Transcript Pulse and Pulse Processing in Experiments

Measure what is measurable,
and make measurable what is not so.
- Galileo Galilei
PULSE AND PULSE PROCESSING
Supriya Das
Department of Physics
&
Centre for Astroparticle Physics and Space Science
Bose Institute
[email protected]
6th. Winter School on Astroparticle Physics (WAPP 2011)
Mayapuri, Darjeeling
Pulse : How does it appear?
Direct detection
Indirect detection
Flow through the processing electronics
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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
Signal width
Rise time / Fall time
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Unipolar / Bipolar
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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
Rise time – hundreds of nanoseconds or
greater
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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
ECL
Logic 1
(high)
2–5V
- 1.75 V
Logic 0
(low)
0 – 0.8 V
-0.90 V
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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
Vout = -(R2/R1) Vin
Voltage sensitive
Vin
Cd
Vout
Vout = - Q/Cf
Charge sensitive
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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 )
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Signal Transmission (contd.)
Characteristic Impedance :
Z0 
K
L
 60 m 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 signa
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
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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
No pile up
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Pulse Shaping (contd.)
Pulse shaping : How does it work?
CR Differentiator : High pass filter
RC Integrator : Low pass filter
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Pulse Shaping (contd.)
CR-RC Shaping
Pole zero cancellation
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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
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Pulse Shaping (contd.)
Baseline Shift
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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
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Pulse Shaping (contd.)
More advancement : Semi-Gaussian Shaping
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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
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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.
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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)
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ADC (contd.) Wilkinson ADC
Operation
Timing diagram
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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
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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
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Timing circuits
Discriminator : Generates digital pulse corresponding to analog pulse
Combination of comparator and mono-shot.
Vth
Comparator
Monoshot
Problem : Time walk
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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
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Timing circuits (contd.)
Solution 2: Constant Fraction Trigger
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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)
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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
Back plane contains power bus as
well as data bus
Station 24 & 25 reserved for the
controller
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Pulse processing - instruments
VME – Versa Module Eurocard (Europa)
Developed in 1981 by Motorola
Much more compact, high speed bus
Fiber optic communication possible
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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
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Thank You
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