A photoacoustic spectrum signal detection circuit and quartz tuning fork sensor

Abstract

The present invention discloses a photoacoustic spectrum signal detection circuit and a quartz tuning fork sensor. The photoacoustic spectrum signal detection circuit mainly includes a symmetrical quartz tuning fork sensor, a high-stability active crystal oscillator, a DDS signal generation circuit, a digital phase-locked chip, a counter, a transimpedance amplifier, a resistor, a shaping circuit, a first bandpass filter, a second bandpass filter, a digital-to-analog conversion chip, a microcontroller, a frequency measurement circuit, and a voltage-controlled amplifier circuit. The circuit injects a sinusoidal signal into one cantilever of the quartz tuning fork sensor and excites an acoustic signal, while the other cantilever receives the acoustic signal and converts it into a sinusoidal electrical signal. Laser light passes through the middle of the quartz tuning fork sensor, and as the gas concentration and other properties between the two cantilevers of the quartz tuning fork change, the received sinusoidal signal also changes. The relatively changing frequency signal is tracked by digital lock, and the change in the frequency signal is measured to establish a corresponding relationship with the gas concentration and other properties.

Description

Technical Field

[0001] The present invention relates to the field of photoacoustic spectrum detection and sensors, and in particular to a photoacoustic spectrum signal detection circuit and a quartz tuning fork sensor. Background Art

[0002] Photoacoustic spectroscopy is an indirect absorption spectroscopy technique based on the photoacoustic effect. It has a wide range of applications in trace gas detection, biodiagnosis, chemical analysis, atmospheric monitoring, and other fields. One of the current mainstream photoacoustic spectroscopy detection technologies uses a quartz tuning fork as a signal receiving sensor. Its principle is to superimpose a modulated sinusoidal signal and a scanning triangular wave signal to drive a DFB laser. The modulated scanning laser signal is injected into the gas chamber and produces a photoacoustic effect, causing the quartz tuning fork in the gas chamber to vibrate. The weak piezoelectric signal (nA level) is converted into a voltage signal by a transimpedance amplifier. Through subsequent phase-locked amplifier and demodulation processing, the harmonic signal and absorption peak are obtained. Based on the harmonic signal and absorption peak, the concentration of the gas to be measured is inverted. According to the references "Tong Yao, Ma Yufei. Research progress of trace gas sensing technology based on quartz enhanced photoacoustic spectroscopy [J]. Journal of Liaocheng University (Natural Science Edition), 2019, 32(2): 34-41", "Li Biao, Dong Lei, Wu Hongpeng. Study on the influence of tuning fork quartz crystal resonance frequency on QEPAS sensor performance [J]. Spectroscopy and Spectral Analysis, 2019, 39(10): 3056-3060", "Sun Shanwen, Yi Hongming, Wang Guishi, et al. Influence of water vapor content on the performance of methane gas detection based on QEPAS [J]. Chinese Laser, 2012, 39(7): 209-214. DOI: 10.3788 / CJL201239.0715001", it can be seen that the existing photoacoustic spectroscopy technology based on quartz tuning fork has the following problems:

[0003] The maximum response of a quartz tuning fork is at its resonant frequency (Q value), and the frequency band response bandwidth is very small. The quartz tuning fork is in passive resonance. When the measured signal is overloaded (such as when there is a large amount of water vapor in the measured gas), the resonant frequency of the quartz tuning fork will significantly deviate from the center modulation frequency, resulting in a rapid decrease in sensitivity, and even causing the quartz tuning fork to be unable to oscillate, resulting in measurement failure. Due to the individual inconsistency of quartz tuning fork parameters and the influence of the exposure environment, the resonant frequencies of different tuning forks are different. When measuring using a pre-set center modulation frequency, the resonant frequency of the quartz tuning fork often deviates from the resonant frequency of the quartz tuning fork, resulting in a smaller resonant amplitude of the quartz tuning fork and a smaller output piezoelectric signal, which in turn affects the measurement sensitivity and accuracy. In addition, it is susceptible to interference from external environments such as thermal noise and electromagnetic interference. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of the photoacoustic spectroscopy measurement method based on quartz tuning forks and provide a quartz tuning fork photoacoustic spectroscopy signal detection circuit and a quartz tuning fork sensor based on frequency measurement. The technical solution of the present invention is as follows:

