CN111257266A - Fourier transform infrared spectrum processing device and method - Google Patents
Fourier transform infrared spectrum processing device and method Download PDFInfo
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Abstract
The invention discloses a Fourier transform infrared spectrum processing device and a method, wherein the device comprises a detector, an operational amplifier, a Butterworth band-pass filter and an upper computer, the Butterworth band-pass filter comprises a second-order high-pass filter and a second-order low-pass filter, and the detector, the operational amplifier, the second-order high-pass filter, the second-order low-pass filter and the upper computer are sequentially connected; the invention has the advantages that: provided are a Fourier transform infrared spectrum processing device and method, which can restrain out-of-band interference noise of signals.
Description
Technical Field
The invention relates to the field of infrared signal acquisition and processing, in particular to a Fourier transform infrared spectrum processing device and method.
Background
The Fourier transform infrared spectroscopy is widely applied to the fields of industry, environmental protection, medicine and the like due to the characteristics of real-time, continuous and online measurement, nondestructive detection of samples and the like. In an infrared spectrogram obtained by infrared spectrometry, besides an absorption peak generated by a sample, interference noise caused by various factors is also included, and when the noise is large or the absorption peak of the sample is weak, the noise and the absorption peak are mixed and difficult to distinguish, so that the qualitative and quantitative analysis of the sample is influenced finally.
In a fourier transform spectrometer, there are various noise sources, such as noise caused by a data acquisition system, an operational amplifier, a data transmission interface, interferometer optical elements (such as a beam splitter/compensator), an optical window, a detector, etc. in an electronic system of the spectrometer, which can reduce the signal-to-noise ratio of a recovered spectrum and affect the accuracy of qualitative and quantitative analysis of a sample.
Chinese patent publication No. CN107655845A discloses an infrared spectrum acquisition method based on Fourier transform infrared spectrum superposition peak shape, which generates an infrared signal by a laser infrared light source, and the infrared signal sequentially passes through an interferometer, a sample chamber and a detector to generate an infrared interference spectrum; sampling the infrared interference spectrum by a computer host through a sampling device to obtain an infrared interference signal; and carrying out Fourier transform on the sampled infrared interference signal, carrying out peak shape superposition on the infrared interference signal through a superposition function, further acquiring an infrared percentage transmittance spectrum, and displaying the infrared percentage transmittance spectrum through a display device. The infrared spectrum acquisition method based on the Fourier transform infrared spectrum superposition type peak shape achieves the technical effects of improving the infrared spectrum resolution by one time and enhancing each peak signal by one time. However, the method provided by the invention cannot inhibit the out-of-band interference noise of the signal.
Disclosure of Invention
The technical problem to be solved by the invention is how to suppress the out-of-band interference noise of the signal.
The invention solves the technical problems through the following technical means: a Fourier transform infrared spectrum processing device comprises a detector, an operational amplifier, a Butterworth band-pass filter and an upper computer, wherein the Butterworth band-pass filter comprises a second-order high-pass filter and a second-order low-pass filter, the detector, the operational amplifier, the second-order high-pass filter, the second-order low-pass filter and the upper computer are sequentially connected, a signal of a Fourier transform infrared spectrometer enters the detector through a Michelson interferometer to generate an infrared interference signal, the operational amplifier performs primary preprocessing on the infrared interference signal, the preprocessed infrared interference signal enters the second-order high-pass filter to filter low-frequency noise, then enters the second-order low-pass filter to filter high-frequency noise, the finally processed signal is collected and transmitted to the upper computer, the conversion from the interference signal to a spectrum signal is completed through an FFT algorithm, and data are, and (4) quantitatively analyzing the characteristic absorption of the recovered spectrogram, and inverting the component and structure information of the substance to be detected.
According to the invention, by utilizing a negative feedback principle, a second-order high-pass filter is designed to filter low-frequency noise, a second-order low-pass filter is designed to filter high-frequency noise, and a Butterworth band-pass filter combining the second-order high-pass filter and the second-order low-pass filter limits the bandwidth of a signal preliminarily processed by an operational amplifier, inhibits out-of-band interference noise, prevents aliasing of sampled data, and improves the signal-to-noise ratio of an instrument.
Preferably, the model of the operational amplifier is OP 37.
