Threshold discriminator circuit applied to time-of-flight mass spectrometry

Abstract

The invention discloses a threshold discriminator circuit applied to a time-of-flight mass spectrometry. The threshold discriminator circuit comprises a power supply module, a signal amplification module, a threshold setting module, a first voltage comparison module, a high-speed D trigger, a second voltage comparison module, a pulse width modulation module and a differential amplification module, the signal amplification module amplifies the input mass spectrum signal; the threshold value setting module outputs a set threshold value; the first voltage comparison module compares the threshold value with the amplified mass spectrum signal, and converts the mass spectrum signal into a digital pulse signal of a digital ELC level; the high-speed D trigger shortens the pulse width of the digital pulse signal to output a spike pulse signal; the pulse width modulation module adjusts a baseline of the spike pulse signal; the second voltage comparison module is used for comparing the peak pulse signal subjected to baseline modulation with a reference level and outputting a small-amplitude pulse signal; and the differential amplification module performs differential amplification on the small-amplitude pulse signal and then outputs a final pulse signal. According to the invention, mass spectrum signals generated by the time-of-flight mass spectrometer can be rapidly identified and screened.

Description

Technical Field

[0001] The invention relates to the field of mass spectrometer signal acquisition, and in particular to a threshold discriminator circuit applied to time-of-flight mass spectrometry. Background Art

[0002] Constant fraction discriminator (CFD), also known as threshold discriminator, is a hardware circuit that can identify pulse spike signals with peak-to-base widths ranging from a few nanoseconds to tens of nanoseconds. The function of this circuit can be summarized as: judging whether the signal peak is greater than the set threshold, and converting the signal peak into a digital pulse signal with the smallest possible transmission delay. The digital pulse signal generated by the conversion can be identified, counted and accumulated by TDC (TDC is a high-frequency counter that counts the generated digital pulse signal).

[0003] After entering the TOFMS analyzer, the ions go through a series of flight processes and finally hit the MCP detector. After a series of processes such as electron multiplication, the MCP detector finally outputs an analog signal composed of a series of Gaussian peaks of different intensities. The time difference between the peak at a certain moment and the moment when the repulsion voltage arrives contains the mass-to-charge ratio information of the ions. The repulsion voltage is a series of equally spaced and equally duty-cycled high-voltage pulse trains that provide the particles with flight kinetic energy in TOFMS. TDC is a device used to identify events and convert their occurrence time into digital output. It is usually used to measure the time interval between events and is also called a time counter in some applications. TDC has many working modes and usually requires at least two signals, namely the start signal and the stop signal (reflected in CFD as the rising edge starting point and the falling edge end point of the digital pulse signal), which control the start and stop of the measurement cycle respectively. TDC has many application areas, from measuring the flight time and life span of various particles in atomic energy and high-energy physics to the optical ranging sensors commonly used in mobile phones in our lives.

[0004] Since TDC has a poor response to mass spectrometry signals generated by trace elements, the mass spectrometry signals generated by trace elements refer to Gaussian peaks with intensities ranging from a few millivolts to tens of millivolts. In addition, some existing TDCs in China cannot directly identify negative mass spectrometry signals. If the output signal of MCP is directly connected to TDC, there will be a poor response to trace elements and the negative signal cannot be identified. Based on this situation, we need a hardware design that can identify trace signals and can uniformly convert Gaussian peaks of different intensities into digital pulse signals with equal amplitudes that can always be recognized by TDC. That is, a threshold discriminator.

[0005] The existing threshold discriminators for mass spectrometry Gaussian peak signals can identify the minimum signal threshold (the existing threshold discriminators on the market have a minimum threshold of 100mV) and generally do not meet the requirements of mass spectrometry signals. For high-frequency mass spectrometry signals, many domestic TDCs have built-in threshold discriminators that are implemented through programming and require complex hardware processing, and cannot achieve ultra-low signal delay. Therefore, leakage points will occur during the acquisition and conversion of signals, resulting in errors in acquisition accuracy. In addition, many threshold discriminators on the market cannot control the working state of the threshold discriminator through external control signals. Therefore, the research and development of low-cost threshold discriminators for small mass spectrometry signals is imminent. Summary of the invention

[0006] The main purpose of the present invention is to overcome the defects of the TDC in the prior art that cannot directly identify mass spectrometry signals of lower levels, the existing threshold discriminator (CFD) cannot achieve extremely low threshold settings for time-of-flight mass spectrometry trace element signals, and has no external signal control module, high cost, small frequency band range, long dead time, etc. A threshold discriminator circuit applied to time-of-flight mass spectrometry is proposed, which converts mass spectrometry signals exceeding the set threshold into digital pulse signals with equal amplitude and pulse width and meeting the requirements of TDC acquisition equipment with extremely low signal transmission delay, thereby reducing costs.

