CN112782774A - Method for automatically tuning and quickly locking frequency - Google Patents
Method for automatically tuning and quickly locking frequency Download PDFInfo
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Abstract
The invention discloses a method for automatically tuning and quickly locking frequency, which adopts a brand-new automatic tuning mode, improves the adaptability of hardware, extracts the maximum effective frequency amplitude value and matches resonance frequency by acquiring the contrast of a background magnetic field signal and an actual magnetic field signal in a region with larger environmental interference without the help of a global magnetic field intensity map and through FFT (fast Fourier transform algorithm) operation, and can achieve the aim of quickly acquiring the magnetic field of the current region with strong anti-interference capability, high reliability and full-automatic maximum resonance amplitude.
Description
Technical Field
The invention relates to the technical field of geological weak magnetic field measurement, in particular to a method for automatically tuning and quickly locking frequency.
Background
The proton magnetometer is an instrument for measuring the earth magnetic field, protons in a magnetic probe convert magnetic field signals into frequency signals through Larmor precession, and the frequency value is measured during measurement. The earth magnetic field signal is very weak, so that in the measuring circuit, a multi-level capacitor is required to be introduced to form an RC tuning circuit to carry out resonance amplification on the amplitude of the magnetic field frequency signal, so that the subsequent circuit can acquire a reliable magnetic field frequency value at the maximum signal-to-noise ratio. The resonance circuit has resonance frequency, if the input frequency is the same as the resonance frequency, the resonance circuit can resonate, and the output of the input signal amplitude is enhanced by a certain multiple, and if the input frequency is different from the resonance frequency, the input signal is not enhanced but is greatly attenuated. In actual measurement, effective frequency signals need to be subjected to resonance amplification, and meanwhile, ineffective frequency signals need to be attenuated, so that the resonance frequency is particularly important, but in actual initial measurement, earth magnetic fields generated by different geological environments in different regions have differences, so that the capacitance in the tuning circuit needs to be adjusted according to the current magnetic field value to match the resonance frequency.
The traditional tuning mode has two types, one type is manual tuning, namely, the magnetic field value of the corresponding area is checked according to a global magnetic field intensity graph, the magnetic field value is manually input in an instrument, and the corresponding resonant frequency is matched. The other is automatic tuning, namely, the multi-gear tuning circuit is matched gear by gear, and then the signal intensity is detected through amplitude measurement to judge the optimal tuning frequency. Although the first mode can effectively match the resonant frequency under a certain condition, the resonant frequency can be effectively matched only by professional measuring personnel with a large amount of experience due to the overlarge area in a global magnetic field intensity map and the low fine degree of a position magnetic field, and the measurement efficiency and the measurement precision are influenced by the manual resonant frequency adjustment for many times for a measurement region with large magnetic field change.
In the second method, when the number of tuning steps is large, if step-by-step matching is required, a large amount of matching time is required, and particularly in a measurement environment with large magnetic field variation, multiple times of matching are required, so that a large amount of time is consumed for matching, and when the amplitude of the environmental interference frequency is large, matching errors are likely to occur, so that the value of the ineffective magnetic field is measured.
For example, patent application CN 201910578198-a system and method for improving tuning accuracy and signal-to-noise ratio of a proton precession sensor discloses a method for tuning a fast locking frequency, which uses a combination of a PCA algorithm and an SVD algorithm to perform principal component analysis and singular value decomposition processing on an inductive FID signal output by the proton precession sensor, so as to further suppress unknown noise and improve the signal-to-noise ratio of the FID signal; the final tuning center frequency is obtained by adopting a three-time tuning mode, however, a hysteresis comparison circuit and a narrow-band filter circuit are required to be arranged in a measurement calculation circuit of the system, the measurement method shown in fig. 3 is complex in calculation, high in requirements on the computing capability and modeling precision of a processor, large in calculation amount, and limited in improvement on the measurement precision and efficiency.
Therefore, it is urgently needed to design a frequency measurement and calculation method to overcome the defects of long automatic tuning time, large locking deviation and failure in effectively eliminating interference frequency, and to reduce the complexity and duration of calculation.