Preferably, the second-order high-pass filter includes a capacitor C1, a capacitor C2, a resistor R1, a resistor R2, a resistor R3, a resistor R4, and an operational amplifier U1, one end of the capacitor C1 is connected to the output end of the operational amplifier, the other end of the capacitor C1 is connected to one end of the capacitor C2 and one end of the resistor R1, the other end of the capacitor C2 is connected to the non-inverting end of the operational amplifier U1, the other end of the resistor R1 is connected to the output end of the operational amplifier U1, one end of the resistor R4 is connected to the non-inverting end of the operational amplifier U1, one end of the resistor R3 is connected to the inverting end of the operational amplifier U1, one end of the resistor R2 is connected to one end of the resistor R3, the other end of the resistor R2 is connected to the output end of the operational amplifier U874.
Preferably, the second-order low-pass filter includes a resistor R5, a resistor R6, a resistor R7, a resistor R8, a capacitor C3, a capacitor C4 and an operational amplifier U2, one end of the resistor R5 is connected to the output end of the operational amplifier U1, the other end of the resistor R5 is connected to one end of a resistor R6 and one end of a capacitor C3, the other end of the resistor R6 is connected to the non-inverting end of the operational amplifier U2, the other end of the capacitor C3 is connected to the output end of the operational amplifier U2, one end of the capacitor C4 is connected to the other end of the resistor R6, one end of the resistor R8 is connected to the non-inverting end and the inverting end of the operational amplifier U8, one end of the resistor R8 is connected to one end of the resistor R8, the other end of the resistor R8 and the other end of the resistor R8 are grounded, and the output end of the operational amplifier U.
The present invention also provides a fourier transform infrared spectrum processing method applied to any one of the above fourier transform infrared spectrum processing apparatuses, the method including: the method comprises the steps that signals of the Fourier transform infrared spectrometer enter a detector through a Michelson interferometer to generate infrared interference signals, an operational amplifier conducts primary pretreatment on the infrared interference signals, the pretreated infrared interference signals enter a second-order high-pass filter to filter low-frequency noise, then enter a second-order low-pass filter to filter high-frequency noise, finally processed signals are collected and transmitted to an upper computer, the interference signals are converted into spectrum signals through an FFT algorithm, data are stored, quantitative analysis is conducted through characteristic absorption of a restored spectrogram, and component and structure information of a substance to be detected is inverted.
Preferably, a laser signal is used as a reference in the acquisition process of the infrared interference signal in the Fourier transform infrared spectrometer, a He-Ne laser is adopted, the wavelength is 632.8nm, the output power is 2mW, and the stability of the output power is less than or equal to +/-5%/1000 h; modulation frequency of He-Ne laser is 5KHz, and spectral measurement is carried outThe wave band range is 400-7000 cm-1The frequency range is 126.4 Hz-2.2 KHz.
Preferably, in the butterworth band-pass filter, the tolerance of the selected resistance value is 1%, the tolerance of the selected capacitance value is 5%, the cutoff frequency is 40% of the actual cutoff frequency, and the error is 75.84Hz to 3.08 KHz.
Preferably, the voltage transfer function of the second-order high-pass filter is:
wherein the resistance value of the resistor R2 is R8The resistance of the resistor R3 is R7，Aup(s)＝1+R8/R7The capacitance values of the capacitor C1 and the capacitor C2 are both ChThe resistance values of the resistor R1 and the resistor R4 are RhAnd s is a complex frequency domain variable.
Preferably, the voltage transfer function of the second-order low-pass filter is:
wherein the resistance value of the resistor R7 is R11The resistance of the resistor R8 is R12，Aup(s)＝1+R11/R12The resistance values of the resistor R5 and the resistor R6 are RlThe capacitance values of the capacitor C3 and the capacitor C4 are both ClAnd s is a complex frequency domain variable.
Preferably, A isupThe value of(s) is greater than 3.
The invention has the advantages that: according to the invention, by utilizing a negative feedback principle, a second-order high-pass filter is designed to filter low-frequency noise, a second-order low-pass filter is designed to filter high-frequency noise, and a Butterworth band-pass filter combining the second-order high-pass filter and the second-order low-pass filter limits the bandwidth of a signal preliminarily processed by an operational amplifier, inhibits out-of-band interference noise, prevents aliasing of sampled data, and improves the signal-to-noise ratio of an instrument.
