Wideband magnetic field sensor

Abstract

This application relates to the field of geophysical exploration technology, specifically to a broadband magnetic field sensor. The sensor includes a sensing unit, a first signal processing channel, a second signal processing channel, a programmable switching switch, a chopper clock generator, and a timing control circuit. The first channel uses a chopper modulator, amplifier, and chopper demodulator to process low-frequency signals; the second channel uses a broadband low-noise amplifier to process high-frequency signals. The timing control circuit receives a mode selection command, first enabling or disabling the chopper clock generator, delaying for a predetermined time, and then driving the programmable switching switch to select the corresponding channel. This invention achieves broadband detection through the same sensing unit and employs a "chopper operation first, channel switching later" timing control, effectively suppressing switching transient interference and chopper clock crosstalk. It has the advantages of wide bandwidth, low noise, low power consumption, and high reliability, and can be widely applied to magnetotelluric, audio-frequency magnetotelluric, and controlled-source audio-frequency magnetotelluric exploration methods.

Description

Technical Field

[0001] This application relates to the field of geophysical exploration technology, specifically to a broadband magnetic field sensor. Background Technology

[0002] The ground electromagnetic method is a geophysical method that uses electromagnetic fields as a source to detect the resistivity structure of underground surfaces. Its working principle is to calculate the impedance tensor by simultaneously measuring the mutually orthogonal electric and magnetic field components on the surface to invert the underground electrical structure.

[0003] Depending on the operating frequency band, ground electromagnetic methods are mainly divided into magnetotelluric (MT) and audio-frequency magnetotelluric / controlled-source audio-frequency magnetotelluric (AMT / CSAMT). Magnetotelluric methods utilize natural electromagnetic fields to detect deep targets (typical frequency band: 10). -4 The frequency range is 0.1 Hz to 10³ Hz, while audio magnetotellurics and controlled-source audio magnetotellurics are used to detect shallow targets (typical frequency band: 0.1 Hz to 10 kHz). All three methods place high demands on the sensitivity, noise level, and bandwidth of the magnetic field sensor, requiring the magnetic field sensor to have the key characteristics of high sensitivity, low noise, and wide bandwidth.

[0004] Conventional inductive magnetic field sensors cannot simultaneously operate in both high-frequency and low-frequency bands, typically requiring dedicated sensors designed for each band. That is, sensors suitable for magnetotellurics and those suitable for audio-frequency magnetotellurics / controlled-source audio-frequency magnetotellurics are two separate types of sensors, each with different performance parameters. During field exploration, when broadband detection is required, operators need to frequently change magnetic field sensors, which is not only time-consuming and labor-intensive but also affects detection accuracy and data reliability.

[0005] Existing technologies have attempted to combine low-frequency and high-frequency circuits simply by switching them together. However, low-frequency signals are significantly affected by 1 / f noise, requiring chopping amplification techniques for suppression, while high-frequency signals require wideband, low-noise amplification. The circuit design, noise characteristics, and signal processing methods for these two types of signals differ fundamentally. Furthermore, the gain of low-frequency circuits is typically fixed, while the gain and bandwidth of high-frequency circuits need to be adjusted according to the different frequencies of the signal. Simple combinations cannot meet the requirements of both, and the switching itself may introduce signal interference and noise, affecting the stability and accuracy of the system.

[0006] Therefore, how to design a wideband low-noise magnetic field sensor that can simultaneously meet the requirements of low-frequency and high-frequency detection and can smoothly switch between the two modes is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] In view of this, embodiments of this application are intended to provide a broadband magnetic field sensor.

[0008] This application provides a broadband magnetic field sensor, comprising:

[0009] The sensing unit is used to sense external magnetic fields and output corresponding electrical signals. The first signal processing channel has its input end coupled to the output end of the sensing unit through a switching unit. The first signal processing channel includes a chopper modulator, an amplifier, and a chopper demodulator, and is used to process signals in the first frequency band. The second signal processing channel has its input terminal coupled to the output terminal of the sensing unit through the switching unit. The second signal processing channel includes a broadband low-noise amplifier for processing signals in a second frequency band, wherein the frequency of the second frequency band is higher than that of the first frequency band. A programmable switching switch is connected to the sensing unit, the first signal processing channel, and the second signal processing channel respectively, and is used to control the sensing unit to communicate with the first signal processing channel or the second signal processing channel; A programmable switching switch, the common terminal of which is connected to the output terminal of the sensing unit, the first contact of which is connected to the first signal processing channel, and the second contact of which is connected to the second signal processing channel; A chopper clock generator is used to generate a chopper clock signal and provide it to the first signal processing channel; The timing control circuit receives mode selection instructions at its input terminal, its first output terminal is connected to the enable terminal of the chopper clock generator, and its second output terminal is connected to the drive terminal of the programmable switching switch. The timing control circuit is configured to: when receiving an instruction to select the first signal processing channel, first enable the chopper clock generator through the first output terminal, and after a predetermined delay, drive the programmable switch through the second output terminal to select the first signal processing channel; when receiving an instruction to select the second signal processing channel, first disable the chopper clock generator through the first output terminal, and after a predetermined delay, drive the programmable switch through the second output terminal to select the second signal processing channel.