Disclosure of Invention
Technical problem to be solved
Based on the above, aiming at the defects of the prior art, the invention provides a method for automatically tuning and quickly locking frequency, which adopts a brand-new automatic tuning mode, extracts the maximum effective frequency amplitude value and matches the resonant frequency by acquiring the contrast of a background magnetic field signal and an actual magnetic field signal in an area with larger environmental interference without the help of a global magnetic field intensity map and through FFT operation, and achieves the aim of quickly acquiring the magnetic field of the current area with the maximum resonant amplitude, strong anti-interference capability, high reliability and full automation.
(II) technical scheme
In order to solve the technical problems, the invention adopts the main technical scheme that:
a method for automatically tuning and rapidly locking frequency is characterized in that a measuring loop of a proton magnetometer comprises a resonant circuit, a multistage amplifying circuit, an ADC (analog to digital converter) acquisition circuit, a shaping circuit, a frequency measuring circuit, an amplitude measuring circuit and an MCU (microprogrammed control unit) processor, wherein the resonant circuit is composed of a probe and a multi-gear tuning capacitor set, the resonant circuit is connected with the input end of the multistage amplifying circuit, the output end of the multistage amplifying circuit is respectively connected with the ADC acquisition circuit, the shaping circuit and the amplitude measuring circuit, the frequency measuring circuit is connected with the shaping circuit, the MCU processor is respectively connected with the frequency measuring circuit, the ADC acquisition circuit and the amplitude measuring circuit, and the method for automatically tuning and rapidly locking frequency by the MCU processor comprises the following steps 1-5:
step 1: when the probe is not polarized, the capacitance value of the multi-gear tuning capacitor bank is adjusted to be minimum so as to adjust the bandwidth of the tuning circuit to be maximum, then the background signal is acquired through the ADC acquisition circuit, and the amplitude is recorded;
step 2: and polarizing the probe, adjusting the capacitance value of the multi-gear tuning capacitor bank to be the minimum, and acquiring the signal through the ADC acquisition circuit again, wherein the difference is that the background signal is acquired for the first time, the background signal and the magnetic field signal are acquired at this time, and the amplitude is recorded.
And step 3: subtracting the acquired signals of the two ADC acquisition circuits to eliminate background signal interference, performing fast FFT operation on the subtracted signals to obtain a first amplitude net value, and calculating a spectrogram;
and 4, step 4: because different gear capacitance values of the multi-gear tuning capacitor bank correspond to different resonant frequencies, the maximum frequency of the amplitude in the calculated spectrogram is the effective magnetic field frequency, namely the tuning frequency, and therefore the tuning capacitance value is quickly set according to the value of the tuning frequency;
and 5: and after setting the capacitance value of the multi-gear tuning capacitor bank as a tuning capacitance value, polarizing again, finally measuring the magnetic field value, recording the amplitude, and obtaining a second amplitude net value, wherein the second amplitude net value is compared with the first amplitude net value, and if the change of the two amplitude net values is obviously increased, the frequency value is the current environment earth magnetic field value.
Furthermore, the probe is a specially-made proton magnetic field measuring probe, the proton magnetic field measuring probe and the multi-gear tuning capacitor bank form an RC resonant circuit, and the MCU processor can adjust the capacitance value and the switch signal of the multi-gear tuning capacitor bank so as to control the proton magnetic field measuring probe to polarize.
Furthermore, 8 groups of capacitor banks connected in parallel are adopted in the multi-gear tuning capacitor bank, and four-stage amplification is adopted in the multi-stage amplification circuit.
Further, the ADC acquisition circuit acquires the amplified signals by using the ADS1271, and the MCU processor adopts an STM32F4 series single chip microcomputer.
Further, step 1 specifically includes: in each measurement process, firstly, the capacitance value of the multi-gear tuning capacitor bank is adjusted to be minimum to adjust the bandwidth of the tuning circuit to be maximum, then, when the probe is not polarized, time domain waveform data after full-wave rectification and integration of the amplitude measuring circuit is collected through the multichannel ADC collecting circuit and stored, the collecting time is t, and the collected time domain waveform data is summed and divided by the number of sampling points to obtain a voltage amplitude V1.
Further, step 2 specifically includes: keeping the capacitance value of the multi-gear tuning capacitor bank unchanged, polarizing the probe, collecting time domain waveform data after full-wave rectification and integration of the amplitude measuring circuit after the first polarization of the probe is collected again through the multichannel ADC collecting circuit, storing the time domain waveform data, collecting the time domain waveform data with the collecting time t, summing the collected time domain waveform data, and dividing the summed time domain waveform data by the number of sampling points to obtain a voltage amplitude V2.
