A New Dead Time Compensation Method and Its Implementation
2019-03-23 02:05:10
Abstract : Using the counting method to solve the peak loss problem of the pulse amplitude analyzer system and then compensating for the number of missing pulses is an effective method to solve the dead time problem in the amplitude analysis system. This article describes in detail the principle of dead-time loss compensation and its application in practice.
Key words: Pulse amplitude analysis system Dead time Introduction Pulse amplitude analysis system is an important part of nuclear radiation detection equipment. According to the principle of analog-digital conversion, the analog signal output by the front circuit (sensor + preamp + main amplifier) Convert to digital information that is proportional to its peak value. The main device for signal conversion is an analog-to-digital converter. During the signal conversion and memory storage of the analog-to-digital converter, the signal output by the front circuit will be lost. Usually, the minimum time interval between two pulses can be called the dead time of the counting system. For the pulse amplitude analysis system, the dead time mainly depends on the sum of the conversion time of the ADC and the storage time of the memory. One approach is to accumulate the dead time for each count, and then compensate for the count loss due to dead time by extending the measurement time. This dead time loss compensation method will cause large errors due to inaccurate dead time and other reasons. At the same time, when the count rate is not large, the conversion speed of the analog-to-digital converter is fast enough, the count is not lost, and the dead time is basically Still, the dead-time loss compensation at this time is obviously unreasonable. For this reason, we have adopted a new dead time loss compensation method that can effectively overcome the above drawbacks.
Methodology Commonly used counting systems There are two types of dead time characteristics: extended response and non-expanded responses. These models embody the idealized dead time characteristics. The response of the actual counting system is often very similar to one of the two. Assume that each true event that occurs within the "live time" of the counting system follows a fixed dead time τ. Although the true event during the dead time is not counted, following the missing event extends another dead time period τ , called extended response dead time. Assuming that each true event that occurs within the counting system “live time†follows a fixed dead time τ, true events that occur within this dead time will be lost and have no effect on the counting system characteristics, called non-expanding responses Dead time.
The dead time of the pulse amplitude analysis system is the non-expanded dead time, because one pulse causes the pulse to enter the system after the counted resolution time will not cause the count, but it will not further increase the dead time. The correction formula for dead time is:
N0=N/(1-NÏ„)
Among them, N0 is the total number of pulses entering the system per unit of time; N is the recorded count rate; Ï„ is the system dead time; the physical meaning of NÏ„ is the sum of the dead time per unit time. In fact, the dead time of the pulse amplitude analysis system is an important technical indicator of the system, but the ultimate purpose of calculating the dead time is to compensate for the dead time loss. If the method is used to extend the measurement time, the dead time is compensated. There will be a new dead time in the extended measurement time, which is obviously unfavorable to the correction of the dead time. For this reason, we use a pulse number compensation method that compensates for the number of pulses lost during the dead time accumulated during the measurement. Let n0 be the total number of pulses entering the system during the measurement time; n the number of counts recorded during the measurement time. The new dead time loss compensation method is to use the method of compensating the number of lost pulses (ie, n0-n). The specific method is: use the T1 timer of the one-chip computer to accumulate and count up the number of pulses output by the front circuit to obtain n0; at the same time, the number of pulses converted by the analog-to-digital converter is implemented in the single-chip microcomputer program (the counter of the single-chip microcomputer may also be used). Accumulated count, get n; when the set measurement time arrives, the difference between the two is the number of system missing pulses (ie n0-n), at this time the system continues to work, when the ADC converts n0- After n pulses, the system stops working and displays the measurement time to achieve dead-time loss compensation.
Obviously, this pulse number compensation method is also applicable to extended dead time compensation corrections.
Hardware Design Figure 1 shows the control circuit of the system, which can also be regarded as the buffer circuit of the input signal. It is used to identify the rising edge of the signal and provide the peak information of the input signal. Among them: Capacitor C1 is the holding capacitance of the signal peak; diode D1 guarantees that C1 collects the peak value of the input signal to prevent it from changing following the falling of the input signal; U1:A is the signal start judgment comparator, and its reverse end accesses a Reference voltage 0.5V, positive phase termination input signal; U1:B is signal peak amplitude judgment comparator, reverse termination capacitor C1, positive phase termination input signal, during the rising edge of the signal, the voltage of the inverting terminal It is 0.3V lower than the positive phase terminal voltage; the waveforms at the output terminals A, B, and C of the two comparators are shown in FIG. 2 . Point C provides the rising edge of the signal, which is used to control the subsequent peak hold circuit. U1:A is actually a threshold voltage discrimination comparator. Signals smaller than 0.5V are considered as noise, point C does not provide control information, and ADC does not provide control information. For conversion, the output A of U1:A can be used as the input of the total count of signals, and receive the T1 timer (P3.5) of the microcontroller.
