A threshold discriminator circuit for time-of-flight mass spectrometry

Abstract

A threshold discriminator circuit applied to time-of-flight mass spectrometer includes a power supply module, 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; the signal amplification module amplifies the input mass spectrum signal; the threshold setting module outputs the set threshold; the first voltage comparison module compares the threshold and the amplified mass spectrum signal, converts the mass spectrum signal into a digital pulse signal of digital ELC level; the high-speed D flip-flop shortens the pulse width of the digital pulse signal to output a sharp pulse signal; the pulse width modulation module adjusts the baseline of the sharp pulse signal; the second voltage comparison module compares the baseline-modulated sharp pulse signal with a reference level to output a small amplitude pulse signal; and the differential amplification module differentially amplifies the small amplitude pulse signal to output the final pulse signal, so that the mass spectrum signal generated by the time-of-flight mass spectrometer can be quickly identified and screened.

Description

Technical Field

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

[0002] A constant fraction discriminator (CFD), also known as a threshold discriminator, is a hardware circuit that can identify pulse spikes with peak widths ranging from a few nanoseconds to tens of nanoseconds. The function of this circuit can be summarized as: determining whether the signal peak value exceeds a set threshold, and converting the signal peak into a digital pulse signal with minimal transmission delay. This converted digital pulse signal can then be used by a high-frequency counter (TDC) for identification, counting, and accumulation.

[0003] After entering the TOFMS analyzer, ions undergo a series of flight processes before finally striking the MCP detector. The MCP detector, after undergoing electron multiplication and other processes, outputs a series of analog signals composed of Gaussian peaks of varying intensities. The time difference between a peak at a given moment and the arrival of the repulsion voltage contains the ion's mass-to-charge ratio information. The repulsion voltage is a series of equally spaced high-voltage pulses with equal duty cycles in the TOFMS that provide the particle with kinetic energy. A Time-Digital Counter (TDC) is a device used to identify events and convert their occurrence time into a digital output. It is typically used to measure the time interval between events and is sometimes referred to as a time counter. TDCs have various operating modes and usually require at least two signals: a start signal and a stop signal (represented in CFD as the start and end of the rising and falling edges of a digital pulse signal), which control the start and stop of the measurement cycle, respectively. TDCs have numerous applications, ranging from measuring the flight time and lifetime of various particles in atomic and high-energy physics to the optical ranging sensors commonly used in mobile phones.

[0004] Because TDCs (Transient Voltage Detectors) have poor response to mass spectrometry signals generated by trace elements (which are Gaussian peaks with intensities ranging from a few millivolts to tens of millivolts), and some existing domestic TDCs cannot directly recognize negative mass spectrometry signals, directly connecting the output signal of an MCP (Medium Voltage Detector) to a TDC will result in poor response to trace elements and an inability to recognize negative signals. Therefore, we need a hardware design that can recognize trace signals and uniformly convert Gaussian peaks of different intensities into digital pulse signals of equal amplitude that can always be recognized by the TDC. This is a threshold discriminator.

[0005] Existing threshold discriminators for Gaussian peak signals in mass spectrometry generally do not meet the minimum signal threshold (100mV for most commercially available threshold discriminators) required for mass spectrometry signals. For high-frequency mass spectrometry signals, many domestically produced threshold discriminators (TDCs) with built-in programmable implementations require complex hardware processing and cannot achieve ultra-low signal delays. This can lead to missed signals during acquisition and conversion, resulting in errors in acquisition accuracy. Furthermore, many current threshold discriminators cannot be controlled externally. Therefore, the development of low-cost threshold discriminators for small-signal mass spectrometry signals is urgently needed. Summary of the Invention

[0006] The main objective of this invention is to overcome a series of shortcomings in the existing technology, such as the inability of TDC to directly identify low-level mass spectrometry signals, the inability of existing threshold discriminators (CFD) to achieve extremely low threshold settings for trace element signals in time-of-flight mass spectrometry, the lack of external signal control modules, high cost, small frequency range, and long dead time. This invention proposes a threshold discriminator circuit for time-of-flight mass spectrometry that converts mass spectrometry signals exceeding the set threshold into digital pulse signals with equal amplitude and pulse width that meet 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 includes a power supply module, and further 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, all powered by the power supply module. The signal amplification module amplifies the input mass spectrometry signal. The threshold setting module outputs a set threshold value. The first voltage comparison module compares the input threshold value with the amplified mass spectrometry signal and converts the amplified mass spectrometry signal into a digital pulse signal at a digital ELC level. The high-speed D 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 baseline-modulated spike pulse signal with a reference level and outputs a small-amplitude pulse signal. The differential amplification module differentially amplifies the small-amplitude pulse signal and outputs the final pulse signal.

