Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Pulse and Pulse Processing Supriya Das Centre for Astroparticle Physics and Space Science Bose Institute [email protected] 4th. Winter School on Astro-Particle Physics (WAPP 2009) Mayapuri, Darjeeling Measure what is measurable, and make measurable what is not so. - Galileo Galilei Pulse : How does it appear? Indirect detection Direct detection Flow through the processing electronics Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 2 Pulse : Where are the information? Brief surges of current or voltage in which information may be contained in one or more of its characteristics – polarity, amplitude, shape etc. Baseline Pulse height or Amplitude Leading edge / Trailing edge Supriya Das, Bose Institute Signal width Rise time / Fall time WAPP 2009, Mayapuri, Darjeeling Unipolar / Bipolar 3 Pulse : How do they look? Analog or digital? Fast or slow? Amplitude or shape varies continuously Proportionately with the information • signal from microphone • signal from proportional chamber Rise time – a few nanoseconds or less Quantized information in discrete number of states (practically two) • pulse after discriminator Supriya Das, Bose Institute Rise time – hundreds of nanoseconds or greater WAPP 2009, Mayapuri, Darjeeling 4 Logic standards Nuclear Instrumentation Module (NIM) Slow positive NIM Fast negative NIM O/P must deliver I/P must accept Logic 1 (high) -14 mA to -18 mA -12 mA to -36 mA Logic 0 (low) -1 mA to +1 mA -4 mA to +20 mA O/P must deliver I/P must accept Logic 1 (high) +4 V to +12 V +3 V to +12 V Logic 0 (low) +1 V to -2 V +1.5 V to -2 V Transistor-Transistor Logic (TTL) and Emitter Coupled Logic (ECL) TTL Supriya Das, Bose Institute ECL Logic 1 (high) 2–5V - 1.75 V Logic 0 (low) 0 – 0.8 V -0.90 V WAPP 2009, Mayapuri, Darjeeling 5 Signal transmission Signal is produced at the detector – one needs to carry it till the Data Acquisition system – How? What are the things one needs to keep in mind? • transmission of large range of frequencies uniformly and coherently over the required distance, typically a few meters. For transmitting 2-3 ns pulse the transmission line have to be able to transmit signals with frequency up to several 100 MHz. One solution (the best one), Coaxial cable : Two concentric cylindrical conductors separated by a dielectric material – the outer conductor besides serving as the ground return, serves as a shield to the central one from stray electromagnetic fields. L ln( b / a) 2 Typically C ~ 100 pF/m and L ~ few tens of H/m 2 C ln( b / a) Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 6 Signal Transmission (contd.) Characteristic Impedance : Z0 Km L 60 ln( b / a) C Ke Q. All coaxial cables are limited to the range between 50 – 200 W. Why? Reflection, Termination, Impedance matching: Reflection occurs when a traveling wave encounters a medium where the speed of propagation is different. In transmission lines reflections occur when there is a change in characteristic impedance. Reflection coefficient r = (R-Z)/(R+Z) , where R is the terminating impedance. if R > Z, the polarity of the reflected signal is the same as the propagating signal and the amplitude of reflected signal is same or less as of that of the propagating signal in limiting case of infinite load (i.e. open circuit), the amplitude of the reflected signal is the same of the propagating signal if R < Z, the polarity of the reflected signal is the opposite to the propagating signal and the amplitude of reflected signal is same or less as of that of the propagating signal in limiting case of zero load (i.e. short circuit), the amplitude of the reflected signal is the same of the propagating signal More on all these during the practical session with Atul Jain Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 7 Preamplification Pre-amplifier (Preamp) : (i) Amplify weak signals from the detector (ii) Match the impedance of the detector and next level of electronics. R2 Cf R1 Vin Vout Vin Vout = -(R2/R1) Vin Vout Vout = - Q/Cf Voltage sensitive Supriya Das, Bose Institute Cd Charge sensitive WAPP 2009, Mayapuri, Darjeeling 8 Pulse Shaping Amplifier : Amplifies signal from preamp (or from detector) to a level required for the analysis / recording. When you’re performing pulse height analysis i.e. you’re interested in the energy information – the amplifier should have shaping capabilities. Pulse shaping: Two conflicting objectives Improve the signal to noise (S/N) ratio – increase pulse width Avoid pile up – shorten a long tail Pile up Supriya Das, Bose Institute No pile up WAPP 2009, Mayapuri, Darjeeling 9 Pulse Shaping (contd.) Pulse shaping : How does it work? CR Differentiator : High pass filter RC Integrator : Low pass filter Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 10 Pulse Shaping (contd.) CR-RC Shaping Pole zero cancellation Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 11 Pulse Shaping (contd.) CR-RC Shaping Fixed differentiator