[0007] The present invention adopts the following technical solution:

[0008] A threshold discriminator circuit for time-of-flight mass spectrometry comprises a power supply module, a signal amplification module powered by the power supply module, a threshold setting module, a first voltage comparison module, a high-speed D-type flip-flop, a second voltage comparison module, a pulse width modulation module and a differential amplification module; the signal amplification module is used to amplify an input mass spectrum signal; the threshold setting module is used to output a set threshold; the first voltage comparison module is used to compare the input threshold value with the amplified mass spectrum signal and convert the amplified mass spectrum signal into a digital pulse signal of a digital ELC level; the high-speed D-type flip-flop shortens the pulse width of the digital pulse signal to output a spike pulse signal; the pulse width modulation module adjusts the baseline of the spike pulse signal; the second voltage comparison module compares the spike pulse signal modulated by the baseline with a reference level and outputs a small-amplitude pulse signal; the differential amplification module performs differential amplification on the small-amplitude pulse signal and outputs a final pulse signal.

[0009] The signal amplification module includes an amplifier chip, the input end of which is connected in series with a gas discharge tube, a capacitor C8, a resistor R7 and a capacitor C9 in sequence; capacitor C8 is connected in parallel with capacitor C4 to filter the DC component of the mass spectrometry signal, and the two ends of capacitor C4 are respectively connected to resistors R2 and R3; capacitor C9 is connected in parallel with capacitor C5; a resistor R11 is also connected between resistor R7 and capacitor C9, and the amplification factor is adjusted by adjusting the resistance values ​​of resistors R7 and R11; the output end of the amplifier chip is connected to capacitor C11, and capacitor C12 is connected in parallel with both ends of capacitor C11 to filter the DC component of the mass spectrometry signal and then output to the first voltage comparison module.

[0010] The threshold setting module includes a sliding rheostat RP1, a resistor R18, a resistor R20, a resistor R21, a resistor R22, a resistor R23, a resistor R24, a resistor R27, a resistor R28 and a capacitor C13; one end of the resistor R18 is connected to the input end of the first voltage comparison module, and the other end is connected to the sliding end of the sliding rheostat RP1, a fixed end of the sliding rheostat RP1 is connected to the resistor R27, and the other fixed end of the sliding rheostat RP1 is connected to the power supply module and is connected in series with resistors R21, R22, R23 and R24 in sequence; one end of the resistor R28 is connected to the resistor R27, and the other end is connected between the resistor R21 and the resistor R22; one end of the resistor R20 is connected to one end of the resistor R18, and the other end is grounded, and the capacitor C13 is connected in parallel with the resistor R20.

[0011] In the first voltage comparison module, when the mass spectrum signal is greater than the threshold, the positive output port of the first voltage comparison module outputs an ELC high level; when the mass spectrum signal is less than the threshold, the positive output port of the first voltage comparison module outputs an ECL low level.

[0012] The high-speed D trigger adopts a high-speed D trigger, which receives the digital pulse signal of the digital ELC level generated by the first voltage comparison module, and generates a spike pulse signal through high-speed self-shutdown and extremely short metastable recovery time.

[0013] It also includes a signal suppression module, which is connected to the enable end of the high-speed D flip-flop to control its operation; the signal suppression module is powered by the power supply module.

[0014] The pulse width modulation module includes a transistor Q8, an inductor L9, a resistor R58, a resistor R59, a capacitor C29 and a sliding resistor RP2; the base of the transistor Q8 is connected to the output end of the high-speed D trigger, the emitter of the transistor Q8 is connected in series with the inductor L9, the resistor R58 and the resistor R59 in sequence, one end of the capacitor C29 is connected to the connecting point of the inductor L9 and the resistor R58 and is connected to the second voltage comparison module; the sliding end of the sliding resistor RP2 is connected to a fixed end of the sliding resistor RP2, one end of the resistor R59 and the power supply module, and the other fixed end of the sliding resistor RP2 is connected to the other end of the resistor R59.

[0015] The second voltage comparison module is provided with a fixed resistor to provide the reference level, the fixed resistor is powered by the power supply and includes a resistor R61, a resistor R63, a resistor R66 and a resistor R67; the resistor R61, the resistor R63 and the resistor R66 are connected in series in sequence, the resistor R67 is connected in parallel with the resistor R66, and the connected end of the resistor R63 and the resistor R66 outputs the reference level.

[0016] The input end of the differential amplifier module connected to the negative output port of the second voltage comparison module is connected in parallel with a capacitor C10. The power storage of the capacitor C10 accelerates the conduction of the subsequent transistor, thereby reducing the response time of the circuit.

[0017] It can be seen from the above description of the present invention that, compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. In the present invention, a signal amplification module, a threshold setting module, a first voltage comparison module, a high-speed D flip-flop, a second voltage comparison module, a pulse width modulation module and a differential amplification module are used in combination to convert a mass spectrometry signal exceeding a set threshold into a digital pulse signal with equal amplitude and pulse width and meeting the requirements of a TDC acquisition device with extremely low signal transmission delay, thereby reducing costs.