[0010] Optionally, it also includes: A common amplifier, whose input is coupled to the output of the first signal processing channel and the second signal processing channel, is used to amplify and output the signal of the selected channel.

[0011] Optionally, it also includes: The feedback coil is coupled to the output of the common amplifier through a feedback resistor. The feedback coil is coupled to the induction coil in the induction unit to form a magnetic flux negative feedback.

[0012] Optionally, the chopper clock generator is a controlled NOT gate ring oscillator.

[0013] Optionally, the NOT gate ring oscillator includes an RC timing network, wherein the resistors in the RC timing network include a first resistor and a second resistor connected in series, the first resistor and the second resistor having opposite temperature coefficients of resistance.

[0014] Optionally, the timing control circuit includes a delay circuit for generating a delay of the predetermined time.

[0015] Optionally, the relay is a magnetic latching relay.

[0016] Optionally, the gain adjustment terminal of the common amplifier is connected to a first variable resistor for adjusting the gain of the common amplifier.

[0017] Optionally, a second variable resistor is connected in series in the feedback loop connected to the feedback coil to adjust the feedback depth.

[0018] Optionally, the sensing unit includes a magnetic core and an induction coil wound on the magnetic core, wherein the aspect ratio of the magnetic core is 50 to 100.

[0019] The solution provided in this application optimizes the processing of low-frequency MT signals and high-frequency AMT / CSAMT signals respectively by setting two parallel signal processing channels on the same sensing unit. This enables the use of a single sensor to cover an ultra-wide bandwidth, eliminating the need to replace sensors in the field and significantly improving exploration efficiency and data consistency. The timing control circuit implements a control logic of "first enabling / disabling the chopper clock generator, then driving the relay after a predetermined delay," effectively avoiding the impact of relay contact jitter on the signal path, ensuring the smoothness of the switching process, and solving the technical problem of output jumps and long recovery times caused by channel switching transients. In high-frequency mode, the timing control circuit disables the chopper clock generator output, keeping the chopper switch stationary. This reduces system power consumption and fundamentally eliminates crosstalk from the chopper clock and its harmonics to weak high-frequency signals, solving the technical problem of ripple and crosstalk introduced by the chopper clock in high-frequency mode. Attached Figure Description

[0020] The above and other objects, features, and advantages of this application will become more apparent from the more detailed description of the embodiments of this application in conjunction with the accompanying drawings. The drawings are provided to further illustrate the embodiments of this application and form part of the specification. They are used together with the embodiments of this application to explain this application and do not constitute a limitation thereof. In the drawings, the same reference numerals generally represent the same components or steps.

[0021] Figure 1 This is a schematic diagram of the structure of a broadband magnetic field sensor provided in one embodiment of this application; Figure 2 This is a schematic diagram illustrating the timing control and chopper signal generation principle provided in one embodiment of this application; Figure 3 This is a circuit diagram of a timing control and chopper signal generator provided in one embodiment of this application; Figure 4 This is a partial circuit diagram of a first signal processing channel and a second signal processing channel provided in one embodiment of this application; Figure 5 This is a schematic diagram of the amplification, filtering, demodulation, and filtering output circuit in the first signal processing channel of this application; Figure 6 This is a circuit diagram of the common amplifier section in this application; Figure 7 These are measured resistivity curves and impedance phase curves obtained from field exploration using the magnetic field sensor described in this application. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] like Figure 1 As shown, this embodiment provides a broadband magnetic field sensor, including: a sensing unit 100, a programmable switching switch 200, a first signal processing channel 300, a second signal processing channel 400, a chopper clock generator 500, a timing control circuit 600, a common amplifier 700, and a feedback coil 800.

[0024] The sensing unit is used to sense an external magnetic field and output a corresponding electrical signal. Specifically, the sensing unit includes a magnetic core and an induction coil wound on the magnetic core. When an external alternating magnetic field passes through the magnetic core, an induced voltage is generated at both ends of the induction coil, which is the electrical signal output by the sensing unit.

[0025] The first signal processing channel is used to process signals in the first frequency band (e.g., low-frequency signals). Its input is coupled to the output of the sensing unit via a switching unit (such as a programmable switch). The first signal processing channel includes a chopper modulator, an amplifier, and a chopper demodulator (see the specific circuit diagram). Figure 5 The chopper modulator modulates the weak low-frequency input signal to near the chopping frequency, converting it into an AC signal; the amplifier amplifies this AC signal; and the chopper demodulator demodulates the amplified signal back to the original baseband, thus recovering the low-frequency signal. Through this chopper amplification principle, the impact of low-frequency 1 / f noise on signal quality can be effectively suppressed.

[0026] The second signal processing channel is used to process signals in a second frequency band (e.g., high-frequency signals), where the frequency of the second frequency band is higher than that of the first frequency band. The input of the second signal processing channel is also coupled to the output of the sensing unit via the switching unit (e.g., a programmable switch). The second signal processing channel includes a broadband low-noise amplifier for directly amplifying the high-frequency signal output by the sensing unit. Since the 1 / f noise in the high-frequency band is not significant, chopping amplification technology is not required.