Further, step 3 specifically includes: and calculating a polarized first amplitude net value V1 ═ V2-V1, performing FFT operation on the time domain waveform data subjected to the first polarization and the filtering, and calculating a response signal spectrum subjected to the first polarization.
Further, the step 4 of rapidly setting the tuning capacitance value according to the tuning frequency specifically includes:
1) finding out a frequency value corresponding to the maximum amplitude value in the spectrogram through a bisection method;
2) and finding out a corresponding tuning capacitance value from a prestored frequency-capacitance value data table through a table look-up method, and adjusting the tuning capacitance to a corresponding gear.
Further, step 5 specifically includes: the magnetic field frequency is identified through a magnetic field frequency identification algorithm, the tuning capacitor is adjusted to a corresponding gear, the probe is polarized again, the probe is subjected to secondary polarization through a multi-channel high-speed AD acquisition for the third time, time domain waveform data after full-wave rectification and integration are obtained through an amplitude measuring circuit, the acquisition time is t, the acquired time domain waveform data are summed and divided by the number of sampling points to obtain a voltage amplitude V3, and a second amplitude net value V2' after the polarization is calculated to be V3-V1.
Further, step 5 further comprises: solving the amplitude change rate k which is V2 '/V1', and according to the sensitivity and practical effect of the frequency measurement circuit, when k is greater than N, achieving the purpose of tuning, wherein N is greater than or equal to 50; otherwise, the process is repeated and polarization is performed again.
(III) advantageous effects
Compared with the prior art, the method for automatically tuning and quickly locking the frequency has the following beneficial effects: .
1. Firstly, the hardware characteristics of each hardware submodule are effectively utilized, and the environmental interference magnetic field frequency can be effectively filtered through two times of ADC acquisition of background signals and actual magnetic field signals before and after polarization, so that the real magnetic field identification capability is effectively improved; performing fast FFT calculation after processing the two times of ADC acquired data, quickly calculating a signal frequency spectrum, accurately identifying the frequency of a magnetic field according to the maximum amplitude frequency, and quickly tuning a capacitance value to match a tuning frequency; comparing the three amplitude measurement amplitude records, whether the tuned circuit is matched or not can be effectively observed, and whether the tuned capacitor gear is matched with the actual magnetic field frequency or not can be effectively determined and is the true magnetic field frequency value; through polarization before polarization and after polarization and repolarization, the tuning frequency can be quickly and reliably obtained through the three processes, the time is effectively saved, the efficiency is improved, automatic tuning can be automatically carried out again in a region with large magnetic field change, the construction cost and the measurement time are greatly saved, and the stability and the reliability of data are ensured; in addition, the weak magnetic field signal is amplified in a multi-stage low-noise mode, the signal-to-noise ratio and the dynamic range are improved, and the reliable acquisition of the ADC and the stable measurement of the frequency are guaranteed.
2. In addition, the method of the invention is to continuously collect the magnetic field signals before and after polarization, and then to process the collected data to obtain the effective spectrogram, and then to quickly determine the tuning frequency. Experiments prove that under the condition that strong interference frequency exists outside and the amplitude of the interference frequency is larger than that of the actual magnetic field frequency, if the traditional automatic tuning gear-by-gear matching is carried out, the interference frequency can be matched, and after the method is adopted, the strong interference frequency can be directly weakened or eliminated after the collected signals before and after polarization are compared and processed, so that the effective magnetic field frequency is obtained. The method can stably and reliably obtain the actual magnetic field value of the current environment by only two times of collection, quickly lock the tuning capacitance value, neglect the limitation of the number of tuning capacitance groups, greatly shorten the locking time, have extremely low requirements on operators, and only need one-key operation, so that the method has strong advantages in both measurement efficiency and measurement data quality, and can ensure the measurement precision and rapidity.
3. Finally, the method has simple calculation mode and small calculation amount, does not need complex modeling and other work, can realize the scheme only by adding hardware such as a conventional shaping circuit, an amplitude measuring circuit and the like, and has low software and hardware improvement cost.