[0005] A photoacoustic spectrum signal detection circuit includes a symmetrical quartz tuning fork sensor, a high-stability active crystal oscillator, a DDS signal generation circuit, a digital phase-locked chip, a counter, a transimpedance amplifier, a resistor R1, a shaping circuit, a first bandpass filter, a second bandpass filter, a digital-to-analog conversion chip, a microcontroller, a frequency measurement circuit, and a voltage-controlled amplifier circuit.

[0006] One pin of the symmetrical quartz tuning fork sensor is connected to the positive input of the transimpedance amplifier. The output of the transimpedance amplifier is connected to the second bandpass filter and fed back to the positive input through resistor R1. The negative output of the transimpedance amplifier is grounded. The output signal of the second bandpass filter is sent to the shaping circuit, and the output signal of the shaping circuit is sent to the ΦA1 pin of the digital phase-locked chip. The output of the high-stability active crystal oscillator is connected to the DDS signal generation circuit, and the output of the DDS signal generation circuit is connected to the KCP and IDCP pins of the digital phase-locked chip. The XORPT pin of the digital phase-locked chip is connected to the D / U pin, and the IDOUT pin is connected to the CLK of the counter. The A, B, C, and D pins of the digital phase-locked chip are controlled by a microcontroller. The CLKOUT of the counter is connected to the ΦB pin of the digital phase-locked chip and the first bandpass filter. The output of the first bandpass filter is connected to the frequency measurement circuit and the input of the voltage-controlled amplifier circuit. The output of the frequency measurement circuit is connected to the microcontroller. The microcontroller is connected to the control digital-to-analog conversion chip. The output voltage of the digital-to-analog conversion chip is connected to the voltage control terminal of the voltage-controlled amplifier circuit. The output of the voltage-controlled amplifier circuit is connected to the other pin of the symmetrical quartz tuning fork sensor.

[0007] Furthermore, the second photoacoustic spectrum signal detection circuit includes a symmetrical quartz tuning fork sensor, a high-stable active crystal oscillator, a DDS signal generating circuit, a digital phase-locked chip, a counter, a transimpedance amplifier, a resistor R1, a shaping circuit, a first bandpass filter, a second bandpass filter, a digital-to-analog conversion chip, a microcontroller, a frequency measurement circuit, a voltage-controlled amplifier circuit, a true effective value conversion chip, a differential amplifier, a PI circuit, and a low-pass filter.

[0008] One pin of the symmetrical quartz tuning fork sensor is connected to the positive input of the transimpedance amplifier, the output of the transimpedance amplifier is connected to the second band-pass filter and fed back to the positive input through resistor R1, the negative output of the transimpedance amplifier is grounded, the output signal of the second band-pass filter is sent to the shaping circuit and the true effective value conversion chip, the output signal of the shaping circuit is sent to the ΦA1 pin of the digital phase-locked chip, the output of the high-stable active crystal oscillator is connected to the DDS signal generating circuit, the output of the DDS signal generating circuit is connected to the KCP and IDCP pins of the digital phase-locked chip, the XORPT pin of the digital phase-locked chip is connected to the D / U pin, the IDOUT pin is connected to the CLK of the counter, and the A, B, C, and D pins of the digital phase-locked chip are connected by a micro The controller controls the counter, the CLKOUT of the counter is connected to the ΦB pin of the digital phase-locked chip and the first band-pass filter, the output of the first band-pass filter is connected to the frequency measurement circuit and the input end of the voltage-controlled amplifier circuit, the output of the frequency measurement circuit is connected to the microcontroller, the microcontroller is connected to the control digital-to-analog conversion chip, the output voltage of the digital-to-analog conversion chip is connected to the negative input end of the differential amplifier, the output of the true effective value conversion chip is connected to the positive input end of the differential amplifier, the output end of the differential amplifier is connected to the input end of the PI circuit, the output end of the PI circuit is connected to the low-pass filter, the output end of the low-pass filter is connected to the voltage control end of the voltage-controlled amplifier circuit, and the output end of the voltage-controlled amplifier circuit is connected to the other pin of the symmetrical quartz tuning fork sensor.

