CN110995173A - Signal processing method and system for pulse Faraday cup signal measurement - Google Patents
Signal processing method and system for pulse Faraday cup signal measurement Download PDFInfo
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- CN110995173A CN110995173A CN201911380809.0A CN201911380809A CN110995173A CN 110995173 A CN110995173 A CN 110995173A CN 201911380809 A CN201911380809 A CN 201911380809A CN 110995173 A CN110995173 A CN 110995173A
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Abstract
The invention provides a signal processing method and a signal processing system for measuring a pulse Faraday cup signal. The method belongs to the field of mass spectrometry, and is characterized in that atoms are ionized into ions by pulse laser resonance excitation in an ion source, and the ions reach a Faraday cup detector after passing through a mass analyzer, and the method comprises the following steps: (1) the electrical signal from the Faraday cup detector is connected into a broadband high-gain preamplifier, so that the signal in the carrier frequency range is effectively amplified; (2) the amplified signal is accessed into a third-order high-pass filter, and the low-frequency part of the signal and introduced low-frequency interference and noise are filtered; (3) the filtered signal is digitally collected by a synchronous analog-to-digital converter, the collection process can play a role of demodulating from carrier frequency to low frequency, and finally, the source signal is recovered by a digital low-pass filter. The invention can effectively filter the low-frequency interference and noise of the detector and the preamplifier, thereby improving the signal-to-noise ratio of the measurement signal.
Description
Technical Field
The invention relates to the field of weak electric signal measurement in a mass spectrometer, in particular to a method for measuring a pulse Faraday cup signal.
Background
In a mass spectrometer, a faraday cup is a classical reliable ion detector, the detection efficiency of which is close to 100%, and the faraday cup is a typical ion detector for high-precision measurement. In a measuring system consisting of a common Faraday cup, a high-gain preamplifier and a high-precision analog-to-digital converter, test data is often influenced by various interferences such as external electromagnetic signals, vacuum free charges, mechanical vibration, ambient temperature and the like, and also influenced by various factors such as high-resistance thermal noise, 1/f noise and bias current noise of a main chip of an amplifier.
The investigation and analysis show that: in most of the existing ion signal measurement systems adopting Faraday cup detectors, a method of increasing the resistance value of a feedback resistor to reduce thermal noise current is adopted to improve the signal-to-noise ratio besides enhancing shielding and selecting a low-noise amplifier chip. However, since the interference and noise are located in the low frequency band, the signal-to-noise ratio is only improved to a limited extent (in the low frequency range, these interference and noise are amplified as well as the signal and are difficult to distinguish from the signal).
Disclosure of Invention
The purpose of the invention is: for the measurement of the pulse Faraday cup signal, the signal to noise ratio is further improved by a method for filtering low-frequency noise.
The inventors have realized in a laser resonance ionization mass spectrometry system, when analyzing resonance ionized ion signals: in laser resonance ionization mass spectrometry instruments, a pulsed laser is typically selected as the source of resonance excitation for the ion source. Multiple wavelength tunable laser pulses interact with a thermally evaporated/atomized species under test in an ion source, the atoms are ionized in a few tens of ns, and in most cases, the ionization is saturated. If sample evaporation is considered as a low frequency signal source, the process of resonance ionization can be considered as pulse sampling/modulation. Despite the temporal broadening of the pulses of the ion transport process, the electrical signal of an ion retains its pulse modulated characteristics when it reaches the detector.
Based on the method, in order to filter the low-frequency interference and noise of the detector and the preamplifier, high-pass filtering is added behind the broadband high-gain preamplifier matched with the Faraday cup detector, and the purpose of filtering the low-frequency interference and noise is achieved by utilizing synchronous acquisition and digital low-pass filtering.
The specific technical scheme is as follows:
a signal processing method for pulsed faraday cup signal measurement, comprising the steps of:
1) amplifying a Faraday cup detector signal containing a carrier signal;
2) carrying out high-pass filtering on the amplified signal to filter out the low-frequency part of the signal and introduced low-frequency interference and noise;
3) recovering the high-frequency region measurement signal obtained after the high-pass filtering to a measurement signal which contains low-frequency components, high-frequency carrier waves and harmonic components thereof and has no low-frequency interference and noise through synchronous digital acquisition;
4) and finally, filtering out a high-frequency part corresponding to the pulse carrier and harmonic waves thereof through digital low-pass filtering to obtain a low-frequency measurement signal without low-frequency interference and noise.