Drawings
The features and advantages of the present invention will be more clearly understood by reference to the accompanying drawings, which are illustrative and not to be construed as limiting the invention in any way, and in which:
fig. 1 is a system block diagram of an auto-tune fast lock frequency system of the present invention.
Fig. 2 is a block diagram of an example of a detailed design of an auto-tuning fast lock frequency system of the present invention.
Fig. 3 is a flow chart of the method of automatically tuning the fast lock frequency according to the present invention.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein, but rather should be construed as broadly as the present invention is capable of modification in various respects, all without departing from the spirit and scope of the present invention.
As shown in fig. 1-2, the system used in the method for automatically tuning the fast locking frequency of the present invention is improved based on the measurement loop of the original proton magnetometer, and the improved measurement loop of the proton magnetometer mainly comprises: the device comprises a resonant circuit, a multistage amplifying circuit, an ADC (analog to digital converter) acquisition circuit, a shaping circuit, a frequency measuring circuit, an amplitude measuring circuit and an MCU (microprogrammed control unit) processor, wherein the resonant circuit is formed by a probe and a multi-gear tuning capacitor bank, the resonant circuit is connected with the input end of the multistage amplifying circuit, the output end of the multistage amplifying circuit is connected with the ADC acquisition circuit, the shaping circuit and the amplitude measuring circuit, the frequency measuring circuit is connected with the shaping circuit, the MCU processor is connected with the frequency measuring circuit, the ADC acquisition circuit and the amplitude measuring circuit, and the MCU processor can adjust the capacitance value and the switching signal of the multi-gear tuning capacitor bank so as to control the proton magnetic field measuring probe to carry out.
The following is a functional description of each module:
1. probe head
The probe uses a special proton magnetic field measuring probe, the basic parameters of the probe are internal resistance of 10 omega, the inductance is 30mH, and after the polarization of a direct current power supply, internal protons generate weak single-frequency alternating current signals which are gradually attenuated by a Larmor precession effect.
2. Multi-gear tuning capacitor bank
The multi-gear tuning capacitor bank and the proton magnetic field measuring probe are connected to form a resonance circuit, and effective frequency can be resonantly amplified and ineffective frequency is attenuated by adjusting the capacitance value of the multi-gear tuning capacitor bank.
As shown in fig. 2, an RC resonant circuit is formed by a proton magnetic field measuring probe (referred to as "probe") and a multi-stage programmable tuning capacitor bank (referred to as "multi-stage tuning capacitor bank") to perform resonant amplification on the amplitude of the magnetic field frequency signal. Preferably, the capacitor bank adopts 8 capacitor banks, 256 gear capacitance values can be formed, and different resonant frequencies can be generated by selecting different capacitance values to combine with the probe. If the fine degree of the resonant frequency needs to be improved, the number of the capacitor sets can be increased, and the relationship between the number of the sets and the gear of the capacitance value is as follows: capacity value shift is 2n(n is the number of groups). The multi-stage tuning capacitor bank comprises S0-S7 switches, and S0 is polarizationAnd the switch controls the S0 to be connected with VCC during polarization, polarizes the probe, and after the polarization of the probe is finished, the switch controls the S0 to be connected with the multi-gear tuning capacitor bank to receive signals generated after polarization. The switches from S1 to S7 are position switches, and are not described in detail since many descriptions of the circuit are already available in the prior art.
3. Multi-stage amplifying circuit
The signal generated by the probe is weak, and is still small even though resonance occurs, so that in order to amplify the magnetic field weak voltage signal and improve the signal-to-noise ratio and the dynamic range, multistage amplification is needed, and the four-stage amplification is preferentially matched with a multi-stage tuning capacitor group with 256 gear capacitance values, and the amplification is 100000 times in total.
4. ADC acquisition circuit
The ADS1271 is used for collecting the amplified signals so as to collect voltage analog signals generated by a magnetic field, the ADC has 24-bit resolution, the highest sampling rate is 128ksps, and the analog signals can be collected at high precision.
5. Shaping circuit
The probe generates a single-frequency alternating current signal for a magnetic field, the amplified alternating current signal needs to be shaped for measuring the frequency, and a square wave signal is output after shaping. Since the self-comparison circuit and the Schmitt trigger are mature circuits, the description is omitted.