time constant 100ns Integrator time constant 10, 30, 100 ns Fixed integrator time constant 10 ns Differentiator time constant inf, 100, 30, 10 ns Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 12 Pulse Shaping (contd.) Baseline Shift Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 13 Pulse Shaping (contd.) Bipolar pulse : Double differentiation or CR-RC-CR shaping Two advantages : (i) solution to baseline shift (ii) zero-crossing trigger for timing Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 14 Pulse Shaping (contd.) More advancement : Semi-Gaussian Shaping Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 15 Digitization of pulse height and time Analog to Digital Conversion - ADC Input is applied to n comparators in parallel Switching thresholds are set by resistor chains 2n comparators for n bits Vref Digital output Advantage: Short conversion time (<10 ns) Disadvantages: o limited accuracy o power consumption Flash ADC Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 16 ADC (contd.) Pulse stretcher Comparator Control Logic Register + DAC Advantage: speed is still nice ~ s high resolution can be fabricated on monolithic ICs Disadvantages: o starts with MSB Successive approximation ADC Starts with MSB (2n). Compares the input with analog correspondent of that bit (from DAC) ands sets the MSB to 0 or 1. Successively adds the next bits till the LSB (20). n conversion steps for 2n bit resolution. Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 17 ADC (contd.) Advantage: excellent linearity – continuous conversion Disadvantage: o slow : Tconv = Nch/fclock Typically for fclock ~ 100MHz and Nch = 8192, Tconv ~ 10 s Wilkinson ADC Nch is proportional to pulse height Charge memory capacitor till the peak Do the following simultaneously: 1. Disconnect the capacitor from input 2. Switch the current source to linearly discharge the capacitor 3. Start the counter to count the clock pulses till the capacitor is discharged fully (decision comes from comparator) Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 18 ADC (contd.) Wilkinson ADC Timing diagram Operation Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 19 ADC (contd.) Analog to Digital Conversion – Hybrid technology Use Flash ADC for coarse conversion : 8 out of 13 bits Successive approximation or Wilkinson type ADC for fine resolution Limited range, short conversion time 256 channels with 100 MHz clock – 2.6 s Result: 13 bit conversion in 4 s with excellent linearity Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 20 Digitization of time (contd.) Time Digitization : TAC, TDC Counter: Very simple : count clock pulses between START and STOP. Limitation : speed of counter, currently possible 1 GHz - time resolution ~ 1 ns Analog Ramp: charge a capacitor through current source START : turn on current source , STOP : turn off current source use Wilkinson ADC to digitize the storage charge/voltage Time resolution ~ 10 ps Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 21 Timing circuits Discriminator : Generates digital pulse corresponding to analog pulse Combination of comparator and mono-shot. Vth Comparator Monoshot Problem : Time walk Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 22 Timing circuits (contd.) Solution 1 : Fast zero crossing Trigger Take the bipolar O/P from shaper/amplifier Trigger at zero crossing point Advantage : The crossing point is independent of amplitude Disadvantage : Works only when the signals are of same shape and rise time Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 23 Timing circuits (contd.) Solution 2: Constant Fraction Trigger Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 24 Pulse processing - instruments NIM Physical/mechanical parameters : • width – 19” (full crate) • width of the slot – 1.35” • height – 8.75” Electrical parameters : +/- 24 V, +/- 12 V, +/- 6 V, +/- 3 V (sometimes) Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling connector 25 Pulse processing - instruments CAMAC – Computer Automated Measurement and Control Main difference with NIM – computer interface Once again 19” wide crate with 25 slots/stations 2U fan tray Supriya Das, Bose Institute Back plane contains power bus as well as data bus Station 24 & 25 reserved for the controller WAPP 2009, Mayapuri, Darjeeling 26 Pulse processing - instruments VME – Versa Module Eurocard (Europa) Developed in 1981 by Motorola Much more compact, high speed bus Fiber optic communication possible Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 27 References Many of the diagrams you’ve seen here are from Radiation Detection and Measurement – G.F. Knoll Techniques for Nuclear and Particle Physics Experiments – W.R. Leo Nuclear Electronics – P.W. Nicholson Radiation Detection and Signal processing (lecture notes) – H. Spieler (http://www-physics.lbl.gov/~spieler/Heidelberg_Notes/) ORTEC Documentation - www.ortec-online.com Supriya Das, Bose Institute WAPP 2009, Mayapuri, Darjeeling 28