[0009] Preferably, the digital phase-locked lock chip model is CD74ACT297M96, or a chip with similar functions;

[0010] Preferably, the counter is implemented using CPLD or a similar functional chip;

[0011] Preferably, the voltage-controlled amplifier circuit is implemented using an AD603 chip, or a chip with similar functions;

[0012] Preferably, the true effective value conversion chip is AD737, or a chip with similar functions.

[0013] Furthermore, another structure of the quartz tuning fork sensor consists of a first cantilever, a second cantilever, and a rectangular block at the top of the second cantilever. The first cantilever is wider than the second cantilever, forming an asymmetric structure. The cantilever lengths of the first and second cantilever are 3-5mm and the thicknesses are 0.2-0.3mm. The width of the first cantilever is 0.3-0.8mm, and the width of the second cantilever is 0.1-0.3mm. The gap between the first and second cantilever is 0.3-1mm. The rectangular block at the top of the second cantilever is 0.2-0.3mm thick, 0.5-1.2mm wide, and 0.5-1mm high.

[0014] The beneficial effects of the present invention are:

[0015] ⑴ It only needs to scan the laser wavelength, without the need for additional modulation signal, high-precision ADC and phase-locked amplifier circuit. The circuit structure is simpler and the cost is lower.

[0016] (2) When using the traditional photoacoustic spectrum detection method, the quartz tuning fork is in passive resonance and the output signal is very weak. When the signal deviates greatly from the center resonant frequency (such as signal overload), it is significantly attenuated, the sensitivity drops rapidly, and even the quartz tuning fork cannot oscillate. The present invention uses active excitation to resonate the quartz tuning fork, which can automatically track the resonant frequency of the quartz tuning fork and only measures the frequency signal instead of the amplitude signal. Therefore, the detection sensitivity and accuracy are less affected by the vibration amplitude, Q value and bandwidth of the quartz tuning fork, and the probability of cessation of vibration is small.

[0017] ⑶ Because the measured signal is the frequency signal, the measurement sensitivity and accuracy will not be seriously affected by the inconsistent performance of different quartz tuning forks.

[0018] ⑷ Due to the use of a highly stable signal source and an active excitation method with a digital phase-locked function, it has a good suppression effect on various environmental interference factors such as thermal noise and electromagnetic interference, and the system stability is higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Attachment Figure 1 This is the principle block diagram of the first photoacoustic spectrum signal detection circuit.

[0020] Attachment Figure 2 This is the principle block diagram of the second photoacoustic spectrum signal detection circuit.

[0021] Attachment Figure 3 Schematic diagram of the asymmetric quartz tuning fork sensor structure.

[0022] Attachment Figure 4 This is the output signal curve when measuring the water vapor concentration (humidity) at a specific methane concentration using the traditional photoacoustic spectroscopy signal detection method.

[0023] Attachment Figure 5 This is the relationship between the water vapor concentration (humidity) and the detection output signal peak value at a specific methane concentration using the traditional photoacoustic spectroscopy signal detection method.

[0024] Attachment Figure 6 This is the relationship between water vapor concentration (humidity) and frequency output at a specific methane concentration using the traditional photoacoustic spectroscopy signal detection method.

[0025] Attachment Figure 7 The first photoacoustic spectrum signal detection circuit principle block diagram used in the present invention shows the relationship between water vapor concentration (humidity) and frequency output at a specific methane concentration.