[0027] The programmable switch is the core component for channel selection. The programmable switch is connected to the sensing unit, the first signal processing channel, and the second signal processing channel, respectively, and is used to control the connection between the sensing unit and either the first signal processing channel or the second signal processing channel. Specifically, the programmable switch may internally include multiple selection switches. Each line of the sensing unit that needs to connect to the first signal processing channel and the second signal processing channel can be connected through a selection switch. The common terminal of the selection switch is connected to an output terminal of the sensing unit, its first contact is connected to the corresponding input terminal of the first signal processing channel, and its second contact is connected to the corresponding input terminal of the second signal processing channel. The programmable switch is used to selectively connect the sensing unit to the first signal processing channel or the second signal processing channel according to an external control signal. The programmable switch can be any device capable of channel switching controlled by an electrical signal, including but not limited to electromagnetic relays (such as magnetic latching relays), solid-state relays, CMOS analog switches, or transmission gates. In a specific embodiment...

[0028] Reference Figure 3The chopper clock generator is used to generate a chopper clock signal and provide it to the first signal processing channel. Specifically, the output of the chopper clock generator is connected to the control terminals of the chopper modulator and chopper demodulator in the first signal processing channel, providing them with a synchronized modulation and demodulation clock. The chopper clock generator has an enable terminal. When the enable terminal receives a valid level, the chopper clock generator starts working and outputs a chopper clock signal; when the enable terminal receives an invalid level, the chopper clock generator stops working and does not output a chopper clock signal.

[0029] The timing control circuit is the core module for controlling channel switching. Its input receives external mode selection commands (e.g., TTL level signals), its first output is connected to the enable terminal of a chopper clock generator, and its second output is connected to the drive terminal of a programmable switch. The timing control circuit is configured to perform channel switching operations according to a specific timing sequence to address the problems of relay contact bounce and chopper clock crosstalk in existing solutions.

[0030] Specifically, when the timing control circuit receives an instruction to select the first signal processing channel (i.e., to switch to low-frequency mode), it first outputs a valid level to the enable terminal of the chopper clock generator through its first output terminal, enabling the chopper clock generator to start outputting a stable chopper clock signal. Then, after a predetermined delay, the timing control circuit outputs a drive signal to the programmable switch driver terminal through its second output terminal, controlling the programmable switch to connect the output terminal of the sensing unit to the first signal processing channel. This timing control, which "enables the chopper clock first, then switches the programmable switch," ensures that the chopper clock has been stably established and the chopper modulator and demodulator have entered normal operating condition when the contacts in the programmable switch switch switch switch. This avoids the impact of contact jitter in the programmable switch switch on the signal channel establishment process.

[0031] When the timing control circuit receives an instruction to select the second signal processing channel (i.e., to switch to high-frequency mode), it first outputs an invalid level to the enable terminal of the chopper clock generator through its first output terminal, disabling the chopper clock generator and stopping it from outputting the chopper clock signal. The chopper modulator and demodulator then cease operation. After a predetermined delay, the timing control circuit then drives the programmable switch through its second output terminal, connecting the output of the sensing unit to the second signal processing channel. This timing control, which "disables the chopper clock first, then switches the programmable switch," ensures that the chopper clock has completely disappeared and the chopper switch is stationary when a high-frequency signal is input. This fundamentally eliminates crosstalk between the chopper clock and its harmonics and weak high-frequency signals, while also reducing system power consumption.

[0032] The aforementioned delay time is determined by the delay circuit inside the timing control circuit. By selecting appropriate resistor and / or capacitor parameters, a delay time (e.g., 1 to 10 milliseconds) that meets the requirements of the programmable switching switch operation can be set.

[0033] Through the above technical solution, the broadband magnetic field sensor of this embodiment can acquire low-frequency and high-frequency signals through the same sensing unit, and can smoothly switch between the two modes without changing the sensor. At the same time, it effectively suppresses switching transient interference and chopper clock crosstalk, ensuring signal quality and system stability.

[0034] Furthermore, the broadband magnetic field sensor in this embodiment also includes a common amplifier. The input terminal of this common amplifier is coupled to the output terminals of both the first and second signal processing channels, and is used to amplify the signal output from the currently selected channel before outputting it. Specifically, when the programmable switch selects the first signal processing channel, the common amplifier further amplifies the low-frequency signal after chopping and amplification; when the programmable switch selects the second signal processing channel, the common amplifier further amplifies the high-frequency signal after being amplified by a broadband low-noise amplifier. The output terminal of the common amplifier serves as the signal output terminal of the entire sensor.

[0035] Furthermore, to avoid mutual interference between the outputs of the first signal processing channel and the second signal processing channel, the solution provided in this application also includes a programmable switch. The outputs of both the first and second signal processing channels are connected to this programmable switch. The common terminal of the programmable switch is connected to the input terminal of a common amplifier. This programmable switch is also connected to the second output terminal of the timing control circuit, used to control the output of the first signal processing channel to connect to the common amplifier when the sensing unit is connected to the first signal processing channel, and to control the output of the second signal processing channel to connect to the common amplifier when the sensing unit is connected to the second signal processing channel.

