Time Pick-off Techniques Jean-Francois Genat CNRS/IN2P3/LPNHE Paris IEEE Nuclear Science Symposium and Medical Imaging Conference October 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 23 d 2011, Valencia, Spain

Fast detectors and signals Moving 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 vt. qE /m electric field Fast: - Vacuum devices - Electron multiplication - Low capacitance - High electric fields Bias Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Fast detectors Rise-time Timing Regular PMTs 2-5ns 500 ps Silicon APDs, PIN300 ps 50 ps Silicon PMs700 ps 100 ps 3D Silicon 500ps ? Vacuum Multi-anode/mesh PMTs 200ps 50 ps MCP PMTs 150 ps ps Multi anodes MCP PMTs 30 ps ? 1 ps ? Nanosecond: Photo-Multipliers Sub-nanosecond: Silicon Photo-Multipliers Pico-second ? Micro-Channel Plates, Streak camera Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Fast signals Example: 5 x 5 cm2 Micro-Channel Plate signal Rise time = 400 ps Bandwidth = 0.35/rise-time ~ 1 GHz 10% 90% Rise-time - Rise-time - Waveform - Signal to Noise Leading edge is relevant for timing (first electrons) Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Timing Measurements Oscilloscope traces for timing signals driving a TDC.  t = 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 Pick-off Techniques, October 23 d 2011, Valencia, Spain

Timing Measurements Electronics gain-bandwidth should match: - Detector sensitivity - Detector rise-time Example: Multi-anodes MCP PMTs: Rise-time: 25ps Corresponding Bandwidth: 15 GHz Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Time Pick-off Techniques Outline Fast signals Threshold techniques Sampling techniques (See Eric Delagnes) Conclusion Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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

Single threshold pick-off Threshold Slope dV/dt AA tt noise Effects of Rise-time and Noise on Timing resolution Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Effects of amplitude and rise-time with single threshold Amplitude and/or Rise-time spectra translate into time spread Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Improved Time Pick-off Techniques Extrapolated time Multi-threshold Leading edge errors Leading edge Constant fraction Constant-fraction Pulse sampling and Waveform analysis Sample, digitize, Fit to the (known) waveform Get time and amplitude ANALOG DIGITAL 12 Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Constant Fraction 1 If rise-time proportional to amplitude, use constant-fraction OK Leading edge errors Constant fraction Leading edge If rise-time does not depend If pulse shape independent upon upon amplitude. Leading edge OK amplitude, use Constant Fraction But detector may be under saturation Leading edge Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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-crossing Carefully optimize parameters wrt signals properties Not trivial ! H. Spieler [IEEE NS 29 June 1982 pp ] T.J. Paulus [IEEE NS 32 June 1985 pp ] Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Timing resolution using Waveform Sampling (Stefan Ritt) Since and where whith Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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

Zero crossing Use zero-crossing of signal derivative Detects signal’s maximum Signal delay Reject noise from signal derivative Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Double Threshold High threshold to trigger Low threshold to time Low thresh Hi thresh Delay Signal delay Avoids noise on low threshold Decision on very first signal rise Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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

Time Pick-off Techniques Outline Fast signals Threshold techniques Sampling techniques Conclusion Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

21 Waveform Sampling (See Eric Delagnes talk) … Waveform analysis Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Sampling Electronics for Micro-Channel Plate Detectors Store 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 Noise as small as possible Slope as steep as possible Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

23 Waveform Sampling Analysis (see Eric Delagnes talk) Extract precise time and amplitude from optimal filtering (minimization of  2 ) 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 Pick-off Techniques, October 23 d 2011, Valencia, Spain

Timing resolution using Waveform Sampling (Stefan Ritt) Since and where whith Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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,10-12bit There 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 Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Waveform Sampling: Timing Resolution vs Sampling Rate 26 Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

27 280ps rise-time (Micro-Channel Plate like) Synthesized signals used for simulations Noise: 50% MCP noise + 50% White noise Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Waveform Sampling : Timing resolution vs Analog Bandwidth and Signal to Noise Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

29 Waveform sampling: Timing resolution vs Sampling rate / Analog Bandwidth (simulation) Timing resolution vs Sampling rate / Analog bandwidth Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Timing Pick-off methods compared (simulation) Time resolution vs Number of photo-electrons zoom Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain Inputs

System issues Drifts due to environmental conditions Power supplies drifts and noise Cables/fibers instabilities - Cable has shorter group delay, and even higher bandwidth, may pick-up noise - Micro-coax makes a come-back Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

CMOS ICs Technologies CMOS bandwidths from 90 to 45 nm technology nodes (ITRS 2005) Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Time Pick-off Techniques Outline Fast signals Threshold techniques Sampling techniques Conclusion Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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

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

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

37 Waveform Sampling benefits Pulse 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: Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Fast Micro-Channel Plate signals 11 mm diameter Micro-Channel Plate signal Signal bandwidth: 10 GHz Single Photoelectron Time Transit Spread: 10 ps From Photek TTS= 10ps Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

39 Micro-Channel Plate Detectors Pore diameter 3-25  m Pore aspect ratio: 1:50 1 st gap pores 2d gap Pore diameter: a few  m 200 V 1- 2kV 200 V Anodes (1.6 x 1.6mm 2 pixels) Photo-cathode Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain 10 mm

Example: Large Area Picosecond Photo-detectors Micro-Channel plate structure Fast Readout sampling Electronics (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 Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

Transmission Line Readout Position Resolution 50 Photo-Electrons 41 Oscilloscope Tektronix TDS6154C 25  m pore MCP signal at the output of a ceramic transmission line Laser 408nm, 50 , no amplification Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain

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 rms G. Varner, S. Ritt Chicago-Hawaii: - 130nm CMOS - 15 GS/s sampling rate - Foreseen timing resolutions of a few ps Trigger Timing Pick-off Techniques, October 23 d 2011, Valencia, Spain