[0026] Attachment Figure 8This is the frequency output waveform of the quartz tuning fork sensor after transimpedance amplification and bandpass filtering under specific methane concentration and different water vapor concentration (humidity).

[0027] Attachment Figure 9 The frequency output curves of symmetrical and asymmetrical quartz tuning fork sensors are compared under different water vapor concentrations (humidity). DETAILED DESCRIPTION

[0028] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the scope of protection of the present invention. In addition, it should be understood that after reading the contents of the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by this application.

[0029] like Figure 1 As shown, the first photoacoustic spectrum signal detection circuit includes a symmetrical quartz tuning fork sensor 2, a high-stability active crystal oscillator 3, a DDS signal generating circuit 4, a digital phase-locked chip 5, a counter 6, a transimpedance amplifier 7, a resistor R1, a shaping circuit 8, a first bandpass filter 9, a second bandpass filter 10, a digital-to-analog conversion chip 11, a microcontroller 12, a frequency measurement circuit 13, and a voltage-controlled amplifier circuit 14.

[0030] One pin of the symmetrical quartz tuning fork sensor 2 is connected to the positive input of the transimpedance amplifier 7, the output of the transimpedance amplifier 7 is connected to the second band-pass filter 10 and fed back to the positive input through the resistor R1, the negative output of the transimpedance amplifier 7 is grounded, the output signal of the second band-pass filter 10 is sent to the shaping circuit 8, the output signal of the shaping circuit 8 is sent to the ΦA1 pin of the digital phase-locked chip 5, the output of the high-stability active crystal oscillator 3 is connected to the DDS signal generating circuit 4, the output of the DDS signal generating circuit 4 is connected to the KCP and IDCP pins of the digital phase-locked chip 5, the XORPT pin of the digital phase-locked chip 5 is connected to the D / U pin, and the IDO pin is connected to the D / U pin. The UT pin is connected to the CLK of the counter 6, the A, B, C, and D pins of the digital phase-locked chip 5 are controlled by the microcontroller 12, the CLKOUT of the counter 6 is connected to the ΦB pin of the digital phase-locked chip 5 and the first band-pass filter 9, the output of the first band-pass filter 9 is connected to the frequency measurement circuit 13 and the input end of the voltage-controlled amplifier circuit 14, the output of the frequency measurement circuit 13 is connected to the microcontroller 12, the microcontroller 12 is connected to the control digital-to-analog conversion chip 11, the output voltage of the digital-to-analog conversion chip 11 is connected to the voltage control end of the voltage-controlled amplifier circuit 14, and the output end of the voltage-controlled amplifier circuit 14 is connected to the other pin of the symmetrical quartz tuning fork sensor 2.

[0031] Further, Figure 2 is Figure 1The second improved photoacoustic spectrum signal detection circuit includes a symmetrical quartz tuning fork sensor 2, a high-stable active crystal oscillator 3, a DDS signal generating circuit 4, a digital phase-locked chip 5, a counter 6, a transimpedance amplifier 7, a resistor R1, a shaping circuit 8, a first bandpass filter 9, a second bandpass filter 10, a digital-to-analog conversion chip 11, a microcontroller 12, a frequency measurement circuit 13, a voltage-controlled amplifier circuit 14, a true effective value conversion chip 15, a differential amplifier 16, a PI circuit 17, and a low-pass filter 18.

