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Digital Filters in Radiation Detection and Spectroscopy

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Digital Filters in Radiation Detection and Spectroscopy Digital Filters in Radiation Detection and Spectroscopy Digital Radiation Measurement and Spectroscopy NE/RHP 537 1 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Classical and Digital Spectrometers •Classical Spectrometer Analog Detector Preamplifier Shaping Multichannel Histogram Amplifier Analyzer Memory •Digital Spectrometer High-Speed Digital Pulse Histogram Detector Preamplifier ADC Processing Memory 2 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Spectrometers: Advantages • Pulse processing algorithm is easy to edit • No bulky analog electronics • Post-processing •Digital Spectrometer High-Speed Digital Pulse Histogram • Digital pulse shaping Detector Preamplifier ADC Processing Memory • More cost-effective • The algorithm is stable and reliable, no thermal noise or other fluctuations • Effects, such as pile-up can be corrected or eliminated at the processing level • Signal capture and processing can be based more easily on coincidence criteria between different detectors or different parts of the same detector • Disadvantage: Lack of expert in (Digital Systems + Nuclear Spectroscopy) 3 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Signal Pulses From Scintillation Detectors Integrating Preamp Rf Anode Cf Preamp output output Scintillation Detector NaI PMT 2 A 1 Vmax 4 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Spectrometers: Digital Pulse Processor (Hardware) • Analog Conditioning: o Nyquist filter ( < ) 퐹푎푎푎 푚푚푚 o Offset adjustment퐹 2 o Amplification Gain • High resolution, fast Analog-to-Digital Convertor (ADC) • Processing Device: A typical FPGA-based digital pulse processor for o FPGA (Field-Programmable Gate Array) scintillation detector o DSP (Digital Signal Processor) • Interface: • USB, PCI, Ethernet 5 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Spectrometers: Field-Programmable Gate Array (FPGA) • FPGAs are an array of programmable logic cells interconnected by a network of wires and configurable switches. • A FPGA has a large number of these cells available to form multipliers, adders, accumulators and so forth in complex digital circuits. • FPGAs can be infinitely reprogrammed in-circuit in only a small fraction of a second. • New FPGA devices provide integrated memory component, to be used as histogram memory. 6 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Spectrometers: Inside the FPGA Threshold Trigger FPGA Trigger Preamp ADC Shaping Shaping Filter Time Real Counter Enable Enable Control Unit Peak Detector Detector (State Machine) Enable Address Live Histogram Memory Counter Memory Update 7 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Spectrometers: Trigger Module Threshold Trigger Comparator A Trigger Output From ADC Fast Triangular A<B B State Machine Filter Trigger Module 8 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Spectrometers: MCA State Machine 1. STOP: wait until receiving start pulse Start ? 2. RESET: start MCA by resetting counters, RESET STOP buffers,… TRIGGER? 3. START: wait for a trigger pulse 4. RD-BASE: read baseline START WR-BIN 5. CNT: wait for N clock cycles 6. PEAK: take a sample from the shaped pulse RD-BASE INC-BIN (peak) N ? = 7. AMP: calculate Pileup CNT RD-BIN 8. SET-ADD: locate풂풂풂 and set풂풑 memory풂풑 − 풃풂 풃address풑풃풃풃풑 which corresponds to the pulse amplitude 9. RD-BIN: read current counts from the PEAK AMP SET-ADD located address 10. INC-BIN: increment the count by one MCA State Machine with 11 states 11. WR-BIN: write the incremented count to the same address, go to START state 12. START: wait for another trigger 9 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Convolution Operation • A finite impulse response (FIR) filter is a filter whose impulse response (or response to any finite length input) is of finite duration, because it settles to zero in finite time. This is in contrast to infinite impulse response (IIR) filters, which may have internal feedback and may continue to respond indefinitely. • For FIR digital filters, the input-output h[n] relation involves a finite sum of products • Convolution operation defines how the x[n] convolution y[n] input signal is related to the output signal 10 