[0036] To facilitate customized adjustment of sensor sensitivity according to different exploration needs or different sensing unit parameters, this embodiment connects a first variable resistor (e.g., a potentiometer or digital potentiometer) to the gain adjustment terminal of the common amplifier. This first variable resistor serves as the feedback resistor or gain setting resistor of the common amplifier, and its resistance value can be changed to continuously or incrementally adjust the voltage gain of the common amplifier.

[0037] Specifically, when the resistance of the first variable resistor increases, the gain of the common amplifier increases, and the overall sensitivity of the sensor increases; when the resistance decreases, the gain decreases, and the sensitivity decreases. This design allows for the production of sensor models with different sensitivities on the same hardware platform, or for fine-tuning based on the actual signal strength during field use, greatly improving the flexibility and adaptability of the product.

[0038] Furthermore, the broadband magnetic field sensor in this embodiment also includes a feedback coil. This feedback coil is coupled to the output of the common amplifier via a feedback resistor, and is coupled to the induction coil in the sensing unit, forming a magnetic flux negative feedback structure. Specifically, the voltage signal output by the common amplifier is converted into a feedback current through the feedback resistor, and this feedback current flows through the feedback coil to generate a feedback magnetic field. The winding direction of the feedback coil is opposite to that of the induction coil, therefore the direction of the feedback magnetic field is opposite to the direction of the measured magnetic field. The net magnetic field actually acting on the sensing unit is the difference between the measured magnetic field and the feedback magnetic field.

[0039] Through the aforementioned flux negative feedback structure, the broadband magnetic field sensor of this embodiment can obtain flat amplitude-frequency characteristics and stable sensitivity over a wide frequency range. Specifically, the flux negative feedback makes the sensor's sensitivity approximately constant, and its low-frequency and high-frequency cutoff frequencies are determined by the feedback parameters, thereby achieving linear output within a specific operating frequency band.

[0040] Furthermore, the input terminals of the common amplifier are coupled to the output terminals of the first signal processing channel and the second signal processing channel, respectively. That is, the first input terminal of the common amplifier is connected to the output terminal of the first signal processing channel, and the second input terminal is connected to the output terminal of the second signal processing channel. The common amplifier is used to amplify the signal output by the currently selected channel and uses the amplified signal as the final output of the entire sensor.

[0041] Specifically, when the sensor operates in low-frequency mode, the programmable switch connects the sensing unit to the first signal processing channel. The low-frequency signal output from the first signal processing channel (which has undergone chopper modulation, AC amplification, and chopper demodulation) is sent to the common amplifier. When the sensor operates in high-frequency mode, the programmable switch connects the sensing unit to the second signal processing channel. The high-frequency signal output from the second signal processing channel (which has undergone broadband low-noise amplification) is sent to the common amplifier. The common amplifier further amplifies the input signal to meet the signal amplitude requirements of subsequent acquisition equipment. By setting the common amplifier, not only can the output signals of the two channels be uniformly gain-adjusted, but the output impedance can also be kept stable during channel switching, facilitating matching with external acquisition equipment.

[0042] Furthermore, the broadband magnetic field sensor in this embodiment also includes a feedback coil. This feedback coil is coupled to the output of the common amplifier through a feedback resistor, and the feedback coil is coupled to the induction coil in the sensing unit (for example, the feedback coil is coaxially wound on the inside or outside of the induction coil, forming a tightly coupled transformer structure), thereby forming a magnetic flux negative feedback loop.

[0043] The working principle of flux negative feedback is as follows: The voltage signal Vout output by the common amplifier passes through the feedback resistor R. fb Converted into feedback current I fb = V out / R fb The feedback current flows through the feedback coil, generating a feedback magnetic field proportional to the feedback current. Since the winding direction of the feedback coil is opposite to that of the induction coil, the direction of the feedback magnetic field is opposite to the direction of the measured magnetic field. Therefore, the net magnetic field actually acting on the magnetic core of the induction unit is the difference between the measured magnetic field and the feedback magnetic field.

[0044] By introducing magnetic flux negative feedback, this scheme achieves the following beneficial effects: First, it significantly broadens the operating bandwidth of the sensor, especially extending it to the low-frequency end; second, it eliminates the additional noise caused by relying solely on circuit compensation, thus improving the signal-to-noise ratio; third, it makes the sensor's sensitivity remain flat across a wide frequency band, and the amplitude-frequency characteristics are more stable; fourth, it improves the linearity and dynamic range of the sensor.

[0045] Furthermore, to adjust the strength of the magnetic flux negative feedback, this embodiment incorporates a second variable resistor connected in series in the feedback loop of the feedback coil. This second variable resistor, in series with the fixed feedback resistor, together determines the magnitude of the feedback current. By changing the value of the second variable resistor, the feedback coefficient can be continuously adjusted, thereby altering the strength of the feedback magnetic field.