On the basis of the method, the invention is further optimized as follows:
step 2) preferably three butterworth high-pass filtering. In practice, high-pass filtering of second order and above is generally effective in filtering out low frequency signals.
And 3) performing synchronous digital acquisition, wherein the acquisition frequency is equal to the frequency of the laser pulse of the resonance excitation source serving as the ion source, and corresponding phase delay is set according to the flight time of the ions, so that the pulse modulation signal and the pulse carrier are convoluted and multiplied again.
A signal processing system for pulse Faraday cup signal measurement, the input of Faraday cup detector is ion signal (pulse modulation signal) generated by the resonance of pulse laser; the signal processing system comprises a broadband high-gain preamplifier, a high-pass filter, a synchronous analog-to-digital converter and a digital low-pass filter which are sequentially connected from the output end of the Faraday cup detector; the wide-band high-gain preamplifier is used for amplifying a Faraday cup detector signal containing a carrier signal, and the high-pass filter is used for filtering a low-frequency part of the amplified signal and introduced interference and noise; the synchronous analog-to-digital converter is used for carrying out synchronous digital acquisition to complete demodulation from a carrier frequency to a low frequency; the digital low-pass filter is used for filtering out high-frequency components.
The high-pass filter can adopt a three-order Butterworth high-pass filter.
The three-order Butterworth high-pass filter comprises an operational amplifier, and a capacitor C1, a capacitor C2 and a capacitor C3 which are connected in series between the output end of the broadband high-gain preamplifier and the positive input end of the operational amplifier; the node between the capacitor C1 and the capacitor C2 is grounded via a resistor R1, the node between the capacitor C2 and the capacitor C3 is grounded via a resistor R2, the output end of the operational amplifier and the negative input end of the operational amplifier are connected to the node between the capacitor C3 and the positive input end of the operational amplifier via a resistor R3, and the positive and negative power supply ends of the operational amplifier are respectively connected to positive and negative voltages with equal amplitude.
The synchronous analog-to-digital converter can adopt a high-precision successive comparison type ADC (SAR ADC) suitable for carrier frequency to realize synchronous digital acquisition and recover to a measurement signal which contains low-frequency components and other high-frequency bands and has no low-frequency interference and noise.
The invention has the following beneficial effects:
the invention can effectively filter the low-frequency interference and noise of the detector and the preamplifier, thereby improving the signal-to-noise ratio of the measurement signal.
Drawings
FIG. 1 is a schematic view of the measurement process of the present invention.
Fig. 2 is a schematic diagram of a three-order butterworth high-pass filter.
FIG. 3 is a diagram illustrating a simulation comparison method according to the present invention.
FIG. 4 is a diagram of the output waveform of the simulation comparison method of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the following drawings and specific examples.
In a laser resonance ionization mass spectrometry system, when analyzing ion signals of resonance ionization, the following are found: the ions generated by the resonant ionization of the pulsed laser in the ion source reach the detector, and the current signal itself is a pulse modulated signal (modulated signal). The low frequency interference and noise introduced by the faraday cup detector and amplifier is separated in the frequency domain from the signal that has been pulse carrier modulated to a higher frequency region.
Thus, the basic principle of the invention is shown in fig. 1: amplifying and collecting signals by using a broadband high-gain preamplifier, high-pass filtering and a synchronous analog-to-digital converter (synchronous digital collection) to obtain a measuring signal which contains low-frequency components, high-frequency carriers and harmonic components thereof and has no low-frequency interference and noise; and finally, recovering the source signal by digital low-pass filtering so as to achieve the purpose of inhibiting or even eliminating low-frequency interference and noise.
Wherein:
the broadband high-gain preamplifier amplifies the modulated high-frequency measurement signal, and can specifically adopt a conventional high-bandwidth main operational amplifier chip and the application of a parasitic capacitance compensation measure aiming at a large resistor to realize the broadband high-gain amplification function.
The high-pass filtering part mainly filters low-frequency interference and noise, and meanwhile ensures that a high-frequency range measuring signal can pass through smoothly as much as possible. Particularly preferably, the three-order butterworth high-pass filtering is performed, as shown in fig. 2, a signal amplified by the faraday cup amplifier is accessed from the left end of C1, the right end of the signal is connected with the left end of C2 and the upper end of R1, and the lower end of R1 is grounded. The right end of the C2 is connected with the left end of the C3 and the upper end of the R2, and the lower end of the R2 is connected with the output end of the operational amplifier. The right end of the C3 is connected with the positive end of the operational amplifier and the upper end of the R3, and the lower end of the R3 is grounded. The negative end of the operational amplifier is connected with the output end of the operational amplifier, and the positive end and the negative end of the power supply are respectively connected with +15V and-15V voltages. By designing the three-order Butterworth high-pass filtering, low-frequency interference and noise are filtered out as much as possible, and meanwhile, the high-frequency-band measurement signal is guaranteed to pass through smoothly.