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

[0010] The threshold setting module includes a sliding rheostat RP1, resistors R18, R20, R21, R22, R23, R24, R27, R28, and capacitor C13. One end of resistor R18 is connected to the input terminal of the first voltage comparison module, and the other end is connected to the sliding terminal of the sliding rheostat RP1. One fixed end of the sliding rheostat RP1 is connected to resistor R27, and the other fixed end of the sliding rheostat RP1 is connected to the power supply module and connected in series with resistors R21, R22, R23, and R24. One end of resistor R28 is connected to resistor R27, and the other end is connected between resistors R21 and R22. One end of resistor R20 is connected to one end of resistor R18, and the other end is grounded. Capacitor C13 is connected in parallel with 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 flip-flop receives a digital pulse signal at the digital ELC level generated by the first voltage comparison module, and generates a spike pulse signal through high-speed self-turn-off and an extremely short metastable recovery time.

[0013] 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.

[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 of the high-speed D flip-flop. The emitter of the transistor Q8 is connected in series with the inductor L9, the resistor R58, and the resistor R59. One end of the capacitor C29 is connected to the junction 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 one fixed end of the sliding resistor RP2, one end of the resistor R59, and the power supply module. 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 resistors R61, R63, R66 and R67. Resistors R61, R63 and R66 are connected in series, and resistor R67 is connected in parallel with resistor R66. The connection between resistors R63 and R66 outputs the reference level.

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

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

[0018] 1. In this 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 mass spectrometry signals exceeding the set threshold into digital pulse signals with equal amplitude and pulse width that meet the requirements of TDC acquisition equipment with extremely low signal transmission delay, thereby reducing costs.

[0019] 2. In this invention, the voltage divider circuit structure of the threshold setting module can achieve voltage divider signal modulation from -10mV to -1V by adjusting the resistance value of the sliding rheostat. This reference signal is compared with the mass spectrometry signal amplified tenfold by the first voltage comparator, thereby achieving mass spectrometry signal threshold modulation equivalent to -1mV to -100mV. The ultra-low detection threshold enhances the TDC's ability to respond to small mass spectrometry signals and improves the sensitivity of the instrument.

[0020] 3. In this invention, the pulse width modulation module generates a voltage of a specific amplitude through a resistor voltage divider. In conjunction with a high-speed D flip-flop, it outputs an adjustable pulse spike of 2-30ns. The modulated range of this pulse spike can be adapted to the dead time characteristic parameters of most TDCs on the market, thereby enhancing the TDC's ability to distinguish signals generated by ions with relatively short flight times and improving the resolution of the instrument.

[0021] 4. Each of the present inventions, through the high-speed self-turn-off of the high-speed D flip-flop, rapidly generates a pulse peak with a peak-to-bottom 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 this invention, a specific signal suppression circuit is used to perform level conversion and control a specific pin of the D flip-flop, so that the opening and closing of the entire circuit module can be controlled by changing the external level signal, which facilitates subsequent digital integration and control.

[0023] 6. In this invention, by selecting a chip with extremely low signal delay, signal processing is performed to greatly reduce signal delay, with the overall signal transmission delay being around 10ns.

[0024] 7. In this invention, the signal amplification module can quickly filter the DC component generated by the mass spectrometry signal through the capacitor and apply a positive bias, thereby maximizing the performance of the amplifier chip and realizing the tracking and amplification of mass spectrometry signals over a wider range; and the amplified small signal will be more accurately identified in the subsequent circuit, thereby increasing the sensitivity of the instrument. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the main circuit components of the present invention;

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

[0027] Figure 3 Circuit diagram for setting threshold;

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

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

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

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

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

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

[0034] Figure 10 This is a PCB design rendering of the present invention;

[0035] Figure 11 This is a physical image of the present invention;

[0036] Figure 12 This is the normalized mass spectrometry signal;

[0037] Figure 13 This is the integral curve of the normalized mass spectrometry signal;

[0038] Figure 14 This is a simulation diagram of the threshold comparison module circuit;

[0039] Figure 15 Verification of the threshold comparison module function of the first voltage comparison module;

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

[0041] Figure 17 This is a truth table for the level transitions of a high-speed D flip-flop chip.