[0046] Adjusting the feedback depth can affect several performance indicators of a sensor: a larger feedback depth results in a wider bandwidth and better linearity, but a corresponding decrease in sensitivity; a smaller feedback depth results in higher sensitivity, but may narrow the bandwidth and is prone to generating resonance peaks. By setting a second variable resistor, optimal matching can be achieved during sensor debugging or production based on the core material, coil parameters, and desired frequency response characteristics, enabling the sensor to reach its optimal operating state. The combined use of the first and second variable resistors allows for independent adjustment of sensitivity and bandwidth, greatly facilitating the serialization and development of products.

[0047] In some embodiments, the chopper clock generator is implemented using a controlled NOT gate ring oscillator. This NOT gate ring oscillator consists of an odd number of NOT gates (e.g., three NOT gates) connected in series to form a ring, and then connected to an RC timing network. The oscillator has an enable terminal (EN). When the enable terminal receives a valid level (e.g., a high level), the oscillator starts oscillating and outputs a stable square wave signal as the chopper clock; when the enable terminal receives an invalid level (e.g., a low level), the oscillator stops oscillating, and the output remains at a fixed level (high or low), without generating a clock signal.

[0048] The advantages of using a NOT gate ring oscillator are: the circuit structure is extremely simple, requiring only a few NOT gates and RC components, resulting in low cost; there is no need to use DSP, FPGA, or dedicated clock chips, avoiding high-frequency interference and additional power consumption caused by digital circuits; the oscillation frequency is determined by the RC time constant, making it easy to adjust; the enable control is direct, facilitating the implementation of "enable / disable first, then switch" timing logic in conjunction with timing control circuits.

[0049] Based on the aforementioned NOT gate ring oscillator, this embodiment optimizes the RC timing network design to further improve the stability of the oscillation frequency under conditions of drastic temperature changes in the field. Specifically, the resistors in the RC timing network include a first resistor and a second resistor connected in series, and the first resistor and the second resistor have opposite temperature coefficients of resistance.

[0050] For example, the first resistor is a positive temperature coefficient (PTC) resistor, whose resistance increases with temperature; the second resistor is a negative temperature coefficient (NTC) resistor, whose resistance decreases with temperature. The total resistance of the two resistors connected in series remains essentially constant within a certain temperature range, thus significantly reducing the influence of temperature on the RC time constant and effectively suppressing the drift of the chopper clock frequency. This simple passive compensation method significantly improves the reliability of the sensor in complex field environments without the need for complex temperature sensors and feedback control circuits.

[0051] In some embodiments, the timing control circuit includes a delay circuit for generating a delay of the predetermined time to ensure that sufficient circuit stabilization time is allowed after the chopper clock generator is enabled or disabled and before the programmable switching switch is activated.

[0052] (I) Basic Implementation Method - RC Delay Circuit In a preferred implementation, the delay circuit is an RC delay circuit. This RC delay circuit consists of a resistor and a capacitor connected in series: one end of the resistor is connected to the input terminal of the timing control circuit (receiving mode selection commands), the other end of the resistor is connected to one end of the capacitor, and the other end of the capacitor is grounded. The connection point of the resistor and capacitor serves as the delay output terminal, connected to the drive terminal of the programmable switch.

[0053] Its working principle is as follows: When the level of the mode selection command changes (e.g., from low to high, or from high to high), this change charges or discharges the capacitor through a resistor. The voltage across the capacitor changes exponentially, requiring a transition time determined by the time constant τ = R × C to reach or fall below the operating threshold voltage of the programmable switch. This transition time is the "predetermined time". By appropriately selecting the values ​​of R and C, the delay time (e.g., from 1 millisecond to 10 milliseconds) can be precisely set.

[0054] (II) Simplified Implementation Method – Pure Capacitor Delay Circuit In a simplified implementation, the delay circuit consists of only one capacitor, eliminating the need for a series resistor. Specifically, one end of the capacitor is connected to the input of the timing control circuit (receiving mode selection commands), and the other end is connected to the drive of the programmable switch.

[0055] The working principle of a pure capacitor delay circuit is as follows: When the level of the mode selection command changes (e.g., from low to high, or from high to low), the voltage across the capacitor cannot change abruptly. A new steady-state voltage needs to be gradually established through the charging or discharging process of the capacitor. This charging and discharging process itself generates an inherent delay, which is determined by the capacitance value and the equivalent resistance of the connected input / output ports (e.g., the internal resistance of the timing control circuit output and the input impedance of the programmable switch driver). By selecting an appropriate capacitance value, a delay time that meets the timing requirements of the programmable switch can be obtained.

[0056] Compared to RC delay circuits, pure capacitor delay circuits have the following advantages: First, they are lower in cost, eliminating the need for a resistor and further reducing material costs; second, they are simpler, requiring only one capacitor component and reducing PCB layout area; third, they are more reliable, with fewer components and fewer potential failure points; and fourth, they consume less power, as pure capacitor delay does not generate static power consumption on the resistor.

[0057] Whether using an RC delay circuit or a pure capacitor delay circuit, the core function is to generate a predetermined delay sufficient for the chopper clock generator to enable or disable and enter a stable state after the timing control circuit receives the mode selection command, and then trigger the programmable switching switch. This "chop first, then switch" timing logic is the key to solving the problems of switching transient interference and chopper clock crosstalk in this invention.