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Convolution Operation (Sum of Products) h[n] • x[n] is the input signal • y[n] is the output signal x[n] convolution y[n] • h[n] is impulse response (filter coefficients) • n is the sample number N yn[ ]= ∑ h [ k ]. xn [− k ] • N+1 is the filter size k =0 yn[ ]= h [0]. xn [ ] + h [1]. xn [ −++ 1] ... hN [ ]. xn [ − N ] 11 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Convolution Operation Filter Coefficients Input Signal Clock # h[3] h[2] h[1] h[0] 4 x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] ... Output Signal: y[4]= hxhxhxhx [0]. [4] ++ [1]. [3] [2]. [2] + [3]. [1] h[3] h[2] h[1] h[0] 5 x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] ... Output Signal: y[5]= h [0]. x [5] +++ hx [1]. [4] h [2]. x [3] hx [3]. [2] h[3] h[2] h[1] h[0] 6 x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] ... Output Signal: y[6]= hxhxhxhx [0]. [6] ++ [1]. [5] [2]. [4] + [3]. [3] 12 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Convolution in Hardware (Realization) x[n-1] x[n-2] x[n-3] x[n] Reg Reg Reg h[0] h[1] h[2] h[3] y[n] yn[ ]= h [0]. xn [ ] + h [1]. xn [ −+ 1] h [2]. xn [ − 2] + h [3]. xn [ − 3] 13 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Moving Average Filter: Noise Reduction • Consider a digital filter whose output signal y[n] is the average of the four most recent values of the input signal x[n]: y[n] = ¼ ( x[n] + x[n-1] + x[n-2] + x[n-3] ) • Such a filter is referred to as a Moving Average Filter and is commonly used for noise reduction. • The amount of noise reduction is equal to the square-root of the number of points in the average. For example, a 100 point moving average filter reduces the noise by a factor of 10. 14 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Moving Average Filter: Noise Reduction • Example of a moving average filter. • In (a), a rectangular pulse is buried in random noise. • In (b) and (c), this signal is filtered with 11 and 51 point moving average filters, respectively. • As the number of points in the filter increases, the noise becomes lower; however, the edges becoming less sharp. 15 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Moving Average Filter: Noise Reduction But how to implement a Moving Average Filter? y[n] = ¼ ( x[n] + x[n-1] + x[n-2] + x[n-3] ) Recall the convolution operation (sum of products) yn[ ]= h [0]. xn [ ] + h [1]. xn [ −+ 1] h [2]. xn [ − 2] + h [3]. xn [ − 3] In this example, h[n] should be: h = [ ¼ , ¼ , ¼ , ¼ ] OR h = [ 1/N , 1/N , 1/N , 1/N ] , where N is the number of coefficients in the filter 16 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Trapezoidal Filter: Pulse Shaping (for Pre-Amp pulses) Positive Averaging Negative Averaging Gap over L samples over L samples h = [ .1, .1, .1, .1, .1, .1, .1, .1, .1, .1, 0, 0, 0, 0, 0, -.1, -.1, -.1, -.1, -.1, -.1, -.1, -.1, -.1, -.1 ] Length (L) = 10 Gap (G) = 5 Signal Input 25 Filter Output Peaking Time > Charge Collection Time 20 15 10 5 0 190 195 200 205 210 215 220 225 230 235 17 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Triangular Filter: Trigger • A triangular filter is a special trapezoidal filter with G=0. • Triangular filters with peaking time of 25 nsec and 50 nsec (with T=5 nsec) are: h1 = [ -1, -1, -1, -1, -1, 1, 1, 1, 1, 1 ] h2 = [-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 1, 1, 1, 1 , 1, 1, 1, 1, 1] Triangular Filter (20 nsec peaking time) 18 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Digital Filters: Finite Impulse Response (FIR) Triangular Filter: Pulse Integration (for Anode pulses) Anode Integration Time = L . Sampling Period ; output (The time over which scintillator emits most of its Preamp light ~ 99% ~ 4.6 decay time) output NaI: 230 * 4.6 = 1058 nsec •G=0 (Triangle Filter) •Unity Coefficients (no normalization) L L For pulses with negative polarity … -1 … … 1 … Pulse Integration Baseline Correction For pulses with positive polarity … 1 … … -1 … 19 Digital Radiation Measurement and Spectroscopy, Oregon State University, Abi Farsoni Trapezoidal40 and Triangular Filters 35 … -1/L … ..0.. … 1/L … 30 Preamp25 Output 20 15 ADC Units 10 5 Filter Response Energy Filter (Trapezoidal), L=100,G=20 0 L G L -5 Trigger Filter (Triangular), Triggering Threshold L=5,G=0 -10 -15 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 Time (nsec) 20 Digital Radiation
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