Drawings
Fig. 1 is a block diagram of a fourier transform infrared spectrum processing apparatus according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a Butterworth band-pass filter of a Fourier transform infrared spectrum processing apparatus according to an embodiment of the present invention;
FIG. 3 is a graph of the amplitude and frequency of a Butterworth band-pass filter in a Fourier transform infrared spectroscopy processing method according to an embodiment of the present invention;
FIG. 4 is a phase-frequency curve of a Butterworth band-pass filter in a Fourier transform infrared spectroscopy processing method according to an embodiment of the present invention;
FIG. 5 is a comparison graph of measured spectra obtained by a conventional Fourier transform infrared spectroscopy processing method and a Fourier transform infrared spectroscopy processing method of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
Because factors such as an electronic system, an optical element, mechanical vibration and the like in the Fourier transform infrared spectrum can introduce noise into an interference pattern obtained by sampling, the noise can reduce the signal-to-noise ratio of a restored spectrum, and the accuracy of qualitative and quantitative analysis of a sample is influenced, a method for reducing interference noise and improving the signal-to-noise ratio of an instrument is required to be researched. As shown in fig. 1, a fourier transform infrared spectrum processing apparatus includes a detector 1, an operational amplifier 2, a butterworth band-pass filter 5 and an upper computer 6, where the butterworth band-pass filter 5 includes a second-order high-pass filter 3 and a second-order low-pass filter 4, and the detector 1, the operational amplifier 2, the second-order high-pass filter 3, the second-order low-pass filter 4 and the upper computer 6 are connected in sequence. The model of the operational amplifier 2 is OP 37. Signals of the Fourier transform infrared spectrometer enter the detector 1 through the Michelson interferometer to generate infrared interference signals, the operational amplifier 2 conducts preliminary preprocessing on the infrared interference signals, and the operational amplifier 2 selects the OP37 with low noise and low temperature drift and is very suitable for detecting weak signals. The preprocessed infrared interference signals enter a second-order high-pass filter 3 to filter low-frequency noise, then enter a second-order low-pass filter 4 to filter high-frequency noise, finally, the processed signals are collected and transmitted to an upper computer 6, the conversion from the interference signals to spectral signals is completed through an FFT algorithm, data are stored, quantitative analysis is carried out on characteristic absorption of a restored spectrogram, and component and structure information of the substance to be detected is inverted.
As shown in fig. 2, the second-order high-pass filter 3 includes a capacitor C1, a capacitor C2, a resistor R1, a resistor R2, a resistor R3, a resistor R4, and an operational amplifier U1, one end of the capacitor C1 is connected to the output end of the operational amplifier 2, the other end of the capacitor C1 is connected to one end of the capacitor C2 and one end of the resistor R1, the other end of the capacitor C2 is connected to the non-inverting end of the operational amplifier U1, the other end of the resistor R1 is connected to the output end of the operational amplifier U1, one end of the resistor R4 is connected to the non-inverting end of the operational amplifier U1, one end of the resistor R3 is connected to the inverting end of the operational amplifier U1, one end of the resistor R2 is connected to one end of the resistor R3, the other end of the resistor R2 is connected to the output end of the operational. The capacitor C1, the capacitor C2, the resistor R1 and the resistor R4 mainly form high-pass filtering to filter low-frequency noise; the resistor R2, the resistor R3 and the operational amplifier U1 form a negative feedback form.
The second-order low-pass filter 4 comprises a resistor R5, a resistor R6, a resistor R7, a resistor R8, a capacitor C3, a capacitor C4 and an operational amplifier U2, one end of the resistor R5 is connected with the output end of the operational amplifier U1, the other end of the resistor R5 is connected with one end of a resistor R6 and one end of a capacitor C3, the other end of the resistor R6 is connected with the in-phase end of the operational amplifier U2, the other end of the capacitor C3 is connected with the output end of the operational amplifier U2, one end of a capacitor C4 is connected with the other end of a resistor R6, one end of a resistor R8 is connected with the in-phase end and the anti-phase end of the operational amplifier U8, one end of the resistor R8 is connected with one end of the resistor R8, the other end of the capacitor C8 and the other end of the resistor R8 are grounded, and the output end of the operational. The resistor R5, the resistor R6, the capacitor C3 and the capacitor C4 mainly form low-pass filtering to filter high-frequency noise; the resistor R7, the resistor R8 and the operational amplifier U2 form a negative feedback form.
