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Development of Front-End Electronics for Picosecond Resolution TOF Detectors
Fukun Tang, Enrico Fermi Institute, The University of Chicago
With Henry Frisch, Mary Heintz, Harold Sanders (The University of Chicago), John Anderson, Karen Byrum, Gary Drake (Argonne National Laboratory) and
Jean-Francois Genat (Saclay)
Introduction
With newly designed 2x2 inches, 1024 equal timing anodes micro-channel plate
(MCP) photo detector featuring single pulse rising time in the order of 50ps and
transit time spread (TTS) in the order of 25ps, it has become possible to attempt
a design of a large area detector with pico-second resolution for relativistic
particles.
The picosecond Time-of-Flight electronics system consists of a custom ASIC and
several commercial chips with a MCP photo-detector that is designed to measure
the arrival time of charged particles with a design goal of one picosecond
resolution. Effectively, the system acts as a “digital phototube”; all analog signal
processing is performed directly by the circuits mounted on the MCP such that
the only interfaces that connect to external data collection systems are digital
control, digital data output and power supplies.
The frond-end ASIC chip will be built with IBM 0.13u SiGe BiCMOS 8HP process.
The basic functional circuitry in this chip includes a signal receiver, an ultra low
timing jitter/walk discriminator, a control logic block, a 2Ghz PLL common “stop”
clock generator and an 1:200 time stretcher with a dynamic range of 1ns. DAQ
chip will convert each stretched signals to digital by a 11-bit counter with 200ps
resolution. With 1:200 time stretching ratio, the time-to-digital conversion will give
1ps time resolution.
Fig. 1: MCP’s 1024 anode pads have been grouped to 4 sections, the anode signals in
each section will be summed in the equal time anode board and sent to its time
stretcher, a DAQ chip converts the stretched pulses from 4 stretchers to digital and
also manages the communication with system.
Time Stretcher ASIC Design Considerations
Fig. 3 MCP Output Signal
Fig. 6
We have designed 2 types of PLLs (Analog
and Digital) to ensure if we can achieve a
less than 1ps timing jitter for common
“stop” clock.
APLL (Fig.6) is designed fully based on hetero junction transistors, the very high
speed and ultra low noise transistors can help achieve very low timing jitter in
APLL. The only disadvantage is the narrow pull-in range. Due to the varactor’s
variation process-to-process, this may cause VCO’s initial frequency out of the
PLL’s pull-in range, in this case, the APLL won’t go to lock.
This fast signal requires ASIC to
have very high speed circuits to
process the front-end analog signal .
ASIC Technology and Design Kit
The Time Stretcher will be
built with IBM 0.13u SiGe
BiCMOS 8HP process.
This state-of-art SiGe
process has the heterojunction bipolar transistors
with the transition
frequency as high as
200GHz. The ultra low
noise devices allow us to
build the very high
performance circuits to
meet our requirements.
Circuit simulation with real time data and interface to other simulation tools
The DPLL(Fig.7) recently has been
designed and simulated. From
simulation results, the performance well
meets our requirements.
Fig. 4
Fig. 7
Low Phase Noise VCO Design
Fig. 2
As an evaluation, a preliminary 1:200
time stretching circuit simulation has
been done with IHP SiGe SG25H1
process, the current sources with a ratio
of 1:200 is in behavior model. Fig. 5
shows 1ns pulse has been stretched to
274ns, the extra 74 ns was caused by the
charge injection from the switches.
Fig. 14 showed PLL’s locking range. The red
chart showed PLL locked to 1.9GHz, blue chart
showed PLL locked to 2GHz; the purple chart
showed PLL locked to 2.1GHz. A maximum of
10mV ripple was observed in a loop bandwidth
of 100MHz, which equivalents to the timing
jitter of 0.33ps peak-to-peak. We predict a
lower PLL bandwidth can further reduce the
ripple.
Fig.8 VCO Schematic
A Simulink modeling for fast
simulation has been setup, the
preliminary simulation results well
matched Cadence Spectre simulation.
VCO V-F Transfer Function and Phase Noise
Fig10: VCO Phase Noise Plot
VCO Layout
2GHz Phase Locked Loop Design and Simulation
2GHz PLL is required to generate the very low jitter common “stop” clock for
Time Stretcher chip. We have simulated analog PLL and digital PLL. The
simulation results showed both achieved a timing jitter less than 1ps. The analog
PLL uses much less components than digital PLL, but it has very narrow pull-in
range, as a result, the initial VCO frequency may out of the PLL’s pull-in range
because the VCO varator variations process-to-process. The digital PLL has a
very wide locking range and pull-in range, so that the varactor variations processto-process should not be a problem.
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Fig.14
Simulink Modeling and Simulation
Fig. 9: V-F Transfer Function
Fig. 5: 1:200 pulse stretching pulse
Fig.13
PLL Lock Range and Jitter Prediction
Stretching Circuit Simulation
Fig. 2: The time stretcher receives signal from MCP, a very low timing jitter/walk
discriminator will be implemented to generate a “start” signal. The time to measure is
the difference between “start” and “Stop”, that is a 500ps-1ns time interval pulse .
With the following 1:200 time stretching circuit, a stretched pulse (100n-200ns) then
be sent to DAQ chip for digitizing by a 11 bit counter with 200ps resolution.
Fig.12
Fig. 12 showed digital phase-frequency
detector. The proper reset delay time can
optimize the phase detector speed and
cancel the inherent errors that caused by
the glitch from charge pump mismatch
and reset signal mismatch when both
reference and local clocks arrive in the
same time. As a result, the erroneous
jitter can be reduced when PLL locked in
“zero-phase”.
Fig13. is the simplified schematic of
charge pump, it is a pair of well matched
current steering switches that control the
current mirrors that act as PLL’s charge
pump. This circuit is operated to
minimize the switching charge injection.
Fig. 1
Due to MCP output signal is
in extremely high speed, it is
almost impossible to use
probe to test and measure
real time signals with the
circuits. In this case, a
complete system simulation
is a key factor to build a
successful system. Fig. 4
shows the interface between
circuit simulation with real
time data.
Time Stretcher ASIC Design and Simulations
Digital Phase-Frequency Detector and Charge Pump Design
The Basic Structure of 2GHz PLLs
MCP Photo Detector Output Signal Characteristics
Fig. 3 shows the simulation result of
MCP output signal. The signal rising
time is in the order of 15-20ps, pulse
width is in the order of 40ps in
FWHM.
Time Stretcher ASIC Design and Simulations
Fig. 11:
We expect the PLL’s acquisition time
can be as long as few hundreds of
microseconds or milliseconds if we
run PLL in a bandwidth of 100KHz to
1MHz. In this case, it is impossible for
Spectre to handle so large transit
simulation time scale (miliseconds)
with so small resolution time step (few
ps).
Fig.15
Fig.16
Simulink can be used for the further
simulations to optimize the PLL loop
bandwidth and loop gains for
minimized the PLL jitter.
VCO Layout View.
Conclusion:
Inductors
(1) IBM 0.13um SiGe BiCMOS8HP has been evaluated, it is very user-friendly
design kit.
Varactors
Heterojunction
Transistors
Capacitors
(2) Circuit performance well met to our requirements based on our very low jitter
PLL designs.
(3) It is possible to generate an on-chip PLL to generate a stable, very low jitter
(<1ps) common “stop” clock.
(4) More challenging work need to be done on Time Stretcher chip design
including the ultra low timing jitter/walk discriminator and the dual-slop
ramping time stretching circuits etc.