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]

6

th.

Winter School on Astroparticle Physics (WAPP 2011) Mayapuri, Darjeeling

Pulse : How does it appear?

Direct detection Indirect detection Flow through the processing electronics WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 3

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 Rise time / Fall time Signal width Unipolar / Bipolar WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 4

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 5

Logic standards

Nuclear Instrumentation Module (NIM) Logic 1 (high) Logic 0 (low) Fast negative NIM O/P must deliver -14 mA to -18 mA -1 mA to +1 mA I/P must accept -12 mA to -36 mA -4 mA to +20 mA Logic 1 (high) Logic 0 (low) Slow positive NIM O/P must deliver +4 V to +12 V +1 V to -2 V I/P must accept +3 V to +12 V +1.5 V to -2 V Transistor-Transistor Logic (TTL) and Emitter Coupled Logic (ECL) TTL ECL Logic 1 (high) Logic 0 (low) 2 – 5 V 0 – 0.8 V - 1.75 V -0.90 V WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 6

Preamplification

Pre-amplifier (Preamp) : (i) Amplify weak signals from the detector (ii) Match the impedance of the detector and next level of electronics. R 2 C f R 1 V in C d V out V in V out V out = -(R 2 /R 1 ) V in Voltage sensitive V out = - Q/C f Charge sensitive WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 7

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

  2  ln(

b

/

a

)

C

 2  ln(

b

/

a

) Typically C ~ 100 pF/m and L ~ few tens of  H/m WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 8

Signal Transmission (contd.)

Characteristic Impedance :

Z

0 

L C

 60

K m K e

ln(

b

/

a

) 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 Raghunandan Shukla WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 9

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 10

Pulse Shaping (contd.)

Pulse shaping : How does it work?

CR Differentiator : High pass filter RC Integrator : Low pass filter WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 11

Pulse Shaping (contd.)

CR-RC Shaping Pole zero cancellation WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 12

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 13

Pulse Shaping (contd.)

Baseline Shift WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 14

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 15

Pulse Shaping (contd.)

More advancement : Semi-Gaussian Shaping WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 16

Digitization of pulse height and time

Analog to Digital Conversion - ADC V ref Digital output    Input is applied to n comparators in parallel Switching thresholds are set by resistor chains

2 n

comparators for n bits Advantage: Short conversion time (<10 ns) Disadvantages: o limited accuracy o power consumption Flash ADC WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 17

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 (2

n

).

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 (2

0

). n conversion steps for 2

n

bit resolution. WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 18

ADC (contd.)

Advantage:  excellent linearity – continuous conversion Disadvantage: o slow : T conv = N ch /f clock Typically for f and N ch clock ~ 100MHz = 8192, T conv ~ 10  s   Wilkinson ADC N ch 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) WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 19

ADC (contd.)

Wilkinson ADC Operation Timing diagram WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 20

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 21

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 22

Timing circuits

Discriminator : Generates digital pulse corresponding to analog pulse Combination of comparator and mono-shot. V th Comparator Problem : Time walk Monoshot WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 23

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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 24

Timing circuits (contd.)

Solution 2: Constant Fraction Trigger WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 25

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) WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 26

Pulse processing - instruments

CAMAC – C omputer A utomated M easurement and C ontrol 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 WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 27

Pulse processing - instruments

VME – V ersa M odule E urocard ( E uropa) Developed in 1981 by Motorola Much more compact, high speed bus Fiber optic communication possible WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 28

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

Thank You

WAPP 2011, Mayapuri, Darjeeling Supriya Das, Bose Institute 29