According to the technical scheme, the Fourier transform infrared spectrum processing device provided by the invention designs the second-order high-pass filter 3 to filter low-frequency noise, the second-order low-pass filter 4 to filter high-frequency noise, and the Butterworth band-pass filter combining the second-order high-pass filter 3 and the second-order low-pass filter 4 limits the bandwidth of signals primarily processed by the operational amplifier 2, inhibits out-of-band interference noise, prevents aliasing of sampled data and improves the signal-to-noise ratio of an instrument.
Example 2
The present invention also provides a fourier transform infrared spectrum processing method applied to the fourier transform infrared spectrum processing apparatus described in embodiment 1, the method including: signals of the Fourier transform infrared spectrometer enter the detector 1 through the Michelson interferometer to generate infrared interference signals, the operational amplifier 2 conducts preliminary preprocessing on the infrared interference signals, and the operational amplifier 2 selects the OP37 with low noise and low temperature drift and is very suitable for detecting weak signals. The preprocessed infrared interference signals enter a second-order high-pass filter 3 to filter low-frequency noise, then enter a second-order low-pass filter 4 to filter high-frequency noise, finally, the processed signals are collected and transmitted to an upper computer 6, the conversion from the interference signals to spectral signals is completed through an FFT algorithm, data are stored, quantitative analysis is carried out on characteristic absorption of a restored spectrogram, and component and structure information of the substance to be detected is inverted. The invention aims to process signals of Fourier transform infrared spectrum to reduce out-of-band interference noise, and does not need to describe the quantitative analysis in the later period. The FFT algorithm is fourier transform, which is not described herein in detail.
In the process of collecting infrared interference signals in the Fourier transform infrared spectrometer, the aim of realizing high degree is toSampling is carried out at equal optical path difference intervals to obtain a high-quality spectrogram, and a laser signal with good monochromaticity is used as a reference. Therefore, in the embodiment of the invention, a laser signal is used as a reference in the acquisition process of the infrared interference signal in the Fourier transform infrared spectrometer, a He-Ne laser is adopted, the wavelength is 632.8nm, the output power is 2mW, and the stability of the output power is less than or equal to +/-5%/1000 h; before the upper computer 6 collects interferogram data, a band-pass filter with corresponding bandwidth is designed for filtering according to the actual measurement spectral range, so that the modulation frequency of a He-Ne laser is 5KHz in a Fourier transform infrared spectrometer through an interferometer moving mirror drive control system, and the spectral measurement waveband range is 400-7000 cm-1The frequency range is 126.4 Hz-2.2 KHz.
The frequency range is selected by the following steps: laser modulation frequency f in Fourier transform infrared spectrometermIs composed of
Since the wavelength of He-Ne laser in the spectrometer is 632.8nm, the laser tuning frequency is 5KHz by the drive control of the interferometer moving mirror, and the moving speed V of the interferometer moving mirror is 0.158cm/s according to the formula (1). Sampling mode for bilateral sampling interferogram, optical path difference speed Vopd2V 0.316 cm/s. The selected spectral measurement waveband range is 400-7000 cm-1The frequency range is calculated from equation (2).
From this, the pass band width f of the band pass filter is knownmin:fmaxIs 126.4 Hz-2.2 KHz.
Because the amplitude-frequency response of the Butterworth-type filter has the most open amplitude response and good linear phase in the pass band, and the performance is stable and easy to realize, the form of the Butterworth-type filter is selected, and simultaneously, because the central frequency of the band-pass filter is easy to be influenced by the performance parameters of electronic components, once the frequency shift of the filter occurs, the performance of the filter can be directly influenced, so the invention selects a mode of combining a high-pass filter and a low-pass filter, and the Butterworth-type band-pass filter 5 is formed by a second-order high-pass filter 3 and a second-order low-pass filter 4. In the Butterworth band-pass filter 5, the influence of numerical errors of electronic components on a transfer function is considered, the tolerance of a resistance value is 1 percent, the tolerance of a capacitance value is 5 percent, and the actual component parameter values are deviated from theoretical calculated values, so that the cut-off frequency of the Butterworth band-pass filter 5 is designed to be 40 percent of the actual cut-off frequency, and the error is 75.84 Hz-3.08 KHz in order to avoid the error of the actual cut-off frequency caused by the electronic components.