[0019] 2. In the present invention, the voltage divider circuit structure setting of the threshold setting module can achieve -10mV to -1V voltage divider signal modulation by adjusting the resistance value of the sliding rheostat. The reference signal and the mass spectrometer signal amplified ten times are passed into the first voltage comparator for comparison, thereby achieving a mass spectrometer signal threshold modulation equivalent to -1mV to -100mV. The ultra-low detection threshold enhances the ability of TDC to respond to tiny mass spectrometer signals and improves the sensitivity of the instrument.

[0020] 3. In the present invention, the pulse width modulation module generates a voltage of a specific amplitude in the form of a resistor divider, and cooperates with a high-speed D trigger to output an adjustable pulse spike of 2-30ns. The modulatable range of the pulse spike can adapt to the dead time characteristic parameters of most TDCs on the market, thereby enhancing the TDC's ability to resolve signals generated by ions with a relatively short flight time and improving the resolution of the instrument.

[0021] 4. The present invention, through the high-speed self-shutdown of the high-speed D flip-flop, quickly generates a pulse spike with a peak-to-base width of about 30ns in a short time, thereby greatly reducing the dead time of the threshold discriminator. The frequency of two adjacent pulses can be around 300MHz, that is, the signal dead time is less than 4ns; and the high-speed D flip-flop has a signal transmission delay as low as a few nanoseconds, which is specifically adapted to the high-frequency Gaussian peak characteristics generated by the mass spectrometer.

[0022] 5. In the present invention, level conversion is performed through a specific signal suppression circuit to control a specific pin of a D-type flip-flop, so that the switching of the entire circuit module can be controlled by changing an external level signal, thereby facilitating subsequent digital integration and control.

[0023] 6. In the present invention, by selecting a chip with extremely low signal delay, signal processing can greatly reduce signal delay, and the overall signal transmission delay is about 10ns.

[0024] 7. In the present invention, the signal amplification module can quickly filter the DC component generated by the mass spectrometry signal through the capacitor and apply a forward bias, thereby maximizing the performance of the amplifier chip and achieving tracking and amplification of a larger range of mass spectrometry signals; and the amplified tiny signal will be more accurately identified in the subsequent circuit, thereby increasing the sensitivity of the instrument. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the main component diagram of the circuit of the present invention;

[0026] Figure 2 This is the circuit diagram of the signal amplification module;

[0027] Figure 3 Circuit diagram for threshold setting module;

[0028] Figure 4 is a circuit diagram of a first voltage comparison module;

[0029] Figure 5 This is the circuit diagram of a high-speed D flip-flop;

[0030] Figure 6 This is the circuit diagram of the signal suppression module;

[0031] Figure 7 This is the circuit diagram of the pulse width modulation module;

[0032] Figure 8 is a circuit diagram of a second voltage comparison module;

[0033] Fig. 9 This is the circuit diagram of the differential amplifier module;

[0034] Fig.10 This is the PCB design effect diagram of the present invention;

[0035] Fig.11 This is a physical diagram of the present invention;

[0036] Fig.12 is the normalized mass spectrometry signal;

[0037] Fig.13 is the normalized mass spectrometry signal integration curve;

[0038] Fig.14 This is the circuit simulation diagram of the threshold comparison module;

[0039] Fig.15 Verifying the function of a threshold comparison module of a first voltage comparison module;

[0040] Fig.16 This is the circuit diagram of the signal latch module;

[0041] Fig.17 It is the level transition truth table of the high-speed D flip-flop chip;

[0042] Fig.18 This is the timing diagram of MC10EL31 generating pulses through high-speed self-shutdown;

[0043] Fig.19 It is the spike pulse wave generated by fast disconnection of MC10EL31 pin Q;

[0044] Fig. 20 Circuit simulation and results of signal latch module;

[0045] Fig.21 Physical function verification of signal latch module;

[0046] Fig. 22 Verify the function of the pulse width modulation module;

[0047] Fig.23 The pin Q of the voltage comparator chip generates a spike wave (shown in the red circle) due to insufficient rise time;

[0048] Fig.24 This is the circuit diagram of the reverse amplifier;

[0049] Fig.25 This is a physical picture of an external inverter;

[0050] Fig.26 This is the collection result diagram of the standard sample;

[0051] Fig. 27 This is the performance parameter table.

[0052] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. DETAILED DESCRIPTION

[0053] The present invention is further described below through specific implementation modes.