[0042] Figure 18 The timing diagram shows the pulse generation of the MC10EL31 via high-speed self-turn-off.

[0043] Figure 19 The spike pulse wave generated by fast switching of pin Q of MC10EL31;

[0044] Figure 20 Simulation results of the signal latch module circuit;

[0045] Figure 21 Physical functional verification of the signal latching module;

[0046] Figure 22 Verification of the pulse width modulation module's functionality;

[0047] Figure 23 The voltage comparator chip's pin Q generates a spike (circled in red) due to insufficient rise time;

[0048] Figure 24 This is a circuit diagram of an inverting amplifier;

[0049] Figure 25 This is a picture of an external inverter.

[0050] Figure 26 This is a graph showing the collection results for standard samples;

[0051] Figure 27 This is a performance parameter table.

[0052] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Detailed Implementation

[0053] The present invention will be further described below through specific embodiments.

[0054] A threshold discriminator circuit for time-of-flight mass spectrometry (TOF-MS) is used to determine whether the intensity of a signal peak (typically a Gaussian peak generated by the signal collection device of the TOF-MS) exceeds a set threshold, and then processes the signal peak into a digital pulse signal. Beyond fulfilling its basic functions, the circuit design should further meet the following conditions: ① The set threshold should be adjustable within a certain range, and should accommodate mass spectrometry signals with lower intensity as much as possible. ② The signal processing time should be as short as possible. ③ The instrument can be started and stopped via external signal control. ④ The dead time (the minimum response interval between two adjacent signals) should be as short as possible.

[0055] See Figure 1 Based on the aforementioned basic functional requirements, this invention designs a threshold discriminator circuit for time-of-flight mass spectrometry (TOF-MS) based on the signal flow. The circuit includes a power supply module and, powered by the power supply module, 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. The signal flow is described as follows: The MCP mass spectrum signal enters the signal amplification module, where it is amplified several times (e.g., ten times) before entering the first voltage comparison module. The threshold setting module outputs the set threshold value. The first voltage comparison module compares the amplified mass spectrum signal with the threshold value (i.e., the trigger value) set by the threshold setting module and converts the pre-amplified mass spectrum signal into a digital pulse signal at the digital ELC level.

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

[0057] The primary purpose of the signal amplification module is to amplify minute pulse signals without distortion, thereby improving the accuracy of subsequent voltage comparisons. In the subsequent design of this module, overshoot protection for mass spectrometry signals (typically negative signals) is implemented. Considering the possibility of high voltages due to internal instrument damage, a gas discharge tube is incorporated as a protection device at the signal input stage of this module. The chip used in this module exhibits different amplification responses for signals in different frequency bands, thus effectively distinguishing between trace elements and baseline noise.

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

[0059] Figure 2 Firstly, the input stage protection device includes a resistor R2 to discharge the DC blocking capacitor in case of a sudden power outage, and two capacitors C4 and C8 to isolate the DC current of the mass spectrometry signal. For transient voltages with large rates of change, a gas discharge tube is added to the input stage to protect the circuit from damage to subsequent chips. A bidirectional clamping diode is also used at the input of the amplifier chip to clamp forward mass spectrometry signal overshoot (the diode's forward voltage drop clamps overshoots exceeding 0.7V to +0.7V). Furthermore, by adjusting the values ​​of resistors R7 and R11, the overall signal amplification factor is maintained at 20dB (10x).

[0060] A DC blocking capacitor is added to the output stage of the signal amplification module to ensure that the amplified mass spectrometry signal is completely preserved without DC offset. After filtering out the DC flux, the AC signal exhibits the same positive and negative area integrals. Simplified calculations reveal that the baseline shift of the mass spectrometry signal within 1 second is on the order of hundreds of μV (see details). Figure 12 , Figure 13 This level of signal offset has almost no impact on subsequent signal comparisons. The baseline offset error caused by different MCP assembly methods can be resolved after the signal passes through three sets of DC blocking capacitors (capacitors C4 / C8, C5 / C9, and C11 / C12).

