point

(3) The transformer itself has thermal noise. Therefore, the noise of the flux modulation readout circuit is not better than that of the direct readout readout circuit (especially for the large βc SQUID chip). In addition, the introduction of the transformer 12 also leads to The circuit structure is more complicated and the usability is degraded.

Summary of the invention

In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a direct-reading magnetic flux modulation readout circuit and method for solving the thermal noise and signal amplitude reduction brought about by the introduction of a transformer in the prior art. At the working point

Become smaller and other issues.

To achieve the above and other related objects, the present invention provides a direct reading flux modulation readout circuit, the direct read flux modulation readout circuit comprising at least:

a SQUID device for detecting a measured magnetic flux signal and converting it into a corresponding electrical signal output;

a preamplifier, connected to the SQUID device, for amplifying an electrical signal output by the SQUID device;

a high pass filter coupled to the preamplifier for filtering DC output and low frequency noise of the preamplifier output;

a modem signal generator for generating a flux modulation signal and a magnetic flux demodulation signal;

a demodulator connected to the high pass filter and the modulation and demodulation signal generator, and demodulating an output signal of the high pass filter according to the magnetic flux demodulation signal;

An integrator coupled to the demodulator, integrating an output signal of the demodulator, and outputting a response voltage signal proportional to the measured magnetic flux signal;

And a feedback module connected to the integrator and the modulation and demodulation signal generator, and the response voltage signal is modulated by the magnetic flux modulation signal and fed back to the SQUID device to lock the operating point.

Preferably, the high pass filter comprises a capacitor connected between the preamplifier and the demodulator, and a resistor connected at one end to the capacitor output and grounded at the other end.

Preferably, the feedback module includes a feedback resistor and a feedback coil; one end of the feedback resistor is connected to the output end of the integrator, and the other end is connected to the feedback coil; the other end of the feedback coil is grounded, the feedback resistor And the magnetic flux modulation signal is connected between the feedback coils.

More preferably, the flux modulation signal is a square wave signal having a duty cycle of 50%.

To achieve the above and other related objects, the present invention provides a readout method for the above-described direct-reading magnetic flux modulation readout circuit, and the direct-reading magnetic flux modulation readout method includes at least:

The working point of the SQUID device is set to jump between the first working point and the second working point by a magnetic flux modulation signal, and the changing trend of the first working point and the second working point is opposite, and the detected The measured magnetic flux signal is converted into a corresponding electrical signal, and the electrical signal is amplified and subjected to high-pass filtering to filter out low-frequency noise, and isolate the DC amount, and then demodulate and integrate The SQUID device is then fed back to form a SQUID flux lock loop to lock the operating point.

Preferably, the first working point is different from the second working point by a half cycle.

As described above, the direct reading type magnetic flux modulation readout circuit and method of the present invention have the following beneficial effects:

1. SQUID device and readout circuit are directly connected to realize flux modulation, and noise suppression is superior; modulation frequency selection is wider, which is beneficial to signal-to-noise ratio improvement; overcomes the load effect of traditional flux modulation circuit transformer, SQUID flux voltage The curve has no attenuation.

2. A high-pass filter is introduced after the output of the preamplifier to ensure that the output bias voltage and low frequency noise of the preamplifier are isolated while the signal passes.

3, to avoid the use of transformers, the choice of flux modulation frequency range is wider, for low-frequency signal acquisition, you can use a lower modulation frequency, to avoid the impact of switching transient effects on flux modulation; avoid the heat of the transformer Noise problem.

4. The SQUID device is matched with the preamplifier, which overcomes the bandwidth limitation problem of SQUID and transformer matching in the traditional flux modulation circuit, no loss of higher harmonic components, and less distortion of SQUID flux-voltage curve.

DRAWINGS

Fig. 1 is a schematic view showing the structure of a magnetic flux modulating circuit in the prior art.

2 is a schematic view showing the structure of a direct reading type magnetic flux modulation readout circuit of the present invention.