[0054] A threshold discriminator circuit applied to time-of-flight mass spectrometry is used to determine whether the intensity of a signal peak (generally a Gaussian peak generated by a signal collection device of a time-of-flight mass spectrometer) is greater than a set threshold, and converts the signal peak into a digital pulse signal. On the premise of achieving basic functions, the circuit design should further meet the following conditions: ① The set threshold needs to be adjustable within a certain range, and take into account the mass spectrometer signal with smaller intensity as much as possible. ② The signal processing time needs to be as short as possible. ③ The start and stop of the instrument can be controlled by an external signal. ④ The dead time (the minimum response interval between two adjacent signals) is as short as possible.

[0055] See also Figure 1 According to the basic functions and conditions required to be realized, the present invention designs a threshold discriminator circuit for time-of-flight mass spectrometry based on the flow of signals, which includes a power supply module and a signal amplification module powered by the power supply module, a threshold setting module, a first voltage comparison module, a high-speed D trigger, a second voltage comparison module, a pulse width modulation module and a differential amplifier module. The signal flow is described as follows: the mass spectrum signal of MCP enters the signal amplification module, and after being amplified several times (for example, ten times), it enters the first voltage comparison module. The threshold setting module is used to output the set threshold. The first voltage comparison module compares the amplified mass spectrum signal with the threshold (i.e., the trigger value) set by the threshold setting module, and converts the mass spectrum signal after the previous stage amplification into a digital pulse signal of the digital ELC level.

[0056] The power supply module of the present invention includes a plurality of low ripple voltage-stabilized power supply modules, including a +5V low ripple voltage-stabilized power supply module (6), a -5V low ripple voltage-stabilized power supply module (7), a -2.2V low ripple voltage-stabilized power supply module, etc. The signal amplification module is powered by the +5V low ripple voltage-stabilized power supply module, and the threshold setting module is powered by the +5V low ripple voltage-stabilized power supply module and the -5V low ripple voltage-stabilized power supply module. The pulse signal generated by the first voltage comparison module is passed through the ELC signal generated by the -2.2V low ripple voltage-stabilized power supply module, and then pulled down and passed into the trigger pin of the high-speed D flip-flop.

[0057] The main purpose of the signal amplification module is to amplify tiny pulse signals without distortion, so that the subsequent voltage comparison is more accurate. In the subsequent design of this module, the reverse overshoot of the mass spectrometer signal (generally a negative signal) is also limited. Considering the special situation, the internal damage of the instrument will generate high voltage. A gas discharge tube is set as a protection device at the signal input level of this module. The chip selected for this module has different amplification responses for signals in different frequency bands, so this chip can distinguish trace elements and baseline noise very well.

[0058] For details, see Figure 2 The signal amplification module includes an amplification chip, and the input end of the amplification chip is connected in series with a gas discharge tube, a capacitor C8, a resistor R7 and a capacitor C9 in sequence; the capacitor C8 is connected in parallel with a capacitor C4 to filter the DC component of the mass spectrometry signal, and the two ends of the capacitor C4 are respectively connected to resistors R2 and R3; the capacitor C9 is connected in parallel with a capacitor C5; a resistor R11 is also connected between the resistor R7 and the capacitor C9, and the amplification factor is adjusted by adjusting the resistance values ​​of the resistor R7 and the resistor R11; the output end of the amplification chip is connected to the capacitor C11, and the two ends of the capacitor C11 are connected in parallel with a capacitor C12 to filter the DC component of the mass spectrometry signal and then output to the first voltage comparison module.

[0059] Figure 2 In the input stage, the first is the protection device. A resistor R2 is set to discharge the DC isolation capacitor in the event of a sudden power outage. Two capacitors C4 and C8 are set to isolate the DC amount of the mass spectrometer signal. For instantaneous voltages with a large rate of change, in order to prevent them from damaging the chips of the subsequent circuits, a gas discharge tube is added to the input stage to protect the safety of the circuit. A bidirectional clamping diode chip is also set at the input end of the amplifier chip to clamp the forward mass spectrometer signal overshoot (the forward signal overshoot exceeding 0.7V is clamped at +0.7V through the conduction voltage drop of the diode), and the overall signal amplification factor is adjusted to 20dB (10 times) by adjusting the resistance values ​​of resistors R7 and R11.

[0060] Add a DC blocking capacitor at the output stage of the signal amplifier module to ensure that the amplified mass spectrometer signal is completely retained and has no DC offset. After filtering out the DC amount, the characteristic of the AC signal is that the positive and negative area integrals are the same. After a simplified calculation, it is found that the baseline offset generated by the mass spectrometer signal within 1s is at the level of hundreds of μV (see Fig.12 , Fig.13 ). This magnitude of signal offset has almost no effect on subsequent signal comparison. The baseline offset error caused by different MCP assembly forms can be solved after the signal passes through three sets of DC blocking capacitors (capacitors C4 / C8, capacitors C5 / C9, and capacitors C11 / C12).