[0061] See Figure 12 The baseline offset is calculated by normalizing a single mass spectrometry signal and then integrating it. Figure 13 In the test, when a high-amplitude mass spectrometry signal of 1V is generated, a signal integral of 0.013 is produced in a single repulsion period (50µs). Since the maximum intensity of the mass spectrometry signal from the CFD instrument is 5V, a signal integral of 0.065 is produced in a single repulsion period. Within 1 second, there are 10*14 (10 ion excitations, each collecting 14 repulsions) repulsions totaling 7ms. Using the formula, the average baseline shift within 1 second is 65*7 / 1000 = 450µV.

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

[0063] The threshold setting module of this invention uses a Wheatstone bridge. By adjusting the resistance of the sliding rheostat RP1, the output level of the threshold setting module is adjustable within the range of -10mV to -1000mV. This adjustable level is used in the subsequent first voltage comparison module to compare the voltage with the amplified mass spectrometry signal. A special threshold feedback interface (THFB) is also provided to provide feedback on the level value of C1_N. In this module, the output voltages of THFB and C1_N can be considered approximately equal. To prevent crosstalk between the level signals, the circuit selects to output the feedback value of the comparison voltage through the THFB pin. For detailed circuit design and output simulation diagrams, please refer to... Figure 14 .exist Figure 14 In the simulation, a circuit was built based on the design. Finally, the resistance values ​​of resistors such as R6A, R7A, and R8A were matched and determined according to the requirements (here, resistors R6A, etc., refer to...). Figure 14 The resistors in the simulation are independent of the schematic diagram. Figure 14 In Figures (a) and (b), typical parameter values ​​of RP1 were selected for verification: (a) R3 = 5KΩ, (b) R3 = 250Ω. Figure 14 The voltage measured by the XMM1 multimeter is the THFB voltage value in the schematic diagram. Figure 14 The voltage measured by the XMM2 multimeter is the value of C1_N in the schematic diagram.

[0064] The primary function of the first voltage comparison module is to compare the mass spectrometry signal amplified 10 times with the level of the threshold setting module, and to generate and output an ELC level at the output stage to trigger the CLK pin of the subsequent high-speed D flip-flop. To accelerate the system's response to the mass spectrometry signal and minimize the signal propagation delay in this module, the core comparator chip selected in this invention is the AD96685, which balances performance and cost-effectiveness. Final testing showed that the module's propagation delay is only 2.5 ns.

[0065] Specifically, the first voltage comparison module is implemented as follows: when the mass spectrometry signal is greater than the threshold, the positive output port Q of the first voltage comparison module outputs an ELC high level, i.e., digital signal 1; when the mass spectrometry signal is less than the threshold, the positive output port Q of the first voltage comparison module outputs an ECL low level, i.e., digital signal 0. The level switching caused by the input signal being greater than the threshold is as follows: Figure 15 As shown, the threshold is set to 10mV, as indicated by the dashed line Y4 in the figure. The propagation delay is approximately 30ns due to the wires and the oscilloscope probe.

[0066] A high-speed D flip-flop shortens the pulse width of a digital pulse signal to output a spike pulse signal. The high-speed D flip-flop is a commonly used digital level flip-flop with latching and follow-up functions; certain connection methods can achieve specific functions. The main purpose of this step is to convert a large-pulse-width mass spectrometry signal into a uniform small-pulse-width pulse signal, shortening the pulse width of the digital pulse signal as much as possible (e.g., the half-width at half-maximum can reach about 15ns), thereby reducing the instrument's dead time. In this invention, the high-speed D flip-flop receives the digital pulse signal at the digital ELC level generated by the first voltage comparison module and generates a spike pulse signal through high-speed self-turn-off and an extremely short metastable recovery time.