The synchronous acquisition part carries out digital conversion on the signal, particularly designs synchronous acquisition frequency which is the same as that of the laser pulse, realizes the convolution multiplication of the modulated signal and the pulse carrier again, and the process can replace the signal sampling process so as to restore the high-frequency area measurement signal to the measurement signal which contains low-frequency components, high-frequency carrier waves and harmonic components thereof and has no low-frequency interference and noise. Specifically, 18bit 1MHz high-precision successive comparison ADC (SAR ADC) can be adopted. The acquisition process may function as a demodulation from the carrier frequency to low frequencies.
The digital low-pass filtering is used for filtering high-frequency band components including carrier frequencies and harmonic waves thereof, and finally obtaining a low-frequency measurement signal without low-frequency interference and noise, namely, the recovery of a source signal is realized.
The invention designs a comparison scheme on the aspect of method verification:
by using Simulink simulation software, in simulation verification, two schemes of designing a high-frequency measurement signal with low-frequency noise and without noise interference are compared to verify the correctness and superiority of the method. As shown in fig. 3, the specific scheme is as follows: (1) a sinusoidal Signal with amplitude of 0.3 and frequency f of 10Hz is generated by a Signal Generator (Signal Generator) in Simulink as a baseband Signal, a dc component with amplitude of 1 is generated by a constant (constant), and finally the dc Signal and the baseband Signal are added by an adder as an actual source. (2) Another signal generator generates a pulse signal with amplitude of 1, frequency f of 10kHz and pulse width of 5us as a high frequency carrier. (3) A multiplier is used to connect the high frequency carrier to the actual source to generate a modulated signal. (4) The modulated signal is divided into two paths, the first path is used as the modulated signal which is not interfered by noise, and the second path generates low-frequency noise by adding a noise module (Random Number) and is used as the modulated signal which is interfered by the low-frequency noise. (5) Then the first path is connected with the high-frequency pulse carrier wave with the same frequency again through a multiplier, the second path is connected with the high-frequency pulse carrier wave with the same frequency through a multiplier after being connected with a high-pass filter with the cut-off frequency of 3kHz, the second path is connected with the high-frequency pulse carrier wave with the same frequency through a multiplier (6), then the two paths respectively pass through a low-pass filter with the cut-off frequency of 100Hz, and output signals of the two paths are used as demodulation. (7) And finally, observing an actual information source, a modulated signal which is not interfered by noise, a demodulated output signal which is not interfered by noise, a modulated signal which is interfered by noise and a modulated output signal which is interfered by noise by using an oscilloscope Scope1 respectively.
As shown in fig. 1 and fig. 3, the ion signal generated by the pulsed laser resonance of the sample in fig. 1 is equivalent to the modulated signal in fig. 3 without noise interference; the signal amplified by accessing the corresponding faraday cup amplifier circuit in fig. 1 is equivalent to the process of adding low frequency noise interference in the second path in fig. 3; the high frequency filtering in fig. 1 corresponds to the high pass filtering process in fig. 3; the synchronous acquisition in fig. 1 corresponds to the process of fig. 3 in which the high-frequency pulse carrier of the same frequency is connected again through a multiplier; finally the low-pass digital filtering in fig. 1 corresponds to the low-pass filtering process in fig. 3.
Taking a laser resonance ionization mass spectrometer as an example, the ion signal is generated by resonance ionization of a sample by 10kHz laser pulses with a pulse width of 20 to 40ns, although there is some time domain dispersion during acceleration and flight of the ions through the mass analyzer, this does not prevent us from viewing the ion signal as a signal modulated by a 10kHz pulse carrier. Meanwhile, various interferences and noises are introduced in a low-frequency area during the process of connecting the measurement signal into a corresponding Faraday cup amplifier circuit for signal amplification, such as: the influence of thermal ionization interference (random signal, irrelevant to modulation frequency) of the sample, 50Hz power frequency interference, large resistance thermal noise, 1/f noise and bias current noise of the amplifier main chip and the like. But they are separated in the frequency domain from the measurement signal modulated to the high frequency band by the 10kHz pulse carrier.