6. Amplitude measuring circuit
In order to measure the amplitude of the amplified signal, an amplitude measuring circuit is required to determine whether the measured signal is correct. The amplitude measuring circuit is used for carrying out full-wave rectification on an alternating current signal through an active amplifier and converting alternating current into direct current through an integrating circuit so as to collect the subsequent voltage amplitude.
7. Frequency measuring circuit
And inputting the square wave signal output by the shaping circuit into a frequency measuring circuit. Frequency measurement this example uses equal precision frequency measurement, uses CPLD as the carrier to perform high precision measurement of frequency, and outputs the measured value to the MCU processor. A frequency meter as in the prior art may also replace this frequency measurement circuit.
8. MCU processor
The MCU processor adopts an STM32F4 series single chip microcomputer to control all circuit parts and simultaneously collects data collected by an ADC, signal amplitude and signal frequency so as to effectively and automatically tune and quickly lock the frequency.
In order to plan the operation of each sub-hardware module and realize high-precision and quick frequency automatic locking, the automatic tuning and quick frequency locking method can be realized in the MCU processor. After the effective magnetic field signal amplitude is amplified by the resonance circuit after the probe is polarized, the amplitude needs to be amplified by the multistage amplification circuit because the signal amplitude is uV-level, and the amplified signal is finally intensively controlled and collected by a processor through different functional circuits, so that the resonance frequency is quickly matched. The amplified signal is processed in three aspects, the first aspect uses a high sampling rate ADC to collect an analog signal, the second aspect uses an amplitude measuring circuit to measure the amplitude of the analog signal and output an amplitude value, and the third aspect shapes the analog signal and outputs a square wave to measure the frequency of the signal.
For fast auto-locking the tuning frequency, the following auto-tuning fast locking frequency method is run in the MCU processor, the method comprising the steps of 1-5:
step 1: when the probe is not polarized, the capacitance value of the multi-gear tuning capacitor bank is adjusted to be minimum so as to adjust the bandwidth of the tuning circuit to be maximum, then the background signal is acquired through the ADC acquisition circuit, and the amplitude is recorded.
Further, step 1 specifically includes: in each measurement process, firstly, the capacitance value of the multi-gear tuning capacitor bank is adjusted to be minimum to adjust the bandwidth of the tuning circuit to be maximum, then, when the probe is not polarized, time domain waveform data after full-wave rectification and integration of the amplitude measuring circuit is collected through the multichannel ADC collecting circuit and stored, the collecting time is t, and the collected time domain waveform data is summed and divided by the number of sampling points to obtain a voltage amplitude V1.
Step 2: and polarizing the probe, adjusting the capacitance value of the multi-gear tuning capacitor bank to be the minimum, and acquiring the signal through the ADC acquisition circuit again, wherein the difference is that the background signal is acquired for the first time, the background signal and the magnetic field signal are acquired at this time, and the amplitude is recorded.
Further, step 2 specifically includes: keeping the capacitance value of the capacitor bank unchanged, polarizing the probe, collecting time domain waveform data after full-wave rectification and integration of the amplitude measuring circuit after the first polarization of the probe is collected again through the multichannel ADC collecting circuit, and storing the time domain waveform data, wherein the collecting time is t. And summing the acquired time domain waveform data and dividing the summed time domain waveform data by the number of sampling points to obtain a voltage amplitude V2.
And step 3: subtracting the acquired signals of the two ADC acquisition circuits to eliminate background signal interference, performing fast FFT operation on the subtracted signals to obtain a first amplitude net value, and calculating a spectrogram.
Further, step 3 specifically includes: and calculating a polarized first net amplitude value V1 ═ V2-V1, and performing FFT operation on the time domain waveform data subjected to the first polarization and the filtering to calculate a response signal spectrum subjected to the first polarization.
Because the first acquisition is a background signal containing noise, the second acquisition is a mixed signal containing the background signal and the magnetic field signal, and the time domain data acquired twice are subtracted, the interference signal can be weakened or eliminated to the maximum extent, so that a first amplitude net value corresponding to a clean magnetic field signal is extracted, the purpose of filtering is achieved, and circuits such as a hysteresis comparison circuit, a narrow-band filtering circuit and the like do not need to be arranged as in the prior art. Practice proves that the filtering method has good effect, simple algorithm, easy understanding and low requirement on a hardware platform.