[0067] The main functions of the high-speed D flip-flop module are: ① To receive the ELC level generated by the first voltage comparator module and generate a pulse spike with a peak-to-bottom width of 30ns through high-speed self-turn-off and an extremely short metastable recovery time (metastable state refers to an unknown unstable state in a numerical circuit between high and low levels). This spike is used for pulse width modulation in the subsequent second voltage comparator module. ② To receive the level signal from the signal suppression module, thereby determining the operating state of the entire system. This module also needs to ensure that the system response time is as fast as possible, the delay time is as short as possible, and that it has a fast recovery speed when the signal is in a metastable state.

[0068] First, an analysis is conducted on function ①. Based on the high-speed metastable response characteristics of the D flip-flop, the following design is proposed: Figure 16 The hardware circuit, the truth table of the MC10EL31 chip is as follows: Figure 17As shown, when the CLK pin of the MC10EL31 receives the mass spectrometry signal and generates the ECL signal, the pins of the D flip-flop will rapidly transition and recover. During this process, due to the special external wiring, the D flip-flop will generate a very short pulse spike. The specific timing diagram is shown below. Figure 18 As shown.

[0069] Here is a brief overview of the entire level transition process: When no external signal suppression is set, the D pin is always held at the D level by an external pull-down switch. CLK, connected to the AD96685, receives the ELC square wave pulse signal from the preceding stage. When the rising edge arrives, according to the truth table, Q and Q′ simultaneously undergo level transitions. When Q and Q′ simultaneously enter the undefined level range, the level of SET (connected to Q′) also enters a metastable state (between high and low levels). At this time, the chip's metastable recovery function is triggered, and the circuit system restores the system to a stable level (i.e., Q is high and Q′ is low) as quickly as possible. This rapid switching is represented on the waveform as a spike pulse with an amplitude of 50mV and a maximum pulse width of 30ns. Figure 19 As shown, the purple-red line represents a continuous positive exponential wave analog mass spectrum signal generated by a pulse generator at a frequency of 20MHz and an amplitude of 250mV. The yellow line represents the waveform transformation of the Q′ pin of the D flip-flop, measured using a 200MHz oscilloscope with a carbon pen. Due to its fast turn-off characteristic and ultra-low threshold setting, the D flip-flop can distinguish between pulse spike signals and subsequent small oscillating spikes with smaller amplitudes.

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

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

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

[0073] The ELC level signal then controls the opening and closing of the D flip-flop module, thereby controlling whether the entire CFD outputs.

[0074] Figures 20-21 This is a functional verification of the signal latch module, in which Figure 20 (a) in the diagram verifies the feasibility of the level shifting control circuit through simulation design. After feasibility verification, the specific implementation scheme is shown in the schematic signal latch module. A truth table of the veto voltage setting and the pins of the MC10EL31 D flip-flop chip it controls is summarized. (See diagram for details.) Figure 20As shown in (b) of the diagram, (c) is the input / output truth table. Subsequent experiments further verified the results with a physical prototype. Figure 21 The purple line represents the external signal input to the Veto, while the yellow signal line represents the output of the D flip-flop Q′. During testing, a pulse generator (FE's FY6900 series pulse generator) was used to generate a continuous positive exponential wave analog mass spectrum signal with a frequency of 20MHz and an amplitude of 250mV. However, it is clearly observed that the circuit does not respond to the input signal when a -500mV signal is input to the Veto.

[0075] The high-speed D flip-flop module can be summarized as follows: ① Generates high-speed pulse spikes; input voltage comparison module 2. ② Controls the instrument's operating status.

[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 a power supply module, specifically, the signal suppression module is powered by a -5V low ripple regulated power supply module. Figure 6 This is the circuit diagram for the signal suppression module.

[0077] The pulse width modulation (PWM) module adjusts the baseline of the spike pulse signal. The PWM module includes a transistor Q8, inductor L9, resistors R58 and R59, capacitor C29, and a sliding resistor RP2. The base of transistor Q8 is connected to the positive output of a high-speed D flip-flop. The emitter of transistor Q8 is connected in series with inductor L9, resistor R58, and resistor R59. One end of capacitor C29 is connected to the junction of inductor L9 and resistor R58 and then to the second voltage comparator module. The sliding end of sliding resistor RP2 is connected to one fixed end of sliding resistor RP2, one end of resistor R59, and the power supply module. The other fixed end of sliding resistor RP2 is connected to the other end of resistor R59.

