Transcript alicetime

Timing in the ALICE trigger system
R. Lietava
The University of Birmingham
for the ALICE collaboration
Content
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Introduction
Synchronisation
Alignment
Summary
Timing in the ALICE trigger system
09/2006
Synchronisation and alignment
The synchronisation: adjusts the phase of the input
signals with respect to the local bunch crossing
clock. Delays are within one clock period: (0-25ns)
The alignment: assures that the trigger signals
originating from the same bunch crosing reach
processor logic in the same clock cycle. Delays
are numbers of full clock periods.
Timing in the ALICE trigger system
09/2006
ALICE Trigger System
Timing in the ALICE trigger system
09/2006
Trigger input phase measurement
ADC input
Trigger input
D
Q
BC clock
25 ns
BC clock
Trigger input
∆T
Lx
Lx
• the detectors are
required to generate
the pattern of alternating
zeros and ones,
• ΔT and VADCis
proportional
to the phase
between the trigger
input and BC clock.
ADC input
Vadc
Timing in the ALICE trigger system
09/2006
Delay scan
Synchronisation: set the sampling of the input signal
with the rising or falling edge of the BC clock
to guarantee the safe margin.
The abrupt changes: the transition of the signal concides
with the rising edge of the BC clock.
Timing in the ALICE trigger system
09/2006
Trigger input alignment
Det A trigger signal
Det B trigger signal
- signal present in BC
- signal not present in BC
Det A trigger signal
Det B trigger signal
Timing in the ALICE trigger system
09/2006
Correlation analysis
• ai ,bi – trigger inputs generated by detectors A,B at
time i; Correlation function:
1
C N ( ) 
N
N 
a
i 0
b
i  i
• CN(δ) depends on rate,detector efficiency and noise
• CN(δ) distributed normally (Binomial from small N),
 2  CN / N
Timing in the ALICE trigger system
09/2006
Correlation function example
Det B trigger signal
C48(0)=0
C (  d )  r 2
Det A trigger signal
C (  d )  r
- signal present in BC
- signal not present in BC
Det B trigger signal
C48(4)=4/48
Det A trigger signal
Timing in the ALICE trigger system
09/2006
Separation function
C (  d )  C (  d )
S N
C (  d )  C (  d )
- C(δ=d) is the correlation function for aligned signals
- C(δ≠d) is the correlation function for non-aligned signals
- in the absence of noise:
C (  d )  r a b
C (  d )  r 2 a b
where εa (εb) is detector efficiency for detector A(B).
Timing in the ALICE trigger system
09/2006
Number of BC to collect
va   b
 a   b  0.1
Number of bunch
crossings
to collect in order to
get a given separation
S 1
N  S 2 /[ r (1  r ) 2 (1  a )(1  b ) a b ]
Timing in the ALICE trigger system
09/2006
Beam conditions
Three situation during ALICE commissioning and
data taking:
• No beam: cosmic trigger
• With one beam: beam-gas interactions
• With both beams
Timing in the ALICE trigger system
09/2006
Beam gas interactions
• Probability of interaction in a BC: r=0.001-0.002,
r=nσLNp ,
σ=70 mbarn,
L=20 m (straight LHC section)
n=1014 m-3 (gas concentration at LHC start up),
Np =1011 protons per bunch
• Low detector efficiencies ε ≈ 0.1
• A large noise ν ≈ r
• kb =43 number of filled bunches
Timing in the ALICE trigger system
09/2006
Beam Gas: statistics
108 events necessary for 1 σ separation
Timing in the ALICE trigger system
09/2006
Two beams
• Probability of the interaction in BC: r=0.15 (at
start up)
r=σL/fLHC/kb
L=1030 cm2s-1 (luminosity)
kb=43 (number of filled bunches)
fLHC =11245 s-1
• Detector efficiencies ε ≈ 0.9
• Noise ν ≈ r/10
Timing in the ALICE trigger system
09/2006
Two beams: statistics
104 events necessary for a 1 σ separation:
106*0.1*0.1 for detector efficiencies
Timing in the ALICE trigger system
09/2006
Implementation in ALICE
• Trigger input recording in the CTP (snapshot):
unbiased recording available on L0,L1 and L2 levels
• Trigger class counting: unbiased counting on L0,
biased on L1 and L2 levels since any L0 trigger introduces
deadtime
• Data recording with DAQ: biased on each level,
since L0 trigger introduces deadtime, analysis complicated
Timing in the ALICE trigger system
09/2006
Trigger input recording on CTP
• Each board contains a memory with 1M words
(SnapShot Memory -SSM)
• Time to measure and to read SSM is about 1.25 sec
• Scan done offline
• The time necessary to collect N BC:
T  1.25  N / 2 20 sec
N=109  T=1190 sec
Timing in the ALICE trigger system
09/2006
Trigger class counting
• Trigger Class = Input A * Input B
(e.g. for 4 inputs A,B,C,D coincidences AB,AC and AD are
sufficient, A is the first arriving input)
• Scan done by programmable delay (0-15) available
for every trigger input
• The time necessary to collect N BC:
T  16  N  2.5 10 8 sec
N=109  T = 400 sec
Timing in the ALICE trigger system
09/2006
Summary
• Synchronisation in the ALICE trigger system has
been described
• Monitoring of synchronisation outside physics run
is available
• Alignment method with acceptable timescale
(hours in the worst case) has been proposed
• Continuous monitoring of trigger input alignment
is available
Timing in the ALICE trigger system
09/2006
Back up slides
Timing in the ALICE trigger system
09/2006
Setting procedure for synchronisation
and alignment
1. Synchronise trigger inputs with arbitrary setting
of BC delay (e.g. 0)
2. Align trigger inputs
3. Find the latest arriving trigger input and set BC
delay to sample it as early as possible
4. Using measurement taken at 1. recalculate the
synchronisation parameters
Timing in the ALICE trigger system
09/2006
Synchronisation setting
25 ns
BC clock
Trigger input
synchronise
with rising edge
of BC clock
∆T
Lx
ADC input
Vadc
25 ns
BC clock
synchronise
with falling edge
of BC clock
Trigger input
Lx
ADC input
Vadc
Timing in the ALICE trigger system
09/2006
ALICE trigger system
L0
Busy
4 TTC
TTCmi
partitions
partitions
BC
L0
Busy
L0
Busy
4 TTC
partitions
4 TTC
partitions
Timing in the ALICE trigger system
4 TTC
L0
Busy
Orbit
CTP
RoI
4 TTC
partitions
4 TTC
partitions
L0
Busy
L0
Busy
09/2006
Sub-detector TTC partition
Control Processor (VME)
ALICE Trigger System
consists of tree subsystems:
- Control
- Monitoring
CTP
to FIFO
VME slave
VME master
VMEbus
FAN-OUT
board
LTU
TTCvi
Orbit
Orbit
Pre-pulse
Pre-pulse
- CTP
- L1 Message
- L2r Word
- L2a Message
TTCmi
TTCex
TTCcf
- LTU
L0
L1
L1
L1 Data
- TTC
A
B
L2 Strobe
BUSY
board
L2 Data
BC
BUSY
BC
BC
BC
BC
Orbit
TTCit
Local pulser
1
320
BUSY
L0
TTCrx
L0
Sub-detector readout electronics
Figure 3.3 Context diagram of the LTU board
Timing in the ALICE trigger system
09/2006