Time Pick -off Techniques

Time Pick-off Techniques

Jean-Francois GenatCNRS/IN2P3/LPNHE Paris

IEEE Nuclear Science Symposium and Medical Imaging ConferenceOctober 23d 2011, Valencia, Spain

The lecture will review the major limitations affecting timing measurements and the techniques to overcome them.

Perspectives to achieve time accuracy in the picosecond regime will be discussed

Outline

Fast signals Threshold techniques Sampling techniques Conclusion

Timing Pick-off Techniques, October 23d 2011, Valencia, Spain

Fast detectors and signalsMoving charges:

i(t)= n(t) q v(t)

Rise-time i’(t)= q [ n(t) v’(t) + n’(t) v(t)]

In order to minimize rise-time, maximize:

n electron multiplication PMTs, MCPs

dv/dt qE/m electric field (in vacuum)

dn/dt primary ionisation, multiplication

v t . qE /m electric field

Fast:

- Vacuum devices- Electron multiplication- Low capacitance- High electric fields

Bias


Fast detectors

Rise-time Timing

Regular PMTs 2-5ns 500 ps Silicon• APDs, PIN 300 ps 50 ps• Silicon PMs 700 ps 100 ps• 3D Silicon 500ps ?

Vacuum• Multi-anode/mesh PMTs 200ps 50 ps• MCP PMTs 150 ps 20-30 ps• Multi anodes MCP PMTs 30 ps ? 1 ps ?

Nanosecond: Photo-Multipliers Sub-nanosecond: Silicon Photo-MultipliersPico-second ? Micro-Channel Plates, Streak camera


Fast signals

Example: 5 x 5 cm2 Micro-Channel Plate signalRise time = 400 psBandwidth = 0.35/rise-time ~ 1 GHz

10%

90%

Rise-time- Rise-time- Waveform- Signal to Noise

Leading edge isrelevant for timing(first electrons)


Timing Measurements

Oscilloscope traces for timing signals driving a TDC. Dt = 10ns

- Obtain digital signals from analog detector pulses and feed a Time to Digital Coder- Sample and digitize, then process digitally to get the difference Stop-Start

Waveforms digitized with a digital oscilloscope. ICs can do this today


Timing Measurements

Electronics gain-bandwidth should match:

- Detector sensitivity- Detector rise-time

Example: Multi-anodes MCP PMTs:

Rise-time: 25ps Corresponding Bandwidth: 15 GHz



Outline Fast signals Threshold techniques Sampling techniques (See Eric Delagnes) Conclusion


Single threshold pick-off

Leading edge Threshold: Time spread proportional to noise and rise-time

See Angelo Rivetti ‘s lecture for CMOS IC discriminators implementations


Single threshold pick-off

AThreshold

Slope dV/dtsA

st

)//(1 dVdtAtA sss)//(1 dtdVAt

noise

Effects of Rise-time and Noise on Timing resolution

t


Effects of amplitude and rise-time with single threshold

Amplitude and/or Rise-time spectra translate into time spread


Improved Time Pick-off Techniques

Extrapolated time

Multi-threshold

Leading edge errors

Leadingedge

Constant fraction

Constant-fraction

Pulse sampling and Waveform analysis

Sample, digitize,Fit to the (known) waveformGet time and amplitude

ANALOG

DIGITAL

12Timing Pick-off Techniques, October 23d 2011, Valencia, Spain

Constant Fraction

1

If rise-time proportional to amplitude, use constant-fraction

OK Leading edge errors

Constant fraction

Leadingedge

If rise-time does not depend If pulse shape independent uponupon amplitude. Leading edge OK amplitude, use Constant FractionBut detector may be under saturation

Leadingedge


Constant Fraction implementation

Compare the delayed signal with the attenuated signal (or zero-cross the difference)Need to enable the zero crossing (undefined state before and after operation)(See A. Rivetti’s lecture)

A regular discriminator can be used, the delay line is critical (delay, bandwidth), In ICs, it can be implemented with a low pass filter (See A. Rivetti’s lecture)

Three parameters:

Trigger threshold Delay Fraction

Maximize slope at zero-crossingCarefully optimize parameters wrt signals propertiesNot trivial !

H. Spieler [IEEE NS 29 June 1982 pp1142-1158 ]T.J. Paulus [IEEE NS 32 June 1985 pp 1242-1249]


Timing resolution using Waveform Sampling

abwt

SNSNtt ssr

t35.01

s

(Stefan Ritt)rstnt tt /, ss

Since

and

dtdSn

t /ss

StdtdS r/

n

SSNs

where

whith


Other effects

Walk: Discriminator delay (walk) depends on slope across threshold Resulting rise-time is (detector rise-time + amplifier rise-time) Use an appropriate (gain x bandwidth) technology


Zero crossing

Use zero-crossing of signal derivative Detects signal’s maximum

Signal

delay

Reject noise from signal derivative


Double Threshold High threshold to trigger

Low threshold to timeLow thresh

Hi thresh

Delay

Signal

delay

Avoids noise on low thresholdDecision on very first signal rise


Single threshold + Peak measurement

• If the peak amplitude is measured, single threshold can be compensated off-line for rise-time, and even lead to better results than constant fraction



Outline Fast signals Threshold techniques Sampling techniques Conclusion


21

Waveform Sampling (See Eric Delagnes talk)