The design of microcontroller software related to software design and dead-time loss compensation mainly involves the following modules:
(1) Timing module. The T0 timer of the single-chip microcomputer is used to implement the system measurement and timing. This module is actually an overflow interrupt service routine of the timer T0.
(2) Total signal count accumulation module. Using the T1 timer of the single-chip computer to realize the total signal of the system, this module is actually the overflow interrupt service program of the counter T1.
(3) ADC conversion pulse number accumulation module. The A/D conversion adopts the interrupt query mode. The status port of the ADC is the external interrupt source of the microcontroller INT0. Each time the A/D conversion is completed, the external interrupt 0 service program reads the ADC conversion result and writes it to the memory. At the same time, the number of ADC conversion pulses is increased by one. It can be seen that the pulse number accumulation module for ADC conversion is actually part of the external interrupt 0 service program.
The software implementation flow chart related to the calculation of the dead time is shown in FIG. The console sends the measurement time Ts to the pulse amplitude analysis system and starts the measurement. The pulse of the front circuit enters the analysis system. The timer T0 starts timing (assuming the time is Tn), and the counter T1 starts recording the total number of pulses entering the system. (n0), n is incremented by one each time the ADC converts one pulse. When the measurement time reaches Tn=Ts, the counter T1 stops counting. At this time, n0 is the total number of pulses entering the analysis system within the set measurement time Ts. If the number of pulses converted by n0 and ADC at this time is If n is equal, it indicates that the system has no peak loss and the dead time is 0; otherwise, the ADC needs to convert n0-n pulses again to compensate for the lost pulse.
Conclusion The peak amplitude analysis system in the CD-10 portable X-ray fluorescence full-spectrum measuring instrument developed by Chengdu University of Technology originally used the traditional dead-time calculation method due to the timing reference pulse period (0.25 μs) and each conversion of the ADC. The time (8μs) is in the same order of magnitude, which makes the dead time calculation inaccurate. With the new deadtime calculation method, the peak amplitude analysis system can accurately reflect the system's dead time and improve the accuracy of the instrument. This method is also suitable for other signal peak amplitude analysis occasions, and has great popularization and application value.
(This article is taken from the world of electronic engineering: http://Test_and_measurement/zhzx/200605/2301.html)
Key words: Pulse amplitude analysis system Dead time Introduction Pulse amplitude analysis system is an important part of nuclear radiation detection equipment. According to the principle of analog-digital conversion, the analog signal output by the front circuit (sensor + preamp + main amplifier) Convert to digital information that is proportional to its peak value. The main device for signal conversion is an analog-to-digital converter. During the signal conversion and memory storage of the analog-to-digital converter, the signal output by the front circuit will be lost. Usually, the minimum time interval between two pulses can be called the dead time of the counting system. For the pulse amplitude analysis system, the dead time mainly depends on the sum of the conversion time of the ADC and the storage time of the memory. One approach is to accumulate the dead time for each count, and then compensate for the count loss due to dead time by extending the measurement time. This dead time loss compensation method will cause large errors due to inaccurate dead time and other reasons. At the same time, when the count rate is not large, the conversion speed of the analog-to-digital converter is fast enough, the count is not lost, and the dead time is basically Still, the dead-time loss compensation at this time is obviously unreasonable. For this reason, we have adopted a new dead time loss compensation method that can effectively overcome the above drawbacks.
Methodology Commonly used counting systems There are two types of dead time characteristics: extended response and non-expanded responses. These models embody the idealized dead time characteristics. The response of the actual counting system is often very similar to one of the two. Assume that each true event that occurs within the "live time" of the counting system follows a fixed dead time τ. Although the true event during the dead time is not counted, following the missing event extends another dead time period τ , called extended response dead time. Assuming that each true event that occurs within the counting system “live time†follows a fixed dead time τ, true events that occur within this dead time will be lost and have no effect on the counting system characteristics, called non-expanding responses Dead time.
