Signal Processing for TPCs in High Energy Physics (Part I) Beijing, 9-10 January 2008 Outline Introduction to signal processing in HEP Detector signal processing mode Electronic signal processing Preamplifier and Shaper Analogue to digital conversion Digital signal processing Luciano musa -cern
Beijing, January 2008 Luciano Musa 1 Signal Processing for TPCs in High Energy Physics (Part I) Beijing, 9-10 January 2008 Luciano Musa - CERN Outline • Introduction to signal processing in HEP • Detector signal processing model • Electronic signal processing • Preamplifier and Shaper • Analogue to Digital Conversion • Digital signal processing
Signal Processing in High Energy Physics Introduction Signal Processing is a way of converting an obscure signal into useful information Signal processing includes signal formation due to a particle passage within a detector, signal amplification, signal shaping(filtering) and readout The basic goal is to extract the desired and pertinent information from the obscuring factors (e.g. noise, pile-up) The two quantities of greatest importance to be extracted from detector signals are amplitude energy, nature of the particle, localization time of occurrence localization, nature of the particle Luciano musa
Beijing, January 2008 Luciano Musa 2 Signal Processing in High Energy Physics Introduction ▪ Signal Processing is a way of converting an obscure signal into useful information ▪ Signal processing includes signal formation due to a particle passage within a detector, signal amplification, signal shaping (filtering) and readout ▪ The basic goal is to extract the desired and pertinent information from the obscuring factors (e.g. noise, pile-up) ▪ The two quantities of greatest importance to be extracted from detector signals are: • amplitude: energy, nature of the particle, localization • time of occurrence: localization, nature of the particle
Signal Processing in High Energy Physics Means of detection Each detection method has to extract some energy from the particle to be detected Nearly all detection methods(Cerenkov and Transition Radiation Detector being an exception) make use of ionization or excitation Charged particles: ionization and excitation is produced directly by the interaction of the particle electromagnetic field with the electrons of the detection medium A typical particle energy(today's experiments) is of the order of few 100Mev to Gev, while the energy loss can be below the Mev level. This is an example of a nondestructive method for detection of charged particles All neutral particles must first undergo some process that transfers all or part of their energy to charged particles. The detection method is destructive Beijing January 2008 Luciano musa
Beijing, January 2008 Luciano Musa 3 Signal Processing in High Energy Physics Means of Detection ▪ Each detection method has to extract some energy from the particle to be detected ▪ Nearly all detection methods (Cerenkov and Transition Radiation Detector being an exception) make use of ionization or excitation ▪ Charged particles: ionization and excitation is produced directly by the interaction of the particle electromagnetic field with the electrons of the detection medium ▪ A typical particle energy (today’s experiments) is of the order of few 100MeV to GeV, while the energy loss can be below the MeV level. This is an example of a nondestructive method for detection of charged particles. ▪ All neutral particles must first undergo some process that transfers all or part of their energy to charged particles. The detection method is destructive
Signal Processing in High Energy Physics Detection of Ionization(1/2) In most ionization detectors the total ionization charge is collected using an externally applied electrical field Sometimes an amplification process by avalanche formation in a high electrical field is used. Examples of detectors are a) Proportional chamber(MWPC, GEM, uMegas) b) Time Projection Chamber d quid-argon chamber, d) Semiconductor detector All of them provide a certain amount of charge onto an output electrode The electrode represents a certain capacitance I For signal-processing point of view these detectors are capacitive sources,i.e their output impedance is dominated by the capacitance Luciano musa
Beijing, January 2008 Luciano Musa 4 Signal Processing in High Energy Physics Detection of Ionization (1/2) ▪ In most ionization detectors the total ionization charge is collected using an externally applied electrical field ▪ Sometimes an amplification process by avalanche formation in a high electrical field is used. Examples of detectors are: a) Proportional chamber (MWPC, GEM, mMegas); b) Time Projection Chamber c) Liquid-argon chamber; d) Semiconductor detector. ▪ All of them provide a certain amount of charge onto an output electrode ▪ The electrode represents a certain capacitance ▪ For signal-processing point of view these detectors are capacitive sources, i.e. their output impedance is dominated by the capacitance