As shown in fig. 2, in the butterworth band-pass filter, the second-order high-pass filter 3 mainly includes capacitors C1 and C2, resistors R1, R2, R3, R4, and a core component operational amplifier U1, wherein the capacitors C1 and C2, the resistors R1 and R4 mainly form high-pass filtering to filter low-frequency noise; the resistors R2 and R3 and the operational amplifier U1 form a negative feedback form; according to the node voltage method, the voltage transfer function of the second-order high-pass filter 3 is:
wherein the resistance value of the resistor R2 is R8The resistance of the resistor R3 is R7，Aup(s)＝1+R8/R7The capacitance values of the capacitor C1 and the capacitor C2 are both ChThe resistance values of the resistor R1 and the resistor R4 are RhAnd s is a complex frequency domain variable.
The second-order low-pass filter 4 mainly comprises resistors R5, R6, R7 and R8, capacitors C3 and C4 and a core element operational amplifier U2, wherein the resistors R5 and R6 and the capacitors C3 and C4 mainly form low-pass filtering and filter high-frequency noise; the resistors R7 and R8 and the operational amplifier U2 form a negative feedback form; according to the node voltage method, the voltage transfer function of the second-order low-pass filter 4 is:
wherein the resistance value of the resistor R7 is R11The resistance of the resistor R8 is R12，Aup(s)＝1+R11/R12The resistance values of the resistor R5 and the resistor R6 are RlThe capacitance values of the capacitor C3 and the capacitor C4 are both ClAnd s is a complex frequency domain variable. A is describedupThe value of(s) is more than 3, namely the first-order coefficient of s in the denominator is more than zero, the circuit can stably work without generating self-oscillation.
Fig. 3 and 4 are amplitude-frequency curves and phase-frequency curves of a negative feedback-based butterworth bandpass filter, respectively. It can be seen from the figure that the maximum passband of the Butterworth band-pass filter based on negative feedback is 12.52, the voltage amplification factor corresponding to the low-pass cutoff frequency of 2.2KHz is 11.13, and the passband attenuation is 2.79 dB. The voltage amplification factor corresponding to the high-pass cut-off frequency of 126.4Hz is 11.79, so that the Butterworth band-pass filter with the maximum pass-band attenuation of-2.75 dB meets the requirement.
FIG. 5 shows the measured spectra obtained by the conventional processing method and the processing method of the present invention after adding a Butterworth band-pass filter, wherein the dotted line shows the spectra obtained by the processing method of the present invention passing through 2500--1The spectral signal-to-noise ratio of the wave band range is obtained by calculating a 100% line of the air transmission rate of the wave band range, wherein the signal-to-noise ratio of a measured spectrum obtained without a filter is 21930:1, and the measured signal-to-noise ratio of the measured spectrum obtained after a negative feedback-based Butterworth band-pass filter is added is 42918:1.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. The Fourier transform infrared spectrum processing device is characterized by comprising a detector, an operational amplifier, a Butterworth band-pass filter and an upper computer, wherein the Butterworth band-pass filter comprises a second-order high-pass filter and a second-order low-pass filter, and the detector, the operational amplifier, the second-order high-pass filter, the second-order low-pass filter and the upper computer are sequentially connected;
the method comprises the steps that signals of the Fourier transform infrared spectrometer enter a detector through a Michelson interferometer to generate infrared interference signals, an operational amplifier conducts primary pretreatment on the infrared interference signals, the pretreated infrared interference signals enter a second-order high-pass filter to filter low-frequency noise, then enter a second-order low-pass filter to filter high-frequency noise, finally the processed signals are collected and transmitted to an upper computer, the interference signals are converted into spectrum signals through an FFT algorithm, data are stored, quantitative analysis is conducted through characteristic absorption of a restored spectrogram, and component and structure information of a substance to be detected is inverted.
2. The fourier transform infrared spectroscopy processing apparatus of claim 1, wherein the operational amplifier is of the type OP 37.