[0032] One pin of the symmetrical quartz tuning fork sensor 2 is connected to the positive input of the transimpedance amplifier 7, the output of the transimpedance amplifier 7 is connected to the second bandpass filter 10 and fed back to the positive input through the resistor R1, the negative output of the transimpedance amplifier 7 is grounded, the output signal of the second bandpass filter 10 is sent to the shaping circuit 8 and the true effective value conversion chip 15, the output signal of the shaping circuit 8 is sent to the ΦA1 pin of the digital phase-locked chip 5, the output of the high-stability active crystal oscillator 3 is connected to the DDS signal generating circuit 4, the output of the DDS signal generating circuit 4 is connected to the KCP and IDCP pins of the digital phase-locked chip 5, the XORPT pin of the digital phase-locked chip 5 is connected to the D / U pin, the IDOUT pin is connected to the CLK of the counter 6, the A, B, C, and D pins of the digital phase-locked chip 5 are controlled by the microcontroller 12, and the counting The CLKOUT of the device 6 is connected to the ΦB pin of the digital phase-locked chip 5 and the first band-pass filter 9. The output of the first band-pass filter 9 is connected to the frequency measurement circuit 13 and the input end of the voltage-controlled amplifier circuit 14. The output of the frequency measurement circuit 13 is connected to the microcontroller 12. The microcontroller 12 is connected to the control digital-to-analog conversion chip 11. The output voltage of the digital-to-analog conversion chip 11 is connected to the negative input end of the differential amplifier 16. The output of the true effective value conversion chip 15 is connected to the positive input end of the differential amplifier 16. The output end of the differential amplifier 16 is connected to the input end of the PI circuit 17. The output end of the PI circuit 17 is connected to the low-pass filter 18. The output end of the low-pass filter 18 is connected to the voltage control end of the voltage-controlled amplifier circuit 14. The output end of the voltage-controlled amplifier circuit 14 is connected to the other pin of the symmetrical quartz tuning fork sensor 2.

[0033] Preferably, the digital phase-locked chip 5 is CD74ACT297M96, or a chip with similar functions;

[0034] Preferably, the counter 6 is implemented using CPLD or a similar functional chip;

[0035] Preferably, the voltage-controlled amplifier circuit 14 is implemented using an AD603 chip, or a chip with similar functions;

[0036] Preferably, the true effective value conversion chip 15 is AD737, or a chip with similar functions.

[0037] Furthermore, if Figure 3 The figure shows another structural scheme for a quartz tuning fork sensor 2. The sensor 2 consists of a first cantilever 21, a second cantilever 22, and a rectangular block 23 at the top of the second cantilever 22. The first cantilever 21 is wider than the second cantilever 22, forming an asymmetric structure. The first and second cantilever 21, 22 have a cantilever length of 3-5 mm and a thickness of 0.2-0.3 mm. The first cantilever 21 has a width of 0.3-0.8 mm, while the second cantilever 22 has a width of 0.1-0.3 mm. The gap between the first and second cantilever 21, 22 is 0.3-1 mm. The rectangular block 23 at the top of the second cantilever 22 has a thickness of 0.2-0.3 mm, a width of 0.5-1.2 mm, and a height of 0.5-1 mm.

[0038] Example 1

[0039] like Figure 1 In the first photoacoustic spectrum signal detection circuit shown, a highly stable active crystal oscillator 3 provides a stable timebase signal to a DDS signal generator circuit 4. The signal generated by the DDS signal generator circuit 4 passes through a digital phase-locked chip 5, a counter 6, and a first bandpass filter 9 before being output by a voltage-controlled amplifier circuit 14 and stimulating the other pin of a symmetrical quartz tuning fork sensor 2. A scanning laser 1 injected into the gas chamber passes through the central gap of the symmetrical quartz tuning fork sensor 2, causing one pin of the symmetrical quartz tuning fork sensor 2 to vibrate due to the photoacoustic effect. The output signal passes through a transimpedance amplifier 7 and a second bandpass filter 10 before being fed into a shaping circuit 8. Any changes in the gas concentration in the gas chamber cause the frequency signal output by the shaping circuit 8 to change. This frequency signal is then frequency-locked by the digital phase-locked chip 5 and the counter 6. The frequency-locked signal then passes through the first bandpass filter 9 and the voltage-controlled amplifier circuit 14 before stimulating the other pin of the symmetrical quartz tuning fork sensor 2, forming a closed loop. The frequency signal output by the first bandpass filter 9 is measured by a frequency measurement circuit 13 and then read by a microcontroller 12. The frequency signal measured by frequency measurement circuit 13 can be used to invert the gas concentration in the gas chamber. Microcontroller 12 uses digital-to-analog converter chip 11 to set the voltage gain of voltage-controlled amplifier circuit 14, actively controlling the amplitude of the drive signal from symmetrical quartz tuning fork sensor 2. This prevents measurement failure caused by oscillation failure of symmetrical quartz tuning fork sensor 2 when the measured signal is overloaded.