Signal Processing in High Energy Physics Detection of Ionization (2/2) This common feature of all detectors for particle physics allows a rather unified approach to signal processing Despite of common features among various detectors used in high-energy physics, great differences exist among them The typical charge at the detector output can differ by six orders of magnitude The output capacitances can differ by the same factor Signal dynamics Pulse repetition rate Luciano musa 5
Beijing, January 2008 Luciano Musa 5 Signal Processing in High Energy Physics Detection of Ionization (2/2) ▪ This common feature of all detectors for particle physics allows a rather unified approach to signal processing ▪ Despite of common features among various detectors used in high-energy physics, great differences exist among them • The typical charge at the detector output can differ by six orders of magnitude • The output capacitances can differ by the same factor • Signal dynamics • Pulse repetition rate
Signal Induced by a Moving Charge Example l Parallel Plate lon Chamber Anode(A) Ael-".A(P) q Applying E Green's VA(P d q Theorem P v A constant induced current flows in the external circuit Cathode(C) dQ dx d dt Luciano musa
Beijing, January 2008 Luciano Musa 6 Signal Induced by a Moving Charge + − d x E Vb i QA,el = -q V’A(P) V’A = -q x d QA,ion = q V’A(P) V’A = q x d Anode (A) Cathode (C) A constant induced current flows in the external circuit i = dQA,el dt = - q d dx dt Parallel Plate Ion Chamber Applying Green’s Theorem Example I P(x)
Signal Induced by a Moving Charge Example Cylindrical Proportional Chamber Charged Avalanche particle eglo primary (amplification) ionization cathode inode Electron cloude lectron -ion E≠0 =io/(1tt0) Luciano musa
Beijing, January 2008 Luciano Musa 7 anode cathode Avalanche region (amplification) Charged particle electron – ion pair E ≠ 0 primary ionization Ion cloude Electron cloude gas Signal Induced by a Moving Charge Cylindrical Proportional Chamber i(t) = i0 / (1+t/t0) Example II
Detector Signal Processing Model senes serles white noise 1/ noise noiseless preamplifier e2Fc/fl A signal processor Q. s(t) d 12=b 12 =d f arallel fs()=8(t) para A(Q/(Cd+ci)) hite noise f nois noise power spectral density Ax[a+blo(cd+Ci)2 The detector is modeled as a current source, delivering a current pulse with time profile s(t) and charge Q, proportional to the energy released across the parallel combination of the detector capacitance Cd and the preamplifier input capacitance Ci Beijing January 2008 Luciano musa
Beijing, January 2008 Luciano Musa 8 Q Cd Ci ·s(t) A noiseless preamplifier Detector Signal Processing Model The detector is modeled as a current source, delivering a current pulse with time profile s(t) and charge Q, proportional to the energy released, across the parallel combination of the detector capacitance Cd and the preamplifier input capacitance Ci . signal processor i 2 W=b e 2 W=a parallel white noise series white noise e 2 f=c/|f| i 2 f=d·f series 1/f noise parallel f noise noise power spectral density Ax[a+b/w(Cd+Ci)2] A(Q/(Cd+Ci)) If s(t) = d(t)
Electronic Signal Processing Signal Processor F(n h(f) F(f U(f fo Noise flo f Improved fO Signal/Noise Ratio Example of signal filtering-the figure shows a"typical case of noise filtering In particle physics, the detector signals have very often a very large frequency spectrum The filter(shaper) provides a limitation in bandwidth, and the output signal shape is different with respect to the input signal shape. Beijing January 2008 Luciano musa
Beijing, January 2008 Luciano Musa 9 Electronic Signal Processing F(f) U(f) F(f) f U(f) f h(f) f Noise floor f0 f0 f0 Improved Signal/Noise Ratio Example of signal filtering - the figure shows a “typical” case of noise filtering In particle physics, the detector signals have very often a very large frequency spectrum The filter (shaper) provides a limitation in bandwidth, and the output signal shape is different with respect to the input signal shape. Signal Processor
Electronic Signal Processing AF U(t h(f) t) u(t Noise floor fo Improved fo f Signal/Noise ratio The output signal shape is determined, for each application, by the following parameters Input signal shape(characteristic of detector Filter(amplifier-shaper) characteristic The output signal shape is chosen such to satisfy the application requirements Time measurement Amplitude measurement Pile-up reduction Optimized signal-to-noise ratio Luciano musa
Beijing, January 2008 Luciano Musa 10 Electronic Signal Processing F(f) U(f) f(t) f u(t) f h(f) f Noise floor f0 f0 Improved Signal/Noise Ratio The output signal shape is determined, for each application, by the following parameters: • Input signal shape (characteristic of detector) • Filter (amplifier-shaper) characteristic The output signal shape is chosen such to satisfy the application requirements: • Time measurement • Amplitude measurement • Pile-up reduction • Optimized Signal-to-noise ratio