2 12 25 80 128 50 32 …

Waveform analysisTiming Pick-off Techniques, October 23d 2011, Valencia, Spain

Sampling Electronics for Micro-Channel Plate DetectorsStore the full detector information as with a digital oscilloscope:

- Detector + electronics noise >> quantization noise (LSB/√12)- Sampling frequency > 2 x full Analog Bandwidth (Shannon-Nyquist)

Ideal approach:

Digitize on the fly, if the two above conditions can be fulfilled.If not, loss of precision due to A/D conversion and/or loss of timing information

2 GHz

Fourier spectrum of a 2”x 2” MCP signal

dttdV

Vt)(/ss

Noise as small as possible

Slope as steep as possible


23

Waveform Sampling Analysis (see Eric Delagnes talk)

Extract precise time and amplitude from optimal filtering (minimization of c2)evaluated with a fit to a waveform template deduced from averaged measurements

Real MCP Laser data

Signal Templates

Many techniques B. Cleland and E. Stern, BNL


Timing resolution using Waveform Sampling

abwt

SNSNtt ssr

t35.01

s

(Stefan Ritt)rstnt tt /, ss

Since

and

dtdSn

t /ss

StdtdS r/

n

SSNs

where

whith


Sampling Electronics for Micro-Channel Plate Detectors

A/D converters do not fit the need. State of the art:

8-bit 1GS/s 10-bit 300 MS/s 16-bit 160 MS/s

Need at least 5 GS/s sampling rate,10-12bitThere is no !

Fast analog storage and slower digitization, if rate allows, or dead-time acceptable

See Eric Delagnes talk

Apply the best timing algorithm suited to the detector, get the charge for free


Waveform Sampling: Timing Resolution vs Sampling Rate

26Timing Pick-off Techniques, October 23d 2011, Valencia, Spain

27

280ps rise-time (Micro-Channel Plate like)

Synthesized signals used for simulations

Noise:50% MCP noise +50% White noise


Waveform Sampling : Timing resolution vs Analog Bandwidth and Signal to Noise


29

Waveform sampling: Timing resolution vs Sampling rate / Analog Bandwidth (simulation)

Timing resolution vs Sampling rate / Analog bandwidth


Timing Pick-off methods compared (simulation)

Time resolution vs Number of photo-electrons

zoom


Inputs

System issues Drifts due to environmental conditionsPower supplies drifts and noiseCables/fibers instabilities

- Cable has shorter group delay, and even higher bandwidth, may pick-up noise

- Micro-coax makes a come-back


CMOS ICs Technologies

CMOS bandwidths from 90 to 45 nm technology nodes (ITRS 2005)



Outline Fast signals Threshold techniques Sampling techniques Conclusion


34

Conclusion Timing pick-off requires dedicated analog front-end electronics matched

with the signal characteristics in terms of noise and bandwidth. Many techniques exist matched to different detector and environment conditions

Fast analog and digital challenging signal processing techniques are

needed for precision timing measurements.

• Knowledge of the intrinsic signal properties (waveform, bandwidth, noise) is mandatory.

• Choice can also be dictated by environment constraints such as event rate, number of channels, availability of digital signal processing and ASIC design means, and cost.


35

Extra slides


Time arbitration

In1 In2

out1 out2

Six transistors implementation in CMOS Metastability issues when driven with the same input

[V. Gutnik et al. MIT, IEEE 2000 Symp. on VLSI Circuits]


37

Waveform Sampling benefitsPulse sampling and waveform analysis for 2D delay line readout

• Pico-second timing with fast detectors• Timing along the strip, a few ps obtained (< 1mm) Centroids perpendicular

• Resolve pile-up, mishappened pulses

• Large area detectors can be read in series:


Fast Micro-Channel Plate signals

11 mm diameter Micro-Channel Plate signalSignal bandwidth: 10 GHzSingle Photoelectron Time Transit Spread: 10 ps

From Photek

TTS= 10ps


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Micro-Channel Plate Detectors

Pore diameter 3-25 mmPore aspect ratio: 1:50

1st gap

pores

2d gap

Pore diameter: a few mm200 V

1- 2kV

200 V

Anodes (1.6 x 1.6mm2 pixels)

Photo-cathode


10 mm

Example: Large Area Picosecond Photo-detectors

Micro-Channel plate structure

Fast Readout samplingElectronics (130nm CMOS)

Bandwidth and crosstalk evaluation

Frequency 1GHz

Response dB

Board only

Board + electronics

Crosstalk %

Normalized injection position

1st neighbor

2d neighbor

3d neighbor

3%

2%

1%

-5dB

-10dB

Anodes strip lines


Transmission Line Readout Position Resolution

50 Photo-Electrons

41

OscilloscopeTektronix TDS6154C

25 mm pore MCP signal at the output of a ceramic transmission line Laser 408nm, 50W, no amplification


42

Fast Sampling Switched Capacitor Array

- Sampling frequency- Analog bandwidth - Analog dynamic range- Depth- Readout frequency- Read/Write

Switched capacitor array

Timing generator

Analog input

A/D converter

State of the art: 250nm CMOS 6ps rmsG. Varner, S. Ritt

Chicago-Hawaii: - 130nm CMOS - 15 GS/s sampling rate - Foreseen timing resolutions of a few ps

Trigger


Time Pick -off Techniques

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