[0061] See also Fig.12 ,The calculation of baseline shift is to normalize a single spectrum of mass spectrometry signal and then integrate it; Fig.13 In the figure, when a mass spectrometer signal with a high amplitude of 1V is generated, a signal integral of 0.013 will be generated in a single repulsion cycle (50us), and the maximum intensity of the mass spectrometer signal of the CFD test instrument is 5V, so a signal integral of 0.065 will be generated in a single repulsion cycle. There are 10*14 (10 ion excitations, 14 repulsions will be collected each time) repulsions in 1s, totaling 7ms. The average baseline shift degree in 1s is converted by the formula to a baseline shift of 65*7 / 1000=450uV.

[0062] The threshold setting module includes a sliding rheostat RP1, a resistor R18, a resistor R20, a resistor R21, a resistor R22, a resistor R23, a resistor R24, a resistor R27, a resistor R28 and a capacitor C13; one end of the resistor R18 is connected to the input end of the first voltage comparison module, and the other end is connected to the sliding end of the sliding rheostat RP1, a fixed end of the sliding rheostat RP1 is connected to the resistor R27, and the other fixed end of the sliding rheostat RP1 is connected to the power supply module and is connected in series with resistors R21, R22, R23 and R24 in sequence; one end of the resistor R28 is connected to the resistor R27, and the other end is connected between the resistor R21 and the resistor R22; one end of the resistor R20 is connected to one end of the resistor R18, and the other end is grounded, and the capacitor C13 is connected in parallel with the resistor R20.

[0063] The threshold setting module of the present invention adopts a Wheatstone bridge. By adjusting the resistance of the sliding rheostat RP1, the output level range of the threshold setting module is within -10mV to -1000mV and is adjustable. It is used in the subsequent first voltage comparison module to compare the voltage with the amplified mass spectrometer signal, and a special threshold feedback interface (THFB) is derived to feedback the level value of C1_N. In this module, the output voltages of THFB and C1_N can be approximately considered equal. In order to prevent crosstalk of the level signal, the circuit chooses to derive the feedback value of the comparison voltage through the THFB pin. For details of the specific circuit design and output simulation diagram, see Fig.14 .exist Fig.14 In the simulation, the designed circuit is built, and finally the resistance values ​​of R6A, R7A, R8A and other resistors are matched and determined according to the requirements (the resistors R6A and R8A here refer to Fig.14 Resistors in simulation, not relevant to schematic). Fig.14 In (a) and (b), typical parameter values ​​of RP1 are selected for verification, (a) R3 = 5KΩ, (b) R3 = 250Ω. Fig.14 The voltage measured by the multimeter-XMM1 is the voltage value of THFB in the schematic diagram. Fig.14 The voltage measured by the multimeter - XMM2 is the value of C1_N in the schematic diagram.

[0064] The main function of the first voltage comparison module is to compare the mass spectrometer signal after 10 times amplification with the level of the threshold setting module, and generate and output the ELC level at the output stage to trigger the CLK pin of the subsequent high-speed D flip-flop. In order to speed up the system's response to the mass spectrometer signal and shorten the signal transmission delay time in the module as much as possible, the core comparator chip model selected by the present invention is AD96685, which takes into account both performance and cost performance. The final test shows that the module propagation delay is only 2.5ns.

[0065] Specifically, the first voltage comparison module is implemented as follows: when the mass spectrum signal is greater than the threshold, the positive output port Q of the first voltage comparison module outputs an ELC high level, that is, a digital signal 1; when the mass spectrum signal is less than the threshold, the positive output port Q of the first voltage comparison module outputs an ECL low level, that is, a digital signal 0. The level flip generated by the input signal being greater than the threshold is shown in FIG. Fig.15 As shown, the threshold is set to 10mV, as shown by the dotted line Y4 in the figure, and the propagation delay is about 30ns due to the wire and the oscilloscope probe.

[0066] The high-speed D flip-flop shortens the pulse width of the digital pulse signal to output a spike pulse signal. The high-speed D flip-flop is a commonly used digital level trigger with latching and following functions, and some specific connection methods can realize some special functions. The main purpose of this link is to convert the mass spectrometer signal with a large pulse width into a uniform small pulse width pulse signal, shorten the pulse width of the digital pulse signal as much as possible (for example, the half-peak width can reach about 15ns), thereby reducing the dead time of the instrument. In the present invention, the high-speed D flip-flop receives the digital pulse signal of the digital ELC level generated by the first voltage comparison module, and generates a spike pulse signal through high-speed self-shutdown and extremely short metastable recovery time.

[0067] The main functions of the high-speed D flip-flop module are: ① Accept the ELC level generated by the first voltage comparison module, and generate a pulse spike with a peak-to-bottom width of 30ns through high-speed self-shutdown and extremely short metastable recovery time (metastable refers to an unknown unstable state between high and low levels in a numerical circuit). This spike is used for pulse width modulation in the subsequent second voltage comparator module. ② Receive the level signal of the signal suppression module to determine the working state of the entire system. This module also needs to make the system's response time as fast as possible, the delay time as short as possible, and also requires a faster recovery speed when the signal is in a metastable state.