Figure 3 shows a schematic diagram of a typical SQUID flux-voltage transfer characteristic.

4 is a schematic view showing the principle of the direct reading type magnetic flux modulation reading method of the present invention.

Component label description

1 flux modulation circuit

11 SQUID device

12 transformer

13 preamplifier

14 demodulator

15 integrator

16 modem signal generator

2 direct reading flux modulation readout circuit

21 SQUID device

22 preamplifier

23 high pass filter

24 modem signal generator

25 demodulator

26 integrator

27 feedback module

detailed description

The embodiments of the present invention are described below by way of specific examples, and those skilled in the art can readily understand other advantages and effects of the present invention from the disclosure of the present disclosure. The present invention may be embodied or applied in various other specific embodiments, and various modifications and changes can be made without departing from the spirit and scope of the invention.

Please refer to Figure 2 to Figure 4. It should be noted that the illustrations provided in the present embodiment merely illustrate the basic concept of the present invention in a schematic manner, and only the components related to the present invention are shown in the drawings, instead of the number and shape of components in actual implementation. Dimensional drawing, the actual type of implementation of each component's type, number and proportion can be a random change, and its component layout can be more complicated.

As shown in FIG. 2, the present invention provides a direct-reading magnetic flux modulation readout circuit 2, and the direct-reading magnetic flux modulation readout circuit 2 includes at least:

SQUID device 21, preamplifier 22, high pass filter 23, modem signal generator 24, demodulator 25, integrator 26, feedback module 27.

As shown in FIG. 2, the SQUID device is used to detect a measured magnetic flux signal and convert it into a corresponding electrical signal output.

Specifically, the SQUID device is a superconducting ring formed by two superconducting Josephson junctions connected in parallel, and leads are terminated at both ends of the Josephson junction, and a certain bias current is applied, and the voltage across the SQUID device is induced. The characteristics of the magnetic field change, the typical SQUID flux-voltage transmission characteristic curve is shown in Figure 3. One end of the SQUID device is grounded, and the other end is connected to the input end of the preamplifier 22, and the detected measured magnetic flux signal is converted into a corresponding electrical signal and output to the preamplifier 22.

As shown in FIG. 2, the preamplifier 22 is connected to the SQUID device to amplify an electrical signal output by the SQUID device 21.

Specifically, the preamplifier 22 has a high input impedance, and the SQUID flux-voltage conversion coefficient is not reduced by the load effect, and the preamplifier 22 has a wide bandwidth and does not have a SQUID flux-voltage curve. Produces high harmonic distortion.

As shown in FIG. 2, the high pass filter 23 is connected to the preamplifier 22 for filtering out the direct current and low frequency noise output by the preamplifier 22.

Specifically, the specific structure of the high-pass filter 23 is not limited, and any circuit structure capable of realizing the high-pass filtering function is included. In this embodiment, the high pass filter 23 includes a capacitor C and a first resistor R1 connected between an output end of the preamplifier 22 and an input end of the demodulator 25; One end of the first resistor R1 is connected between the capacitor C and the demodulator 25, and the other end is grounded.

As shown in FIG. 2, the modulation and demodulation signal generator 24 is used to generate a magnetic flux modulation signal and a magnetic flux demodulation signal.

Specifically, the modulation and demodulation signal generator 24 respectively generates a magnetic flux modulation signal and a magnetic flux demodulation signal corresponding to the magnetic flux modulation signal. In this embodiment, the magnetic flux modulation signal is a square wave signal with a duty ratio of 50%, so that the operating point jumps between the first working point and the second working point, wherein the first working point and The trend of change in the second working point is reversed.

As shown in FIG. 2, the demodulator 25 is connected to the high pass filter 23 and the modulation and demodulation signal generator 24, and outputs an output signal of the high pass filter 23 according to the magnetic flux demodulation signal 24. Perform demodulation.

As shown in FIG. 2, the integrator 26 is connected to the demodulator 25, integrates an output signal of the demodulator 25, and outputs a response voltage signal Vf proportional to the measured magnetic flux signal. .