And 4, step 4: because different gear capacitance values of the multi-gear tuning capacitor bank correspond to different resonant frequencies, the maximum frequency of the amplitude in the calculated spectrogram is the effective magnetic field frequency and is also the tuning frequency, and therefore the tuning capacitance value is quickly set according to the frequency value.
Further, step 4 specifically includes: the quickly setting the tuning capacitance value according to the tuning frequency specifically includes:
1) finding out a frequency value corresponding to the maximum amplitude value in the spectrogram through a bisection method;
2) finding out a corresponding tuning capacitance value from a prestored frequency-capacitance value data table through a table look-up method, and adjusting the tuning capacitance to a corresponding gear;
and 5: and after setting the capacitance value of the multi-gear tuning capacitor bank as a tuning capacitance value, polarizing again, finally measuring the magnetic field value, recording the amplitude, and obtaining a second amplitude net value, wherein the second amplitude net value is compared with the first amplitude net value, and if the change of the two amplitude net values is obviously increased, the frequency value is the current environment earth magnetic field value.
Further, step 5 specifically includes:
identifying the magnetic field frequency through a magnetic field frequency identification algorithm, adjusting a tuning capacitor to a corresponding gear, polarizing the probe again, collecting time domain waveform data after full-wave rectification and integration after the probe is subjected to secondary polarization through a multi-channel high-speed AD collecting probe for the third time, wherein the collecting time is t, summing the collected time domain waveform data, and dividing the summed time domain waveform data by the number of sampling points to obtain a voltage amplitude V3, and calculating a second amplitude net value V2' after the polarization is V3-V1;
and solving the amplitude change rate k as V2 '/V1', and according to the sensitivity and practical effect of the frequency measurement circuit, when k is more than 100 (the 100 can also be used for manually setting a threshold N, and N is more than or equal to 50), the purpose of tuning is achieved, otherwise, the process is repeated and the polarization is carried out again. The actual evaluation success rate reaches more than 99.5 percent.
The experiments show that the resonance effect evaluation of the frequency locking method can be realized by simply and comprehensively improving the software and the hardware of the proton magnetometer, the evaluation of the resonance effect of the frequency locking method is mainly realized by the amplitude measuring circuit and the amplitude measuring evaluation algorithm, the operation amount is low, the method is simple, the characteristics of different resonance frequencies corresponding to different gear capacitance values of the multi-gear tuning capacitor bank under the polarization condition are effectively utilized, the automatic tuning is continuously carried out under different magnetic field environments of different regions, the frequency can be quickly locked, and the frequency measurement is carried out by the maximum signal amplitude. Therefore, the current environmental magnetic field frequency can be quickly and effectively locked in the magnetic field measurement of the proton magnetometer.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
Claims (10)
1. A method for automatically tuning and rapidly locking frequency is characterized in that a measuring loop of a proton magnetometer comprises a resonant circuit, a multistage amplifying circuit, an ADC (analog to digital converter) acquisition circuit, a shaping circuit, a frequency measuring circuit, an amplitude measuring circuit and an MCU (microprogrammed control unit) processor, wherein the resonant circuit is composed of a probe and a multi-gear tuning capacitor set, the resonant circuit is connected with the input end of the multistage amplifying circuit, the output end of the multistage amplifying circuit is respectively connected with the ADC acquisition circuit, the shaping circuit and the amplitude measuring circuit, the frequency measuring circuit is connected with the shaping circuit, the MCU processor is respectively connected with the frequency measuring circuit, the ADC acquisition circuit and the amplitude measuring circuit, and the method for automatically tuning and rapidly locking frequency executed in the MCU processor comprises the following steps 1-5:
step 1: when the probe is not polarized, the capacitance value of the multi-gear tuning capacitor bank is adjusted to be minimum so as to adjust the bandwidth of the tuning circuit to be maximum, then the background signal is acquired through the ADC acquisition circuit, and the amplitude is recorded;
step 2: and polarizing the probe, adjusting the capacitance value of the multi-gear tuning capacitor bank to be the minimum, and acquiring the signal through the ADC acquisition circuit again, wherein the difference is that the background signal is acquired for the first time, the background signal and the magnetic field signal are acquired at this time, and the amplitude is recorded.