The dead time of the pulse amplitude analysis system is the non-expanded dead time, because one pulse causes the pulse to enter the system after the counted resolution time will not cause the count, but it will not further increase the dead time. The correction formula for dead time is:
N0=N/(1-NÏ„)
Among them, N0 is the total number of pulses entering the system per unit of time; N is the recorded count rate; Ï„ is the system dead time; the physical meaning of NÏ„ is the sum of the dead time per unit time. In fact, the dead time of the pulse amplitude analysis system is an important technical indicator of the system, but the ultimate purpose of calculating the dead time is to compensate for the dead time loss. If the method is used to extend the measurement time, the dead time is compensated. There will be a new dead time in the extended measurement time, which is obviously unfavorable to the correction of the dead time. For this reason, we use a pulse number compensation method that compensates for the number of pulses lost during the dead time accumulated during the measurement. Let n0 be the total number of pulses entering the system during the measurement time; n the number of counts recorded during the measurement time. The new dead time loss compensation method is to use the method of compensating the number of lost pulses (ie, n0-n). The specific method is: use the T1 timer of the one-chip computer to accumulate and count up the number of pulses output by the front circuit to obtain n0; at the same time, the number of pulses converted by the analog-to-digital converter is implemented in the single-chip microcomputer program (the counter of the single-chip microcomputer may also be used). Accumulated count, get n; when the set measurement time arrives, the difference between the two is the number of system missing pulses (ie n0-n), at this time the system continues to work, when the ADC converts n0- After n pulses, the system stops working and displays the measurement time to achieve dead-time loss compensation.
Obviously, this pulse number compensation method is also applicable to extended dead time compensation corrections.
Hardware Design Figure 1 shows the control circuit of the system, which can also be regarded as the buffer circuit of the input signal. It is used to identify the rising edge of the signal and provide the peak information of the input signal. Among them: Capacitor C1 is the holding capacitance of the signal peak; diode D1 guarantees that C1 collects the peak value of the input signal to prevent it from changing following the falling of the input signal; U1:A is the signal start judgment comparator, and its reverse end accesses a Reference voltage 0.5V, positive phase termination input signal; U1:B is signal peak amplitude judgment comparator, reverse termination capacitor C1, positive phase termination input signal, during the rising edge of the signal, the voltage of the inverting terminal It is 0.3V lower than the positive phase terminal voltage; the waveforms at the output terminals A, B, and C of the two comparators are shown in FIG. 2 . Point C provides the rising edge of the signal, which is used to control the subsequent peak hold circuit. U1:A is actually a threshold voltage discrimination comparator. Signals smaller than 0.5V are considered as noise, point C does not provide control information, and ADC does not provide control information. For conversion, the output A of U1:A can be used as the input of the total count of signals, and receive the T1 timer (P3.5) of the microcontroller.
The design of microcontroller software related to software design and dead-time loss compensation mainly involves the following modules:
(1) Timing module. The T0 timer of the single-chip microcomputer is used to implement the system measurement and timing. This module is actually an overflow interrupt service routine of the timer T0.
(2) Total signal count accumulation module. Using the T1 timer of the single-chip computer to realize the total signal of the system, this module is actually the overflow interrupt service program of the counter T1.
(3) ADC conversion pulse number accumulation module. The A/D conversion adopts the interrupt query mode. The status port of the ADC is the external interrupt source of the microcontroller INT0. Each time the A/D conversion is completed, the external interrupt 0 service program reads the ADC conversion result and writes it to the memory. At the same time, the number of ADC conversion pulses is increased by one. It can be seen that the pulse number accumulation module for ADC conversion is actually part of the external interrupt 0 service program.
The software implementation flow chart related to the calculation of the dead time is shown in FIG. The console sends the measurement time Ts to the pulse amplitude analysis system and starts the measurement. The pulse of the front circuit enters the analysis system. The timer T0 starts timing (assuming the time is Tn), and the counter T1 starts recording the total number of pulses entering the system. (n0), n is incremented by one each time the ADC converts one pulse. When the measurement time reaches Tn=Ts, the counter T1 stops counting. At this time, n0 is the total number of pulses entering the analysis system within the set measurement time Ts. If the number of pulses converted by n0 and ADC at this time is If n is equal, it indicates that the system has no peak loss and the dead time is 0; otherwise, the ADC needs to convert n0-n pulses again to compensate for the lost pulse.
Conclusion The peak amplitude analysis system in the CD-10 portable X-ray fluorescence full-spectrum measuring instrument developed by Chengdu University of Technology originally used the traditional dead-time calculation method due to the timing reference pulse period (0.25 μs) and each conversion of the ADC. The time (8μs) is in the same order of magnitude, which makes the dead time calculation inaccurate. With the new deadtime calculation method, the peak amplitude analysis system can accurately reflect the system's dead time and improve the accuracy of the instrument. This method is also suitable for other signal peak amplitude analysis occasions, and has great popularization and application value.
(This article is taken from the world of electronic engineering: http://Test_and_measurement/zhzx/200605/2301.html)
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