As shown in FIG. 2, the feedback module 27 is connected to the integrator 26 and the modulation and demodulation signal generator 24, and the response voltage signal Vf is modulated by the magnetic flux modulation signal and fed back to the SQUID. Device 21, thereby locking the operating point.

Specifically, the feedback module 27 includes a feedback resistor Rf and a feedback coil Lf, one end of the feedback resistor Rf is connected to the output end of the integrator 26, and the other end is connected to the feedback coil Lf, and the feedback coil Lf is another One end is grounded, and the magnetic flux modulation signal is connected between the feedback resistor Rf and the feedback coil Lf. The response voltage signal Vf and the magnetic flux modulation signal respectively generate a feedback current through the feedback resistor Rf and the second resistor R2, and the feedback current generates a mutual inductance Mf with the SQUID device 21 through the feedback coil Lf, and the mutual inductance Mf cancels the measured flux signal so that the entire negative feedback loop is stable and the operating point is locked.

As shown in FIGS. 2 to 4, the above-described direct reading type magnetic flux modulation readout circuit 2 operates as follows:

The working point of the SQUID device is set to jump between the first working point and the second working point by a magnetic flux modulation signal, and the changing trend of the first working point and the second working point is opposite, and the detected The magnetic flux signal is converted into a corresponding electrical signal, and the electrical signal is amplified and subjected to high-pass filtering to filter out low-frequency noise, and the DC amount is isolated, and then demodulated, integrated, and fed back to the SQUID device to form a SQUID flux lock. The loop thus locks the working point.

Specifically, as shown in FIG. 4, the magnetic flux modulation signal is a square wave signal with a duty ratio of 50%, and its high level and low power are Conversely, the amplitude of the flat is modulated by the flux modulation signal, and the operating point of the SQUID device jumps between the two operating points, the first operating point W+ and the second operating point W. The measured magnetic flux signal detected by the SQUID device 21 is converted into a corresponding electrical signal, and the preamplifier 22 amplifies the electrical signal. The low frequency noise in the output signal of the preamplifier 22 is then filtered by the high pass filter 23 and the amount of DC is isolated. The signal output from the high pass filter 23 is then demodulated by the demodulator 25. The signal output by the demodulator 25 is then integrated by the integrator 26. The signal is fed back to the SQUID device 21 by the feedback module 27 to form a SQUID flux lock loop to lock the operating point. When the direct-reading flux modulation readout circuit 2 is in the locked state, a small change in the measured magnetic flux signal causes a slight change in the SQUID operating point, as shown in FIG. 4, and when the SQUID operates in two, respectively. When different working points (first working point W+ and second working point W-), and the changing trend of the two working points is opposite, such small changes just cancel each other out. In this embodiment, the change trend of the first working point W+ is upward, and the change trend of the second working point W- is downward. In order to ensure that the slopes of the first operating point W+ and the second operating point W-change are the same, the first operating point W+ and the second operating point W- are on the SQUID flux-voltage characteristic curve. The difference is half a cycle; more specifically, in the embodiment, the first operating point W+ and the second operating point W- are located at an intermediate point between the peak and the trough.