[0068] First, we analyze function ① and design the following according to the high-speed metastable response characteristics of the D flip-flop: Fig.16 The hardware circuit of the MC10EL31 chip is shown in the following figure: Fig.17As shown in the figure, when the CLK pin of MC10EL31 receives the mass spectrometer signal and generates the ECL signal, each pin of the D flip-flop will quickly jump and recover. In this process, due to the special external wiring method, the D flip-flop will generate a very short pulse spike. The specific timing diagram is as follows Fig.18 shown.

[0069] Now let's briefly introduce the entire level jump process: When the external signal suppression is not set, the D pin is always kept in the D level state through an external pull-down, and the CLK connected to the AD96685 receives the ELC square wave pulse signal from the previous stage. When the rising edge comes, according to the truth table, Q and Q' flip at the same time. When Q and Q' enter the undefined level interval at the same time, the level of SET (connected to Q') also enters the metastable state (a state between high level and low level). At this time, the metastable recovery function of the chip is triggered, and the circuit system restores the system to a stable level state (that is, Q is high level and Q' is low level) as quickly as possible. This rapid disconnection is manifested in the waveform as a spike pulse with an amplitude of 50mV and a maximum pulse width of 30ns. As shown Fig.19 As shown, the purple-red line uses a continuous positive exponential wave simulation mass spectrometry signal with a frequency of 20MHz and an amplitude of 250mV generated by a pulse generator, and the yellow line is the waveform transformation of the Q' pin of the D trigger measured by a 200MHz oscilloscope charcoal pencil. Due to the fast shutdown characteristics and ultra-low threshold settings, the D trigger can distinguish the pulse peak signal from the subsequent oscillation small peak with a smaller amplitude.

[0070] Secondly, we analyze the function ② of the high-speed D flip-flop module. This function needs to be introduced together with the signal latch module of the circuit. The function of the signal latch module is:

[0071] When the external input is -500mV, the signal latch module outputs the ELC level 1.

[0072] When the external input is -500mV, the signal latch module outputs the ELC level of 0.

[0073] The ELC level signal then controls the on / off of the D flip-flop module, thereby controlling the output of the entire CFD.

[0074] Figure 20-21 It is the functional verification of the signal latch module, where Fig. 20 (a) in the figure verifies the feasibility of the level conversion control circuit through simulation design. After the feasibility verification, the specific implementation plan is shown in the schematic diagram signal latch module. The truth table of the set value of the veto voltage and the various pins of the D flip-flop chip MC10EL31 controlled by it is summarized. Fig. 20(b) is shown in the figure, and (c) is the input-output truth table. The actual object was also verified in subsequent experiments. Fig.21 The purple line in the middle represents the external signal input by Veto, while the yellow signal line represents the output of the D flip-flop Q'. During the test, a pulse generator (FE's FY6900 series pulse generator) was used to generate a continuous positive exponential wave analog mass spectrometry signal with a frequency of 20MHz and an amplitude of 250mV. However, it can be clearly found that when VETO inputs a -500mV signal, the circuit has no response to the input signal.

[0075] The high-speed D trigger module is summarized as follows: ① Generate high-speed pulse spike input voltage comparison module 2. ② Control the working state of the instrument.

[0076] The present invention also includes a signal suppression module, the front stage of the signal suppression module is connected to the logic level of the external input, and the signal output of the rear stage controls whether the high-speed D flip-flop module is enabled or not, thereby controlling the operation of the high-speed D flip-flop. The signal suppression module is powered by the power supply module, specifically, the signal suppression module is powered by a -5V low ripple voltage-stabilized power supply module. Figure 6 This is the circuit diagram of the signal suppression module.

[0077] The pulse width modulation module adjusts the baseline of the spike pulse signal. The pulse width modulation module includes a transistor Q8, an inductor L9, a resistor R58, a resistor R59, a capacitor C29 and a sliding resistor RP2; the base of the transistor Q8 is connected to the positive output end of the high-speed D trigger, the emitter of the transistor Q8 is connected in series with the inductor L9, the resistor R58 and the resistor R59 in sequence, one end of the capacitor C29 is connected to the connection point between the inductor L9 and the resistor R58 and is connected to the second voltage comparison module; the sliding end of the sliding resistor RP2 is connected to a fixed end of the sliding resistor RP2, one end of the resistor R59 and the power supply module, and the other fixed end of the sliding resistor RP2 is connected to the other end of the resistor R59.

