Supriya DasDepartment of Physics &Centre for Astroparticle Physics and Space Science Bose [email protected]
6th. Winter School on Astroparticle Physics (WAPP 2011)Mayapuri, Darjeeling
Pulse : How does it appear?
Supriya Das, Bose InstituteWAPP 2011, Mayapuri, Darjeeling 3
Direct detection Indirect detection
Flow through the processing electronics
Pulse : Where are the information?
Supriya Das, Bose InstituteWAPP 2011, Mayapuri, Darjeeling 4
Brief surges of current or voltage in which information may be contained in one or Brief surges of current or voltage in which information may be contained in one or more of its characteristics – polarity, amplitude, shape more of its characteristics – polarity, amplitude, shape etcetc..
Baseline Pulse height or Amplitude Signal width
Leading edge / Trailing edge Rise time / Fall time Unipolar / Bipolar
Pulse : How do they look?
Supriya Das, Bose InstituteWAPP 2011, Mayapuri, Darjeeling 5
Fast or slow?
Rise time – a few nanoseconds or less
Rise time – hundreds of nanoseconds orgreater
Analog or digital?
Amplitude or shape varies continuouslyProportionately with the information
• signal from microphone• signal from proportional chamber
Quantized information in discrete number of states (practically two)
• pulse after discriminator
Logic standards
O/P must O/P must deliverdeliver
I/P must I/P must acceptaccept
Logic 1 Logic 1 (high)(high)
-14 mA to -14 mA to
-18 mA-18 mA-12 mA to -12 mA to
-36 mA-36 mA
Logic 0 Logic 0
(low)(low)-1 mA to-1 mA to
+1 mA+1 mA-4 mA to-4 mA to
+20 mA+20 mA
O/P must O/P must deliverdeliver
I/P must I/P must acceptaccept
Logic 1 Logic 1 (high)(high)
+4 V to +4 V to
+12 V+12 V+3 V to +3 V to
+12 V+12 V
Logic 0 Logic 0
(low)(low)+1 V to+1 V to
-2 V-2 V+1.5 V to+1.5 V to
-2 V-2 V
TTLTTL ECLECL
Logic 1Logic 1
(high)(high)2 – 5 V2 – 5 V - 1.75 V- 1.75 V
Logic 0Logic 0
(low)(low)0 – 0.8 V0 – 0.8 V -0.90 V-0.90 V
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Nuclear Instrumentation Module (NIM)
Fast negative NIM Slow positive NIM
Transistor-Transistor Logic (TTL) and Emitter Coupled Logic (ECL)
Preamplification
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Pre-amplifier (Preamp) : (i) Amplify weak signals from the detector(ii) Match the impedance of the detector and next level of electronics.
Vin Vout
R1
R2
Vout = -(R2/R1) Vin
Voltage sensitive
Vin Vout
Cf
Cd
Vout = - Q/Cf
Charge sensitive
Signal transmission
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Signal is produced at the detector – one needs to carry it till the DataAcquisition 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.
)/ln(2
abL
)/ln(
2
abC
Typically C ~ 100 pF/m and L ~ few tens of H/m
Signal Transmission (contd.)
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Characteristic Impedance : )/ln(600 abK
K
C
LZ
e
m
Q. All coaxial cables are limited to the range between 50 – 200 Why?
Reflection, Termination, Impedance matching:
Reflection occurs when a traveling wave encounters a medium where the speed of propagationis different.
In transmission lines reflections occur when there is a change in characteristic impedance.
Reflection coefficient = (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 Raghunandan Shukla
Pulse Shaping
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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 theenergy 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 No pile up
Pulse Shaping (contd.)
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Pulse shaping : How does it work?
CR Differentiator : High pass filter
RC Integrator : Low pass filter
Pulse Shaping (contd.)
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CR-RC Shaping
Pole zero cancellation
Pulse Shaping (contd.)
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Fixed differentiator time constant 100nsIntegrator time constant 10, 30, 100 ns
Fixed integrator time constant 10 nsDifferentiator time constant inf, 100, 30, 10 ns
CR-RC Shaping
Pulse Shaping (contd.)
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Bipolar pulse : Double differentiation or CR-RC-CR shaping
Two advantages : (i) solution to baseline shift (ii) zero-crossing trigger for timing
Pulse Shaping (contd.)
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More advancement : Semi-Gaussian Shaping
Digitization of pulse height and time
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Analog to Digital Conversion - ADC
Flash ADC
Vref
Digitaloutput
Input is applied to n comparators in parallel Switching thresholds are set by resistor chains 2n comparators for n bits
Advantage:
Short conversion time (<10 ns)
Disadvantages:
o limited accuracyo power consumption
ADC (contd.)
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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.
Pulse stretcher Comparator Control LogicRegister + DAC
Successive approximation ADC
Advantage:
speed is still nice ~ s high resolution can be fabricated on monolithic ICs
Disadvantages:
o starts with MSB
ADC (contd.)
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Wilkinson ADC
Charge memory capacitor till the peak Do the following simultaneously:
1. Disconnect the capacitor from input2. Switch the current source to linearly discharge the capacitor3. Start the counter to count the clock pulses till the capacitor is discharged fully (decision comes from comparator)
Advantage:
excellent linearity – continuous conversion
Disadvantage:
o slow : Tconv = Nch/fclock
Typically for fclock ~ 100MHz and Nch = 8192, Tconv ~ 10 s
Nch is proportional to pulse height
ADC (contd.)
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Wilkinson ADC
Operation Timing diagram
ADC (contd.)
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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
Digitization of time (contd.)
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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
Timing circuits
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Discriminator : Generates digital pulse corresponding to analog pulseCombination of comparator and mono-shot.
Vth Comparator Monoshot
Problem : Time walk
Timing circuits (contd.)
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Solution 1 : Fast zero crossing Trigger
Take the bipolar O/P from shaper/amplifierTrigger 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
Timing circuits (contd.)
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Solution 2: Constant Fraction Trigger
Pulse processing - instruments
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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) connectorconnector
NIM
Pulse processing - instruments
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Once again 19” wide crate with25 slots/stations
2U fan tray
CAMAC – Computer Automated Measurement and Control
Main difference with NIM – computer interface
Back plane contains power bus as well as data bus
Station 24 & 25 reserved for the controller
Pulse processing - instruments
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VME – Versa Module Eurocard (Europa)
Much more compact, high speed bus
Fiber optic communication possible
Developed in 1981 by Motorola
References
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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
Thank You