[0078] The specific function of the pulse width modulation module is as follows: By adjusting the voltage drop across transistor Q8 and the variable resistor RP2, the baseline of the pulse spike signal generated by the high-speed D flip-flop module and entering the second voltage comparator module is adjustable, serving as the basis for subsequent chopping by the second voltage comparator module. In this invention, through the selection of resistors and transistors, the baseline of the pulse spike signal is adjustable within the range of 230mV to 270mV. Specific test results can be found... Figure 22 In (a1), (a2), (b), and (c), the signal baseline is adjusted from 230mV (a1) to 270mV (a2) by baseline adjustment and transistor voltage drop adjustment. (b) The comparison voltage is set to -260mV by external circuit design. (c) The peak pulse width is measured to be approximately Δx of 50ns.

[0079] The second voltage comparison module compares the baseline-modulated spike pulse signal with the reference level and outputs a small-amplitude pulse signal. (See also...) Figure 8 This is the circuit diagram for the second voltage comparison module. Its specific function is to receive a baseline-modulated pulse spike signal generated by a D flip-flop. This spike is compared with a reference level generated by a voltage divider using fixed resistors (represented as R61, R63, R66, and R67 in the schematic). By adjusting the baseline of the spike signal, the pulse width of the portion of the signal exceeding the reference level is determined. The module then outputs a digital pulse signal with the same pulse width. The fixed resistors, powered by a power supply, include resistors R61, R63, R66, and R67. Resistors R61, R63, and R66 are connected in series, while resistor R67 is connected in parallel with resistor R66. The connection between resistors R63 and R66 outputs the reference level.

[0080] The second voltage comparator module receives a baseline-modulated pulse spike signal generated by the Q pin of a high-speed D flip-flop. Through external circuit design, the baseline of the pulse spike signal is clamped using the voltage drop across a PNP transistor (0.7V) and an external adjustable rheostat, making the baseline adjustable within the range of -230mV to -270mV. Furthermore, the amplitude of the other input is clamped to approximately -261mV using a resistor divider. (See details...) Figure 22 (b). The measured maximum value of a single spike pulse after baseline processing is approximately 50 ns. See details. Figure 22 (c). Therefore, by adjusting the baseline of the pulse spike signal to -261mV to chop the signal, a pulse spike output with a pulse width of 2ns-30ns can be achieved through the comparator output.

[0081] Depend on Figure 8 As can be seen, the high-speed voltage comparator chip used in this hardware, model AD96685, has a level transition time of approximately 120ns. Therefore, within the time frame of 2ns to 30ns, this module cannot complete the entire ELC level transition process from 0 to 1. The resulting waveform will exhibit a digital pulse spike of 2ns to 30ns. Specific test results are as follows... Figure 23 As shown, after chopping and baseline modulation, the spike pulse width is approximately 10 ns. Specifically, pin Q of the voltage comparator chip generates a spike due to insufficient rise time (circled in red).

[0082] The differential amplifier module amplifies the small-amplitude pulse signal differentially and outputs the final pulse signal. A capacitor C10 is connected in parallel to the input terminal of the differential amplifier module, which is connected to the negative output port of the second voltage comparator module. The storage of electricity in 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 as follows: Due to the small amplitude of the spike, it cannot meet the requirements of most threshold discriminators on the market (taking a certain TDC without a built-in threshold discriminator as an example, the trigger threshold of this TDC is approximately +1.5V). In this case, it is necessary to ensure signal shaping, fast response, and amplitude compliance through subsequent circuit design. This invention employs... Figure 9 The differential amplifier circuit shown incorporates a transistor acceleration circuit in the pre-amplifier stage to ensure low system latency. The capacitor C10 stores charge, accelerating the conduction of the subsequent transistor and reducing the circuit's response time. To meet the requirements of positive and negative threshold current distributions (TCDs on the market have both positive and negative thresholds), the output polarity of the CFD is altered by selecting whether to connect an external inverting amplifier circuit.

[0084] Assuming a TDC acquisition board is used, without a built-in threshold discriminator, and a signal of approximately +1.5V is required to trigger the TDC, this invention can also connect an external inverting amplifier (to convert negative digital pulse signals into positive digital pulse signals) for subsequent signal processing, ensuring compatibility with most TDC products on the market. Specific schematic diagrams and physical diagrams are shown below. Figure 24 and Figure 25 As shown. In addition to interference from the inverting amplifier and wire transmission, the propagation delay of this system is approximately 15 ns.