[0040] In order to illustrate the superiority of the present invention, this embodiment is compared with the traditional photoacoustic spectrum signal detection method. The experimental device of the traditional photoacoustic spectrum signal detection method refers to "Li Biao, Dong Lei, Wu Hongpeng. Study on the influence of tuning fork quartz crystal resonance frequency on QEPAS sensor performance [J]. Spectroscopy and Spectral Analysis, 2019, 39(10): 3056-3060". During the test, the mass concentration of 452 mg / m 3The standard CH4 gas is mixed with zero air (the volume ratio of N2 to O2 is 78%:22%). The output voltage of the digital-to-analog conversion chip 11 is set to 5V. Figure 4 This is a graph showing the frequency signal and output signal curves when the water vapor concentration (humidity) changes using the traditional photoacoustic spectroscopy signal detection method. When the water vapor concentration (humidity) is 65%, the signal fluctuates significantly, and when it increases to 75%, no measurement results can be obtained. Figure 5 It will Figure 4 The relationship diagram between the detection output signal peak and water vapor concentration (humidity) is established. Figure 6 It will Figure 4 The relationship between the frequency point where the detection output signal peaks and the water vapor concentration (humidity) is established in the figure. It can be seen that as the water vapor concentration (humidity) increases, the detection output signal peak value decreases and the frequency also decreases. Figure 7 This shows the frequency output changes after changing the moisture concentration (humidity). As can be seen, the frequency signal decreases as the moisture concentration (humidity) increases. Unlike traditional measurement methods, this embodiment can still detect a result when the moisture concentration (humidity) reaches 75%, whereas the traditional method is unable to detect a result.

[0041] Example 2

[0042] like Figure 2In the second photoacoustic spectrum signal detection circuit shown, a highly stable active crystal oscillator 3 provides a stable timebase signal to a DDS signal generator circuit 4. The signal generated by the DDS signal generator circuit 4 passes through a digital phase-locked chip 5, a counter 6, and a first bandpass filter 9 before being output by a voltage-controlled amplifier circuit 14 and stimulating the other pin of a symmetrical quartz tuning fork sensor 2. A scanning laser 1 injected into the gas chamber passes through the central gap of the symmetrical quartz tuning fork sensor 2, causing one pin of the symmetrical quartz tuning fork sensor 2 to vibrate due to the photoacoustic effect. The output signal passes through a transimpedance amplifier 7 and a second bandpass filter 10 before being fed into a shaping circuit 8. Any changes in the gas concentration in the gas chamber cause the frequency signal output by the shaping circuit 8 to change. The changed frequency signal is then frequency-locked by the digital phase-locked chip 5 and the counter 6. The frequency-locked signal then passes through the first bandpass filter 9 and the voltage-controlled amplifier circuit 14 before stimulating the other pin of the symmetrical quartz tuning fork sensor 2, forming a closed loop. The frequency signal output by the first bandpass filter 9 is measured by a frequency measurement circuit 13 and then read by a microcontroller 12. The frequency signal measured by frequency measurement circuit 13 can be used to invert the gas concentration in the gas chamber. The output signal of symmetrical quartz tuning fork sensor 2 passes through second bandpass filter 10 and is then connected to true RMS conversion chip 15. It undergoes differential amplification with the voltage set by microcontroller 12 via digital-to-analog conversion chip 11. Differential amplifier 16 then passes the differentially amplified signal through PI circuit 17 and low-pass filter 18 to form the gain control voltage for voltage-controlled amplifier circuit 14. This actively controls the amplitude of the drive signal for symmetrical quartz tuning fork sensor 2, preventing measurement failure caused by quartz tuning fork oscillation failure when the measured signal is overloaded.