The oscilloscope Scope1 results are shown in FIG. 4: from top to bottom, the actual signal source, the modulated signal without noise interference, the demodulated output signal without noise interference, the modulated signal with noise interference, and the demodulated output signal with noise interference are respectively. As can be seen from fig. 4, except for the difference in amplitude, the frequencies of the demodulation output waveforms without noise interference and with noise interference are substantially consistent with those of the actual signal source, which indicates that the method can correctly demodulate the signal source. Meanwhile, the waveform distortion of the modulated signal interfered by the noise is serious, but the modulated signal is basically consistent with the modulated output signal not interfered by the noise after being demodulated by high-pass filtering, high-frequency pulse carrier and low-pass filtering, which shows that the method can effectively filter the low-frequency noise interference. The correctness and the effectiveness of the invention are verified through simulation comparison.
Claims (7)
1. A signal processing method for pulsed faraday cup signal measurement, comprising the steps of:
1) amplifying a Faraday cup detector signal containing a carrier signal;
2) carrying out high-pass filtering on the amplified signal to filter out the low-frequency part of the signal and introduced low-frequency interference and noise;
3) recovering the high-frequency region measurement signal obtained after the high-pass filtering to a measurement signal which contains low-frequency components, high-frequency carrier waves and harmonic band components thereof and has no low-frequency interference and noise through synchronous digital acquisition;
4) and finally, filtering out a high-frequency part corresponding to the pulse carrier and harmonic waves thereof through digital low-pass filtering to obtain a low-frequency measurement signal without low-frequency interference and noise.
2. The signal processing method for pulsed faraday cup signal measurement as claimed in claim 1, wherein: and 2) adopting three-order Butterworth high-pass filtering.
3. The signal processing method for pulsed faraday cup signal measurement as claimed in claim 1, wherein: and 3) performing synchronous digital acquisition, wherein the acquisition frequency is equal to the frequency of the laser pulse of the resonance excitation source serving as the ion source, and corresponding phase delay is set according to the flight time of the ions, so that the pulse modulation signal and the pulse carrier are convoluted and multiplied again.
4. A signal processing system for measuring a pulse Faraday cup signal is characterized by comprising a broadband high-gain preamplifier, a high-pass filter, a synchronous analog-to-digital converter and a digital low-pass filter which are sequentially connected from the output end of a pulse Faraday cup detector; the wide-band high-gain preamplifier is used for amplifying a Faraday cup detector signal containing a carrier signal, and the high-pass filter is used for filtering a low-frequency part of the amplified signal and introduced interference and noise; the synchronous analog-to-digital converter is used for carrying out synchronous digital acquisition to complete demodulation from a carrier frequency to a low frequency; the digital low-pass filter is used for filtering out high-frequency parts.
5. The signal processing system for pulsed faraday cup signal measurement of claim 4, wherein: the high-pass filter adopts a three-order Butterworth high-pass filter.
6. The signal processing system for pulsed faraday cup signal measurement of claim 5, wherein: the triple Butterworth high-pass filter comprises an operational amplifier, and a capacitor C1, a capacitor C2 and a capacitor C3 which are connected in series between the output end of the broadband high-gain preamplifier and the positive input end of the operational amplifier; the node between the capacitor C1 and the capacitor C2 is grounded via a resistor R1, the node between the capacitor C2 and the capacitor C3 is grounded via a resistor R2, the output end of the operational amplifier and the negative input end of the operational amplifier are connected to the node between the capacitor C3 and the positive input end of the operational amplifier via a resistor R3, and the positive and negative power supply ends of the operational amplifier are respectively connected to positive and negative voltages with equal amplitude.
7. The signal processing system for pulsed faraday cup signal measurement of claim 4, wherein: the synchronous analog-to-digital converter adopts a high-precision successive comparison type ADC (SAR ADC) suitable for carrier frequency to realize synchronous digital acquisition and recover to a measurement signal which contains low-frequency components and other high-frequency bands and has no low-frequency interference and noise.
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| CN115031896A (en) * | 2022-05-20 | 2022-09-09 | 北京晨晶精仪电子有限公司 | Detection device, vacuum gauge and detection method for capacitive thin film vacuum gauge |
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| CN115031896A (en) * | 2022-05-20 | 2022-09-09 | 北京晨晶精仪电子有限公司 | Detection device, vacuum gauge and detection method for capacitive thin film vacuum gauge |
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