[0085] Subsequent testing used a self-made solid-state direct analysis glow discharge mass spectrometer and the existing product FCFR-USB8002TDC to verify the various functions of the threshold discriminator. Figure 26 The resulting spectrum, and obtained through rough measurements, is as follows: Figure 27 Performance parameter table.

[0086] The above are merely specific embodiments of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial modifications made to the present invention using this concept shall be considered as infringing upon the protection scope of the present invention.

Claims

1. A threshold discriminator circuit for time-of-flight mass spectrometry comprising a power supply module, characterised in that, The signal amplification module is used for amplifying the input mass spectrum signal; the threshold setting module is used for outputting a set threshold; the first voltage comparison module is used for comparing the input threshold and the amplified mass spectrum signal and converting the amplified mass spectrum 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 sharp pulse signal; The pulse width modulation module adjusts the baseline of the sharp pulse signal; The second voltage comparison module compares the sharp pulse signal after baseline modulation with a reference level to output a small amplitude pulse signal; The differential amplification module differentially amplifies the small amplitude pulse signal to output a final pulse signal; 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.

2. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as claimed in claim 1, characterised in that, The signal amplification module includes an amplification chip, the input end of the amplification chip is sequentially connected with a gas discharge tube, a capacitor C8, a resistor R7 and a capacitor C9 in series; the capacitor C8 is connected with a capacitor C4 in parallel to filter the direct current component of the mass spectrum signal, and the capacitor C4 is respectively connected with a resistor R2 and a resistor R3 at both ends; the capacitor C9 is connected with a capacitor C5 in parallel; the resistor R7 and the capacitor C9 are further connected with a resistor R11, and the amplification multiple is adjusted by adjusting the resistance values of the resistor R7 and the resistor R11; the output end of the amplification chip is connected with a capacitor C11, and the capacitor C11 is connected with a capacitor C12 in parallel at both ends to filter the direct current component of the mass spectrum signal and then output to the first voltage comparison module.

3. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as defined in claim 1, characterized in that The threshold setting module includes a sliding resistor 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 with the input end of the first voltage comparison module, and the other end is connected with the sliding end of the sliding resistor RP1; one fixed end of the sliding resistor RP1 is connected with the resistor R27, and the other fixed end of the sliding resistor RP1 is connected with the power supply module and sequentially connected with the resistor R21, the resistor R22, the resistor R23 and the resistor R24; one end of the resistor R28 is connected with 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 with one end of the resistor R18, and the other end is grounded; and the capacitor C13 is connected with the resistor R20 in parallel.

4. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as defined in claim 1, wherein, 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 sharp pulse signal through high-speed self-turn-off and extremely short metastable state recovery time.

5. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as claimed in claim 4, characterised in that, The signal suppression module is connected with the enable end of the high-speed D flip-flop to control the work of the high-speed D flip-flop. The signal suppression module is powered by the power supply module.

6. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as defined in claim 1, wherein, The pulse width modulation module comprises a triode Q8, an inductor L9, a resistor R58, a resistor R59, a capacitor C29 and a slide resistor RP2; a base of the triode Q8 is connected to an output of the high-speed D flip-flop, an emitter of the triode 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 a connection position of the inductor L9 and the resistor R58 and is connected to the second voltage comparison module; a slide end of the slide resistor RP2 is connected with one fixed end of the slide resistor RP2, one end of the resistor R59 and the power supply module, and the other fixed end of the slide resistor RP2 is connected with the other end of the resistor R59.

7. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as defined in claim 1, wherein, 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 module and comprises 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 resistor R63 and the resistor R66 are connected at the output end of the reference level.

8. A threshold discriminator circuit for use in a time-of-flight mass spectrometer as defined in claim 1, wherein, The input end of the differential amplification module connected with the negative output port of the second voltage comparison module is connected in parallel with a capacitor C10, and the capacitor C10 is used to store electricity to accelerate the conduction of the triode in the later stage, thereby reducing the response time of the circuit.

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