[0043] In order to illustrate the superiority of this embodiment, this embodiment is compared with Example 1. During the test, the mass concentration was 452 mg / m 3 The standard CH4 gas is mixed with zero air (the volume ratio of N2 to O2 is 78%:22%). The digital-to-analog conversion chip 11 sets the output voltage to 5V. Figure 8The waveforms of the input shaping circuit 8 at different moisture concentrations (humidities) are shown. At a moisture concentration (humidity) of 1%, the signal amplitude of the input shaping circuit 8 in both Example 1 and Example 2 is 92mV (0-200us). At a moisture concentration (humidity) of 65%, using the circuit of Example 1, the signal amplitude of the input shaping circuit 8 (upper half of the figure) is approximately 23mV (200-400us), while using the circuit of Example 2, the signal amplitude of the input shaping circuit 8 (lower half of the figure) is approximately 91mV (200-400us). At a moisture concentration (humidity) of 80%, using the circuit of Example 1, the signal amplitude of the input shaping circuit 8 (upper half of the figure) is distorted, making it impossible to detect the frequency (400-600us). Using the circuit of Example 2, the signal amplitude of the input shaping circuit 8 (lower half of the figure) is approximately 10mV, and the waveform remains relatively intact (400-600us).

[0044] Example 3

[0045] The symmetrical quartz tuning fork sensor 2 in Example 1 or Example 2 is replaced by Figure 3 For the asymmetric quartz tuning fork sensor shown, the corresponding circuit remains unchanged, and other test conditions remain unchanged. Figure 9 Using the circuit of Example 1, measurements were performed using both a symmetrical quartz tuning fork and an asymmetrical quartz tuning fork sensor. As can be seen, when the moisture concentration (humidity) changes from 65% to 75%, the frequency change of the asymmetrical quartz tuning fork sensor is approximately 1 Hz, while the frequency change of the symmetrical quartz tuning fork sensor is approximately 0.5 Hz. Therefore, the asymmetrical quartz tuning fork sensor has higher detection sensitivity.

[0046] The above are only specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that are not conceived through creative work should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection defined in the claims.

Claims

1. A photoacoustic spectrum signal detection circuit, characterized in that: It includes a symmetrical quartz tuning fork sensor, a high-stable active crystal oscillator, a DDS signal generating circuit, a digital phase-locked chip, a counter, a transimpedance amplifier, a resistor R1, a shaping circuit, a first bandpass filter, a second bandpass filter, a digital-to-analog conversion chip, a microcontroller, a frequency measurement circuit, and a voltage-controlled amplifier circuit; One pin of the symmetrical quartz tuning fork sensor is connected to the positive input of the transimpedance amplifier. The output of the transimpedance amplifier is connected to the second bandpass filter and fed back to the positive input through resistor R1. The negative output of the transimpedance amplifier is grounded. The output signal of the second bandpass filter is sent to the shaping circuit, and the output signal of the shaping circuit is sent to the ΦA1 pin of the digital phase-locked chip. The output of the high-stability active crystal oscillator is connected to the DDS signal generation circuit, and the output of the DDS signal generation circuit is connected to the KCP and IDCP pins of the digital phase-locked chip. The XORPT pin of the digital phase-locked chip is connected to the D / U pin, and the IDOUT pin is connected to the CLK of the counter. The A, B, C, and D pins of the digital phase-locked chip are controlled by a microcontroller. The CLKOUT of the counter is connected to the ΦB pin of the digital phase-locked chip and the first bandpass filter. The output of the first bandpass filter is connected to the frequency measurement circuit and the input of the voltage-controlled amplifier circuit. The output of the frequency measurement circuit is connected to the microcontroller. The microcontroller is connected to the control digital-to-analog conversion chip. The output voltage of the digital-to-analog conversion chip is connected to the voltage control terminal of the voltage-controlled amplifier circuit. The output of the voltage-controlled amplifier circuit is connected to the other pin of the symmetrical quartz tuning fork sensor.