And step 3: subtracting the acquired signals of the two ADC acquisition circuits to eliminate background signal interference, performing fast FFT operation on the subtracted signals to obtain a first amplitude net value, and calculating a spectrogram;
and 4, step 4: because different gear capacitance values of the multi-gear tuning capacitor bank correspond to different resonant frequencies, the maximum frequency of the amplitude in the calculated spectrogram is the effective magnetic field frequency, namely the tuning frequency, and therefore the tuning capacitance value is quickly set according to the value of the tuning frequency;
and 5: and after setting the capacitance value of the multi-gear tuning capacitor bank as a tuning capacitance value, polarizing again, finally measuring the magnetic field value, recording the amplitude, and obtaining a second amplitude net value, wherein the second amplitude net value is compared with the first amplitude net value, and if the change of the two amplitude net values is obviously increased, the frequency value is the current environment earth magnetic field value.
2. The method according to claim 1, wherein the probe is a special proton magnetic field measuring probe, the proton magnetic field measuring probe and a multi-step tuning capacitor bank form an RC resonant circuit, and the MCU processor can adjust the capacitance value and the switching signal of the multi-step tuning capacitor bank to control the polarization of the proton magnetic field measuring probe.
3. The method of claim 1, wherein the multi-stage tuning capacitor bank comprises 8 parallel capacitor banks, and the multi-stage amplification circuit uses four stages of amplification.
4. The method of claim 1, wherein the ADC acquisition circuit acquires the amplified signal using ADS1271, and the MCU processor employs an STM32F4 series single chip microcomputer.
5. The method for automatically tuning a fast lock on frequency as claimed in claim 1, wherein step 1 specifically comprises: in each measurement process, firstly, the capacitance value of the multi-gear tuning capacitor bank is adjusted to be minimum to adjust the bandwidth of the tuning circuit to be maximum, then, when the probe is not polarized, time domain waveform data after full-wave rectification and integration of the amplitude measuring circuit is collected through the multichannel ADC collecting circuit and stored, the collecting time is t, and the collected time domain waveform data is summed and divided by the number of sampling points to obtain a voltage amplitude V1.
6. The method for automatically tuning a fast lock on frequency as claimed in claim 5, wherein step 2 specifically comprises: keeping the capacitance value of the multi-gear tuning capacitor bank unchanged, polarizing the probe, collecting time domain waveform data after full-wave rectification and integration of the amplitude measuring circuit after the first polarization of the probe is collected again through the multichannel ADC collecting circuit, storing the time domain waveform data, collecting the time domain waveform data with the collecting time t, summing the collected time domain waveform data, and dividing the summed time domain waveform data by the number of sampling points to obtain a voltage amplitude V2.
7. The method for automatically tuning a fast lock on frequency as claimed in claim 6, wherein step 3 specifically comprises: and calculating a polarized first amplitude net value V1 ═ V2-V1, performing FFT operation on the time domain waveform data subjected to the first polarization and the filtering, and calculating a response signal spectrum subjected to the first polarization.
8. The method of claim 1 or 7, wherein the step 4 of rapidly setting the tuning capacitance value according to the tuning frequency specifically comprises:
1) finding out a frequency value corresponding to the maximum amplitude value in the spectrogram through a bisection method;
2) and finding out a corresponding tuning capacitance value from a prestored frequency-capacitance value data table through a table look-up method, and adjusting the tuning capacitance to a corresponding gear.
9. The method for automatically tuning a fast lock on frequency as claimed in claim 7, wherein step 5 specifically comprises: the magnetic field frequency is identified through a magnetic field frequency identification algorithm, the tuning capacitor is adjusted to a corresponding gear, the probe is polarized again, the probe is subjected to secondary polarization through a multi-channel high-speed AD acquisition for the third time, time domain waveform data after full-wave rectification and integration are obtained through an amplitude measuring circuit, the acquisition time is t, the acquired time domain waveform data are summed and divided by the number of sampling points to obtain a voltage amplitude V3, and a second amplitude net value V2' after the polarization is calculated to be V3-V1.
10. The method for automatically tuning a fast lock on frequency of claim 9, wherein step 5 further comprises: solving the amplitude change rate k which is V2 '/V1', and according to the sensitivity and practical effect of the frequency measurement circuit, when k is greater than N, achieving the purpose of tuning, wherein N is greater than or equal to 50; otherwise, the process is repeated and polarization is performed again.
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