3. The Fourier transform infrared spectrum processing device according to claim 1, wherein the second-order high-pass filter comprises a capacitor C1, a capacitor C2, a resistor R1, a resistor R2, a resistor R3, a resistor R4 and an operational amplifier U1, one end of the capacitor C1 is connected with an output end of an operational amplifier, the other end of the capacitor C1 is connected with one end of a capacitor C2 and one end of a resistor R1, the other end of the capacitor C2 is connected with a non-inverting end of an operational amplifier U1, the other end of the resistor R1 is connected with an output end of an operational amplifier U1, one end of a resistor R4 is connected with an inverting end of the operational amplifier U1, one end of a resistor R3 is connected with an inverting end of an operational amplifier U1, one end of a resistor R2 is connected with one end of a resistor R3, the other end of a resistor R2 is connected with an output end of an operational amplifier U1, and the other end of.
4. A Fourier transform infrared spectrum processing device according to claim 3, wherein the second-order low-pass filter comprises a resistor R5, a resistor R6, a resistor R7, a resistor R8, a capacitor C3, a capacitor C4 and an operational amplifier U2, one end of the resistor R5 is connected with the output end of the operational amplifier U1, the other end of the resistor R5 is connected with one end of a resistor R6 and one end of a capacitor C3, the other end of the resistor R6 is connected with the in-phase end of the operational amplifier U2, the other end of the capacitor C3 is connected with the output end of the operational amplifier U2, one end of the capacitor C4 is connected with the other end of a resistor R6, one end of the resistor R8 is connected with the in-phase end and the anti-phase end of the operational amplifier U8, one end of the resistor R8 is connected with the output end of the operational amplifier U8, the other end of the capacitor C8 and the other end of the resistor R8 are grounded, and the output end of the operational amplifier U.
5. A fourier transform infrared spectroscopy processing method, comprising: the method comprises the steps that signals of the Fourier transform infrared spectrometer enter a detector through a Michelson interferometer to generate infrared interference signals, an operational amplifier conducts primary pretreatment on the infrared interference signals, the pretreated infrared interference signals enter a second-order high-pass filter to filter low-frequency noise, then enter a second-order low-pass filter to filter high-frequency noise, finally processed signals are collected and transmitted to an upper computer, the interference signals are converted into spectrum signals through an FFT algorithm, data are stored, quantitative analysis is conducted through characteristic absorption of a restored spectrogram, and component and structure information of a substance to be detected is inverted.
6. A Fourier transform infrared spectroscopy processing method according to claim 5 wherein a laser signal is used as a reference in the acquisition of the infrared interference signal in the Fourier transform infrared spectrometer, a He-Ne laser is used, the wavelength is 632.8nm, the output power is 2mW, and the stability of the output power is less than or equal to +/-5%/1000 h; the modulation frequency of the He-Ne laser is 5KHz, and the spectral measurement waveband range is 400-7000 cm-1The frequency range is 126.4 Hz-2.2 KHz.
7. A Fourier transform infrared spectroscopy processing method as claimed in claim 6, wherein in said Butterworth band-pass filter, a tolerance of 1% for resistance values, a tolerance of 5% for capacitance values, a cut-off frequency of 40% for the actual cut-off frequency, and an error of 75.84 Hz-3.08 KHz are selected.
8. A Fourier transform infrared spectroscopy processing method according to claim 5 wherein the voltage transfer function of the second order high pass filter is:
wherein the resistance value of the resistor R2 is R8The resistance of the resistor R3 is R7，Aup(s)＝1+R8/R7The capacitance values of the capacitor C1 and the capacitor C2 are both ChThe resistance values of the resistor R1 and the resistor R4 are RhAnd s is a complex frequency domain variable.
9. A Fourier transform infrared spectroscopy processing method according to claim 5 wherein the voltage transfer function of the second order low pass filter is:
wherein the resistance value of the resistor R7 is R11The resistance of the resistor R8 is R12，Aup(s)＝1+R11/R12The resistance values of the resistor R5 and the resistor R6 are RlThe capacitance values of the capacitor C3 and the capacitor C4 are both ClAnd s is a complex frequency domain variable.
10. The method according to claim 9, wherein a is aupThe value of(s) is greater than 3.
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