2. A photoacoustic spectrum signal detection circuit according to claim 1, characterized in that: Including symmetrical quartz tuning fork sensor, high-stable active crystal oscillator, DDS signal generation circuit, digital phase-locked chip, counter, transimpedance amplifier, resistor R1, shaping circuit, first bandpass filter, second bandpass filter, digital-to-analog conversion chip, microcontroller, frequency measurement circuit, voltage-controlled amplifier circuit, true RMS conversion chip, differential amplifier, PI circuit, low-pass filter; One pin of the symmetrical quartz tuning fork sensor is connected to the positive input of the transimpedance amplifier, the output of the transimpedance amplifier is connected to the second band-pass filter and fed back to the positive input through resistor R1, the negative output of the transimpedance amplifier is grounded, the output signal of the second band-pass filter is sent to the shaping circuit and the true effective value conversion chip, the output signal of the shaping circuit is sent to the ΦA1 pin of the digital phase-locked chip, the output of the high-stable active crystal oscillator is connected to the DDS signal generating circuit, the output of the DDS signal generating circuit is connected to the KCP and IDCP pins of the digital phase-locked chip, the XORPT pin of the digital phase-locked chip is connected to the D / U pin, the IDOUT pin is connected to the CLK of the counter, and the A, B, C, and D pins of the digital phase-locked chip are connected by a micro The controller controls the counter, the CLKOUT of the counter is connected to the ΦB pin of the digital phase-locked chip and the first band-pass filter, the output of the first band-pass filter is connected to the frequency measurement circuit and the input end of the voltage-controlled amplifier circuit, the output of the frequency measurement circuit is connected to the microcontroller, the microcontroller is connected to the control digital-to-analog conversion chip, the output voltage of the digital-to-analog conversion chip is connected to the negative input end of the differential amplifier, the output of the true effective value conversion chip is connected to the positive input end of the differential amplifier, the output end of the differential amplifier is connected to the input end of the PI circuit, the output end of the PI circuit is connected to the low-pass filter, the output end of the low-pass filter is connected to the voltage control end of the voltage-controlled amplifier circuit, and the output end of the voltage-controlled amplifier circuit is connected to the other pin of the symmetrical quartz tuning fork sensor.

3. A photoacoustic spectrum signal detection circuit using any one of claims 1-2, characterized in that: The digital phase-locked lock chip model is CD74ACT297M96, or a chip with similar functions.

4. A photoacoustic spectrum signal detection circuit using any one of claims 1-2, characterized in that: The counter is implemented by using CPLD or a chip with similar functions.

5. A photoacoustic spectrum signal detection circuit using any one of claims 1-2, characterized in that: The voltage-controlled amplifier circuit is implemented using an AD603 chip or a chip with similar functions.

6. A photoacoustic spectrum signal detection circuit using the photoacoustic spectrum signal detection circuit according to claim 2, characterized in that: The true effective value conversion chip is AD737, or a chip with similar functions.

7. A photoacoustic spectrum signal detection circuit using any one of claims 1-2, characterized in that: The quartz tuning fork sensor consists of a first cantilever, a second cantilever, and a rectangular block at the top of the second cantilever. The first cantilever is wider than the second cantilever, forming an asymmetric structure. The cantilever lengths of the first cantilever and the second cantilever are 3-5 mm and 0.2-0.3 mm, the width of the first cantilever is 0.3-0.8 mm, the width of the second cantilever is 0.1-0.3 mm, the gap between the first cantilever and the second cantilever is 0.3-1 mm, and the rectangular block at the top of the second cantilever is 0.2-0.3 mm thick, 0.5-1.2 mm wide, and 0.5-1 mm high.

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