[0058] In some embodiments, the programmable switching switch used for channel switching is preferably a magnetic latching relay. A magnetic latching relay is a bistable relay: when a positive pulse current is applied to its drive coil, the relay contacts switch to a first state (e.g., the common terminal is connected to the first contact); when a reverse pulse current is applied, the contacts switch to a second state (the common terminal is connected to the second contact); and after the pulse ends, the contact state is maintained by an internal permanent magnet, requiring no continuous power supply.

[0059] The use of magnetic latching relays has the following advantages: First, the drive circuit can be completely de-energized after switching, maintaining zero power consumption and significantly reducing the overall power consumption of the sensor, which is especially important for field battery-powered exploration equipment; Second, since no continuous current is required, the interference of the magnetic field generated by the drive current on the sensing unit is avoided; Third, the contact on-resistance is extremely low (usually in the milliohm range), which has minimal impact on the attenuation of weak signals and noise contribution, ensuring high signal fidelity; Fourth, the action speed is fast (millisecond level), the switching process is crisp, and the jitter time is short.

[0060] In this embodiment, the sensing unit includes a magnetic core and an induction coil wound on the magnetic core. The magnetic core is made of a soft magnetic material with high permeability, typically including permalloy (such as 1J85), iron-based amorphous alloy, or nanocrystalline alloy. The magnetic core is rod-shaped and formed by stacking multiple long strip-shaped laminations, with insulating layers or air gaps between the laminations to reduce eddy current losses.

[0061] The aspect ratio (the ratio of length L to equivalent diameter d) of the magnetic core is designed to be between 50 and 100. This range is selected based on the following comprehensive considerations: a larger aspect ratio results in higher effective permeability of the magnetic core, a larger output voltage of the induction coil, and lower background noise of the sensor; however, if the aspect ratio is too large, the magnetic core is easily magnetized and saturated by the Earth's magnetic field, causing the operating point to deviate from the linear region and producing nonlinear distortion. When the aspect ratio is between 50 and 100, it is possible to obtain high effective permeability while ensuring that the magnetic core operates in the linear segment of the BH curve in the Earth's magnetic field environment, avoiding saturation. At the same time, this aspect ratio also takes into account the engineering feasibility of the overall sensor size.

[0062] The induction coil is uniformly wound around the outside of the magnetic core using precision enameled wire (such as QA-2 grade polyurethane enameled wire), with the magnetic core and coil isolated by an engineering plastic sheath. To reduce the distributed capacitance of the coil, segmented winding (dividing the total number of turns into several segments with gaps between segments) or quasi-random winding (randomly varying the winding angle) can be used. Reducing the distributed capacitance helps to increase the self-resonant frequency of the sensor, thereby extending the effective operating bandwidth at the high-frequency end. The number of turns, wire diameter, and number of layers of the coil are optimized according to the target frequency band and impedance matching requirements to balance sensitivity, noise, and bandwidth.

[0063] The solution provided in this application will be further described below with reference to the above preferred embodiments: Reference Figure 4 This solution integrates a low-noise circuit corresponding to high frequencies and a chopper amplifier circuit corresponding to low frequencies, forming a wideband low-noise amplifier circuit suitable for the sensing unit (magnetic core and coil) of an inductive magnetic field sensor. However, this integration is not simply a matter of splicing together traditional high-frequency and low-frequency circuits and switching them via a switch. The characteristics of low-frequency and high-frequency signals are very different, especially in terms of noise characteristics, gain requirements, frequency response, and circuit stability. They each have independent design requirements, and simple merging often introduces a lot of interference and cannot guarantee the accuracy and stability of signal processing.

[0064] First, low-frequency signals are typically heavily affected by 1 / f noise, which is quite significant in the low-frequency range. Therefore, conventional low-frequency amplifier circuit designs require the use of chopper amplification technology. This involves modulation and demodulation processes to shift the signal spectrum from the low-frequency band to a higher frequency, thereby avoiding 1 / f noise interference. This necessitates that low-frequency circuits possess high-precision chopper frequency control and noise suppression capabilities to ensure that the quality of the low-frequency signal is not severely compromised.

[0065] High-frequency signals, however, have different requirements. High-frequency signal processing is typically unaffected by 1 / f noise; the focus is more on high-frequency gain, bandwidth stability, and signal distortion. High-frequency circuit design requires low-noise amplifiers to ensure accurate signal amplification while avoiding any nonlinear distortion in the high-frequency bandwidth. Therefore, low noise, high gain, and wide bandwidth are key objectives in high-frequency amplifier circuit design, and these design goals are fundamentally different from those of low-frequency circuits.

[0066] Secondly, the transmission and processing of low-frequency and high-frequency signals involve different circuit topologies. Simply splicing them together will generate unnecessary circuit noise and signal interference during switching, thus affecting the overall system performance. Therefore, this solution introduces an intelligent switching mechanism that enables seamless switching between different frequency bands, ensuring the stability of both low-frequency and high-frequency circuits while avoiding signal aliasing and noise interference. Specifically, the intelligent switching mechanism uses a highly adaptable programmable switch. During switching, it adjusts the switching frequency and current path to avoid the circuit instability and noise problems caused by frequent switching in traditional methods.