[0078] The specific function of the pulse width modulation module is: through the voltage drop of the transistor Q8 and the adjustment of the sliding rheostat RP2, the pulse peak baseline generated by the high-speed D trigger module entering the second voltage comparison module can be adjusted, and this is used as the basis for the subsequent chopping of the second voltage comparison module. In the present invention, after the selection of resistors and transistors. The baseline of the pulse peak signal is adjustable in the range of 230mV to 270mV. For specific test results, see Fig. 22 In (a1), (a2), (b), and (c), the signal baseline is adjusted to 230mV (a1) to 270mV (a2) through baseline adjustment and the voltage drop of the transistor. (b) The comparison voltage is set to -260mV through external circuit design. (c) The peak pulse width measurement △x is about 50ns.

[0079] The second voltage comparison module compares the peak pulse signal modulated by the baseline with the reference level and outputs a small amplitude pulse signal. Figure 8 The circuit diagram of the second voltage comparison module has the following specific functions: receiving the pulse spike signal generated by the D trigger after baseline modulation. Comparing it with the reference level generated by the voltage division of the fixed resistor (R61, R63, R66, R67 in the schematic diagram), and adjusting the pulse width of the part of the signal that is greater than the reference level by adjusting the baseline of the spike signal, the module will produce a digital pulse signal with the same pulse width at the output end. Among them, the fixed resistor is powered by a power supply, and includes resistor R61, resistor R63, resistor R66 and resistor R67; resistor R61, resistor R63 and resistor R66 are connected in series in sequence, resistor R67 is connected in parallel with resistor R66, and the end connected with resistor R63 and resistor R66 outputs the reference level.

[0080] One channel of the second voltage comparison module receives the pulse spike signal generated by the Q pin of the high-speed D trigger after baseline modulation, and through the design of the external circuit, the baseline of the pulse spike signal is clamped by using the tube voltage drop (0.7V) of the PNP triode and the external adjustable sliding resistor, so that the baseline of the pulse spike signal is adjustable in the range of -230mV to -270mV, and the amplitude of the other input is clamped at about -261mV through the resistor voltage divider. Fig. 22 (b). The maximum value of a single spike pulse after baseline processing is about 50ns. Fig. 22 (c) Therefore, by adjusting the baseline of the pulse peak signal at -261mV to chop the signal, a pulse peak wave output with a pulse width of 2ns-30ns can be achieved through the comparator output.

[0081] Depend on Figure 8 It can be seen that the high-speed voltage comparator chip selected by this hardware is model AD96685, and its level jump time is about 120ns. Therefore, this module cannot complete the entire ELC level jump process from 0 to 1 within 2ns to 30ns. The final waveform will appear as a digital pulse spike of 2ns to 30ns. The specific test results are as follows Fig.23 As shown in the figure, after chopping and baseline modulation, the spike pulse width is about 10ns. Among them, the pin Q of the voltage comparator chip generates a spike (shown in the red circle) due to insufficient rise time.

[0082] The differential amplifier module differentially amplifies the small-amplitude pulse signal and outputs the final pulse signal. The input end of the differential amplifier module connected to the negative output port of the second voltage comparison module is connected in parallel with a capacitor C10. The power storage of the capacitor C10 accelerates the conduction of the subsequent transistor, thereby reducing the response time of the circuit.

[0083] The main function of the differential amplifier signal output module is: due to the small amplitude of the spike wave, it cannot meet the requirements of most threshold discriminators on the market (taking a TDC on the market without a threshold discriminator as an example, the trigger threshold of this TDC is about +1.5V). In this case, it is necessary to ensure signal shaping, fast response and amplitude that meet the requirements through the post-stage circuit design. The present invention adopts Fig. 9 The differential amplifier circuit shown in the figure has a transistor acceleration circuit added to the front stage of the differential amplifier circuit in order to ensure low latency of the system. The power storage of capacitor C10 speeds up the conduction of the back stage transistor, thereby reducing the response time of the circuit. In order to meet the needs of positive and negative TDC (some TDCs on the market have positive thresholds and some have negative thresholds), the output polarity of the CFD is changed by selecting whether to connect an external reverse amplifier circuit.

[0084] Assuming that a TDC acquisition board is used, which does not have its own threshold discriminator, a signal of about +1.5V is required to trigger the TDC. Then the present invention can also be connected to an external inverting amplifier (to convert a negative digital pulse signal into a positive digital pulse signal) for subsequent signal processing to ensure compatibility with most TDC products on the market. The specific schematic diagram and physical diagram are as follows: Fig.24 and Fig.25 As shown in Figure 2, coupled with a series of interferences such as the reverse amplifier and wire transmission, the propagation delay of the system is about 15ns.

[0085] Subsequently, the functions of the threshold discriminator were verified using the solid direct analysis glow discharge mass spectrometer made in our laboratory and the existing product FCFR-USB8002TDC. Fig.26 The resulting spectrum is obtained by rough measurement as follows Fig. 27 Performance parameter table.

[0086] The above is only a specific implementation of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial changes to the present invention using this concept shall be deemed as an infringement of the protection scope of the present invention.