In summary, the present invention provides a direct reading flux modulation readout circuit comprising: a SQUID device for detecting a measured magnetic flux signal; a preamplifier for amplifying an output signal of the SQUID device; filtering a DC amount and a high-pass filter for low-frequency noise; a modem signal generator for generating a flux modulation signal and a magnetic flux demodulation signal; a demodulator for demodulating an output signal of the high-pass filter according to the magnetic flux demodulation signal; The output signal of the device is integrated to generate an integrator that responds to the voltage signal; and a feedback module that modulates the response voltage signal through the flux modulation signal and feeds back to the SQUID device. The working point of the SQUID device is set to jump between the first working point and the second working point by a magnetic flux modulation signal, and the changing trend of the first working point and the second working point is opposite, and the detected Measuring the magnetic flux signal into a corresponding electrical signal, After the electrical signal is amplified, high-pass filtering is performed to filter out the low-frequency noise, and the DC amount is isolated, and then demodulated, integrated, and fed back to the SQUID device to form a SQUID flux lock loop to lock the operating point. The direct reading flux modulation readout circuit and method of the present invention achieves isolation of the preamplifier low frequency noise and output DC bias by a high pass filter. And by adjusting the device parameters, a good match between the noise and impedance of the SQUID and the preamplifier can be achieved. The direct-reading magnetic flux modulation readout circuit and method of the present invention avoids the use of a transformer compared to the conventional magnetic flux modulation circuit and method, and achieves suppression of low-frequency noise by magnetic flux modulation. Due to the high input impedance of the preamplifier, the SQUID flux-to-voltage conversion factor is not reduced by the load effect; the preamplifier has a wide bandwidth and does not generate high harmonic distortion to the SQUID flux voltage curve; The thermal noise problem of the transformer. In general, the direct-reading magnetic flux modulation readout circuit and method of the present invention completely solves the negative factors introduced by the transformer in the conventional magnetic flux modulation circuit, and the circuit structure is simpler and more practical. Therefore, the present invention effectively overcomes various shortcomings in the prior art and has high industrial utilization value.

The above-described embodiments are merely illustrative of the principles of the invention and its effects, and are not intended to limit the invention. Modifications or variations of the above-described embodiments may be made by those skilled in the art without departing from the spirit and scope of the invention. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and scope of the invention will be covered by the appended claims.

Claims

A direct-reading magnetic flux modulation readout circuit, characterized in that the direct-reading magnetic flux modulation readout circuit comprises at least:

a SQUID device for detecting a measured magnetic flux signal and converting it into a corresponding electrical signal output;

a preamplifier, connected to the SQUID device, for amplifying an electrical signal output by the SQUID device;

a high pass filter coupled to the preamplifier for filtering DC output and low frequency noise of the preamplifier output;

a modem signal generator for generating a flux modulation signal and a magnetic flux demodulation signal;

a demodulator connected to the high pass filter and the modulation and demodulation signal generator, and demodulating an output signal of the high pass filter according to the magnetic flux demodulation signal;

An integrator coupled to the demodulator, integrating an output signal of the demodulator, and outputting a response voltage signal proportional to the measured magnetic flux signal;

And a feedback module connected to the integrator and the modulation and demodulation signal generator, and the response voltage signal is modulated by the magnetic flux modulation signal and fed back to the SQUID device to lock the operating point.
A direct reading flux modulation readout circuit according to claim 1, wherein said high pass filter comprises a capacitor connected between said preamplifier and said demodulator, and one end is connected to said The resistor at the output of the capacitor and the other end is grounded.
The direct reading flux modulation readout circuit according to claim 1, wherein the feedback module comprises a feedback resistor and a feedback coil; one end of the feedback resistor is connected to the output end of the integrator, and the other end is connected. The feedback coil; the other end of the feedback coil is grounded, and the magnetic flux modulation signal is connected between the feedback resistor and the feedback coil.
The direct-reading magnetic flux modulation readout circuit according to claim 1 or 3, wherein the magnetic flux modulation signal is a square wave signal having a duty ratio of 50%.
A method for reading a direct-reading magnetic flux modulation readout circuit according to any one of claims 1 to 4, wherein the direct-reading magnetic flux modulation readout method comprises at least:

The working point of the SQUID device is set to jump between the first working point and the second working point by a magnetic flux modulation signal, and the changing trend of the first working point and the second working point is opposite, and the detected The magnetic flux signal is converted into a corresponding electrical signal, and the electrical signal is amplified and subjected to high-pass filtering to filter out low-frequency noise, and the DC amount is isolated, and then demodulated, integrated, and fed back to the SQUID device to form a SQUID flux lock. The loop thus locks the working point.
The direct reading flux modulation readout method according to claim 5, wherein the first operating point is different from the second operating point by a half cycle.