[0067] Furthermore, this solution optimizes the design of certain circuit components. For example, referring to... Figure 5In the low-frequency circuit, precise control of the chopper amplifier's gain and feedback loop bandwidth prevents the accumulation of low-frequency noise and improves the sensitivity of low-frequency signals. In the high-frequency circuit, the gain-bandwidth characteristics of the low-noise amplifier are optimized, making signal processing more precise in the high-frequency range, while increasing bandwidth to meet wideband requirements. These optimizations of local circuits not only allow for smooth integration of low-frequency and high-frequency circuits but also ensure signal transmission stability and noise control across the entire system within a wide bandwidth.

[0068] In the design, the inputs and outputs of all local circuits are consistent with the overall circuit diagram, ensuring smooth signal transmission and coordinated circuit operation. This design philosophy ensures that the combination of low-frequency and high-frequency signals is not only technically feasible but also achieves efficient and stable performance in practical applications.

[0069] Therefore, although the circuit design appears to combine low-frequency and high-frequency circuits, this solution successfully addresses the technical challenges of combining the two through intelligent switching mechanisms, precise circuit optimization, and reasonable noise control, ensuring the reliability and efficiency of the broadband magnetic field sensor in practical applications.

[0070] The solution provided in this application employs a magnetic latching relay, ensuring that the channel state is maintained after switching to a high-frequency or low-frequency channel. To reduce overall circuit power consumption, the chopping frequency is only output to control the chopper switch when switching to a low-frequency channel; when switching to a high-frequency channel, the chopping frequency is not output, and the chopper switch does not operate, reducing power consumption and interference between channels. Similarly, to reduce power consumption and simplify circuit design, this solution does not use a DSP or FPGA to generate the chopping frequency signal; instead, it uses a NOT gate oscillator to generate a square wave output chopping frequency signal. A switching control signal is added to the traditional NOT gate oscillator to control whether the NOT gate oscillator starts oscillating.

[0071] like Figure 2 The diagram shows the channel switching control signal and the chopper amplification frequency signal generation circuit. The control signal is generated through... Figure 2 The C2 capacitor outputs the relay control signal. By selecting a suitable capacitor, a short delay can be achieved, allowing the chopper frequency signal to oscillate before the relay switches after the channel switching signal is input to the circuit. The chopper frequency signal is generated using a NOT gate oscillator circuit. The oscillation frequency is determined by the resistor and capacitor; by appropriately selecting the resistor and capacitor, the oscillation frequency can be adjusted to a suitable operating point. Notably, this scheme uses resistors with opposite temperature coefficients in series to reduce the impact of temperature changes on the oscillation frequency, achieving a stable oscillation frequency output. Figure 2R1 and R2 in the diagram are resistors with opposite temperature coefficients, used for temperature compensation to reduce chopping frequency changes caused by temperature variations. It should be noted that the circuit can be configured according to the above principles, but is not limited to the structure shown in the attached diagram.

[0072] Specifically, the sensing unit is a combination of a magnetic core, an induction coil, and a feedback coil, as described in patent CN121634288A.

[0073] First signal processing channel (i.e., low-frequency chopper preamplifier channel): Modulation / amplification / demodulation / low-pass + magnetic flux negative feedback interface; Second signal processing channel (i.e., high-frequency preamplifier channel): wideband low-noise amplification + filtering + magnetic flux negative feedback interface; Magnetic latching relays are used to select the channel output / or channel input connection method. The chopper frequency generation module (including: NOT gate oscillator, RC frequency controller and enable terminal) is used to output the chopper frequency.

[0074] In practical applications, if TTL=0 (i.e., when the instruction indicates that the first signal processing channel is selected): enter low-frequency mode → enable the NOT gate oscillator → output chopper frequency → delay Δt → drive the relay to switch to the low-frequency channel. If TTL=1 (i.e., when the instruction to select the second signal processing channel is given): enter high-frequency mode → turn off the oscillator / disable the output chopper frequency → relay switches to high-frequency channel; In high-frequency mode, the chopping frequency is not output, the switch does not operate, and the oscillator circuit does not work, which can reduce power consumption. Additionally, since the switch is not operating, there is no ripple or crosstalk introduced by the switch. Because the chopping frequency is generally high-frequency (3.5kHz and above) compared to low-frequency magnetic field sensors, and this sub-frequency band falls within the signal frequency range of the high-frequency band, interference from switch operation can affect the effective signal.

[0075] The high-frequency and low-frequency switching modes are controllable time-series switching. High-frequency and low-frequency switching can be achieved at any time by controlling the switch. This sensor mode is perfectly compatible with the broadband electromagnetic detection technology described in our unit's patent: CN113391358A A method and device for acquiring broadband magnetotelluric data, and has been widely used.