Claims

1. A threshold discriminator circuit for time-of-flight mass spectrometry, comprising a power supply module, characterized in that: It also includes a signal amplification module, a threshold setting module, a first voltage comparison module, a high-speed D flip-flop, a second voltage comparison module, a pulse width modulation module and a differential amplification module powered by the power supply module; the signal amplification module is used to amplify the input mass spectrometry signal; the threshold setting module is used to output the set threshold; the first voltage comparison module is used to compare the input threshold with the amplified mass spectrometry signal and convert the amplified mass spectrometry signal into a digital pulse signal of a digital ELC level; The high-speed D flip-flop shortens the pulse width of the digital pulse signal to output a peak pulse signal; The pulse width modulation module adjusts the baseline of the spike pulse signal; The second voltage comparison module compares the peak pulse signal modulated by the baseline with the reference level and outputs a small-amplitude pulse signal; The differential amplifier module performs differential amplification on the small-amplitude pulse signal and then outputs a final pulse signal.

2. A threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: The signal amplification module includes an amplifier chip, the input end of which is connected in series with a gas discharge tube, a capacitor C8, a resistor R7 and a capacitor C9 in sequence; capacitor C8 is connected in parallel with capacitor C4 to filter the DC component of the mass spectrometry signal, and the two ends of capacitor C4 are respectively connected to resistors R2 and R3; capacitor C9 is connected in parallel with capacitor C5; a resistor R11 is also connected between resistor R7 and capacitor C9, and the amplification factor is adjusted by adjusting the resistance values ​​of resistors R7 and R11; the output end of the amplifier chip is connected to capacitor C11, and capacitor C12 is connected in parallel with both ends of capacitor C11 to filter the DC component of the mass spectrometry signal and then output to the first voltage comparison module.

3. A threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: The threshold setting module includes a sliding rheostat RP1, a resistor R18, a resistor R20, a resistor R21, a resistor R22, a resistor R23, a resistor R24, a resistor R27, a resistor R28 and a capacitor C13; one end of the resistor R18 is connected to the input end of the first voltage comparison module, and the other end is connected to the sliding end of the sliding rheostat RP1, a fixed end of the sliding rheostat RP1 is connected to the resistor R27, and the other fixed end of the sliding rheostat RP1 is connected to the power supply module and is connected in series with resistors R21, R22, R23 and R24 in sequence; one end of the resistor R28 is connected to the resistor R27, and the other end is connected between the resistor R21 and the resistor R22; one end of the resistor R20 is connected to one end of the resistor R18, and the other end is grounded, and the capacitor C13 is connected in parallel with the resistor R20.

4. A threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: In the first voltage comparison module, when the mass spectrum signal is greater than the threshold, the positive output port of the first voltage comparison module outputs an ELC high level; when the mass spectrum signal is less than the threshold, the positive output port of the first voltage comparison module outputs an ECL low level.

5. The threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: The high-speed D trigger adopts a high-speed D trigger, which receives the digital pulse signal of the digital ELC level generated by the first voltage comparison module, and generates a spike pulse signal through high-speed self-shutdown and extremely short metastable recovery time.

6. A threshold discriminator circuit for time-of-flight mass spectrometry as claimed in claim 5, characterized in that: It also includes a signal suppression module, which is connected to the enable terminal of the high-speed D flip-flop to control its operation; The signal suppression module is powered by the power supply module.

7. A threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: The pulse width modulation module includes a transistor Q8, an inductor L9, a resistor R58, a resistor R59, a capacitor C29 and a sliding resistor RP2; the base of the transistor Q8 is connected to the output end of the high-speed D trigger, the emitter of the transistor Q8 is connected in series with the inductor L9, the resistor R58 and the resistor R59 in sequence, one end of the capacitor C29 is connected to the connecting point of the inductor L9 and the resistor R58 and is connected to the second voltage comparison module; the sliding end of the sliding resistor RP2 is connected to a fixed end of the sliding resistor RP2, one end of the resistor R59 and the power supply module, and the other fixed end of the sliding resistor RP2 is connected to the other end of the resistor R59.

8. The threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: The second voltage comparison module is provided with a fixed resistor to provide the reference level, the fixed resistor is powered by the power supply and includes a resistor R61, a resistor R63, a resistor R66 and a resistor R67; the resistor R61, the resistor R63 and the resistor R66 are connected in series in sequence, the resistor R67 is connected in parallel with the resistor R66, and the connected end of the resistor R63 and the resistor R66 outputs the reference level.

9. The threshold discriminator circuit for time-of-flight mass spectrometry according to claim 1, characterized in that: The input end of the differential amplifier module connected to the negative output port of the second voltage comparison module is connected in parallel with a capacitor C10. The power storage of the capacitor C10 accelerates the conduction of the subsequent transistor, thereby reducing the response time of the circuit.

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