[0076] Furthermore, refer to Figure 6 The second signal processing channel (i.e., high-frequency CSAMT) and the first signal processing channel (i.e., low-frequency MT section) are combined at the input of the common amplifier through a switch. One end of the common amplifier is connected to a feedback resistor, which amplifies the signal and applies it to the feedback coil. The other end is connected to a filter and an output stage for amplification and output.

[0077] The circuit uses a sliding rheostat to adjust the gain of the amplifier after combining high and low frequencies, which can realize a sensor with customized sensitivity according to customized requirements.

[0078] A sliding rheostat is set in the feedback loop to adjust the feedback coefficient, so that magnetic field sensors with different customized requirements can achieve a stable feedback state and improve reliability.

[0079] In summary, the broadband magnetic field sensor provided in this application has a bandwidth of 5×10⁻⁶. -3 ~10kHz, capable of switching between high-frequency and low-frequency modes via TTL level control, eliminating the need for manual control or sensor replacement, enabling switching between audio magnetotellurics and magnetotellurics methods, and adaptable to broadband electromagnetic exploration applications, making it more flexible and efficient.

[0080] The sensor provided in this application, in conjunction with the broadband electromagnetic detection technology described in our team's patent CN113391358A, "A Method and Device for Acquiring Broadband Magnetotelluric Data," enables broadband magnetotelluric detection. Figure 7 In practical applications, the resistivity and impedance phase curves obtained from a single point of detection are shown in the figure. It can be seen from the figure that the apparent resistivity detection can cover 5 × 10⁻⁶. -3 Up to 10kHz.

[0081] The above description has been given for purposes of illustration and description. Furthermore, this description is not intended to limit the embodiments of this application to the forms disclosed herein. Although numerous exemplary aspects and embodiments have been discussed above, those skilled in the art will recognize certain variations, modifications, alterations, additions, and sub-combinations thereof.

Claims

1. A broadband magnetic field sensor, characterized in that, include: The sensing unit is used to sense external magnetic fields and output corresponding electrical signals. The first signal processing channel has its input end coupled to the output end of the sensing unit through a switching unit. The first signal processing channel includes a chopper modulator, an amplifier, and a chopper demodulator, and is used to process signals in the first frequency band. The second signal processing channel has its input terminal coupled to the output terminal of the sensing unit through the switching unit. The second signal processing channel includes a broadband low-noise amplifier for processing signals in a second frequency band, wherein the frequency of the second frequency band is higher than that of the first frequency band. A programmable switching switch is connected to the sensing unit, the first signal processing channel, and the second signal processing channel respectively, and is used to control the sensing unit to communicate with the first signal processing channel or the second signal processing channel; A programmable switching switch, the common terminal of which is connected to the output terminal of the sensing unit, the first contact of which is connected to the first signal processing channel, and the second contact of which is connected to the second signal processing channel; A chopper clock generator is used to generate a chopper clock signal and provide it to the first signal processing channel; The timing control circuit receives mode selection instructions at its input terminal, its first output terminal is connected to the enable terminal of the chopper clock generator, and its second output terminal is connected to the drive terminal of the programmable switching switch. The timing control circuit is configured to: when receiving an instruction to select the first signal processing channel, first enable the chopper clock generator through the first output terminal, delay for a predetermined time, and then drive the programmable switching switch through the second output terminal to select the first signal processing channel. When an instruction to select the second signal processing channel is received, the chopper clock generator is first disabled through the first output terminal, and after a predetermined delay, the programmable switching switch is driven through the second output terminal to select the second signal processing channel.

2. The broadband magnetic field sensor according to claim 1, characterized in that, Also includes: A common amplifier, whose input is coupled to the output of the first signal processing channel and the second signal processing channel, is used to amplify and output the signal of the selected channel.

3. The broadband magnetic field sensor according to claim 2, characterized in that, Also includes: The feedback coil is coupled to the output of the common amplifier through a feedback resistor. The feedback coil is coupled to the induction coil in the induction unit to form a magnetic flux negative feedback.

4. The broadband magnetic field sensor according to claim 1, characterized in that, The chopper clock generator is a controlled NOT gate ring oscillator.

5. The broadband magnetic field sensor according to claim 4, characterized in that, The NOT gate ring oscillator includes an RC timing network, wherein the resistors in the RC timing network include a first resistor and a second resistor connected in series, the first resistor and the second resistor having opposite temperature coefficients of resistance.

6. The broadband magnetic field sensor according to claim 1, characterized in that, The timing control circuit includes a delay circuit for generating the predetermined delay time.

7. The broadband magnetic field sensor according to claim 1, characterized in that, The relay is a magnetic latching relay.

8. The broadband magnetic field sensor according to claim 2, characterized in that, The gain adjustment terminal of the common amplifier is connected to a first variable resistor, which is used to adjust the gain of the common amplifier.

9. The broadband magnetic field sensor according to claim 3, characterized in that, A second variable resistor is connected in series in the feedback loop connected to the feedback coil to adjust the feedback depth.

10. The broadband magnetic field sensor according to claim 1, characterized in that, The sensing unit includes a magnetic core and an induction coil wound on the magnetic core, wherein the aspect ratio of the magnetic core is 50 to 100.

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