Transcript 1-shot

Background: Use of TKR Trigger One-Shots
•
individual TKR strip channels “true” when analog shaped pulse is above threshold.
hit single strip analog pulse and
threshold level
•
strips in a plane are OR’d together.
binary strip signal (also resulting layer
OR if this is the only strip in the layer
above threshold)
– The resulting layer OR’d signal is the subject of discussion today. A one-shot with
programmable width can be turned on in the circuit. Currently in our testing it is off.
– This signal is used in forming the “3-in-a-row”: when these signals from spatially
consecutive xy*yx*xy are “true”, a TKR trigger request is sent to the global trigger.
•
the global trigger process is started by transition edges, not levels (this is good!).
Consider a situation with a long-duration pulse (e.g., a heavy ion in the TKR):
–
–
–
•
a trigger request level that persists longer than the single-event deadtime will NOT retrigger the
instrument.
the channels involved in a long-time level are effectively dead in the trigger when subsequent
particles pass through them; that situation persists until the level goes away. (see next slides)
in the readout, a ghost of the track persists in any subsequent triggers until the levels go below
threshold.
once a global trigger decision process (“window”) has started, the logic is levelsensitive, not edge-sensitive. (again, this is good)
• Purpose of today’s discussion: what is the most likely flight
configuration for the one-shots: on or off; and what is the stretch
duration? We want to test in the flight configuration, so a timely
decision is necessary.
•
[Many details glossed over here]
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Why does this matter?
• Because the opening of the trigger window is edge-sensitive,
this issue is not very critical (LAT won’t continuously
retrigger). However, we still must make a choice.
• The first, best-guess length of time for the one-shot stretch is
~1 microsecond (see later slide).
– no identified benefits, and some possible complications,
making it longer
• We have found three cases in which turning on/off the oneshots matters:
– impact on handling of heavy ions
– impact on noise triggers
– possible impact on mip trigger efficiency due to jitter (see
talk by Su Dong)
• In all three cases, the impacts are fairly small but potentially
important.
– “choose your poison” – J. Russell
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Heavy Ions
•
•
•
TKR signal from passage of a heavy ion will often persist (pileup) into subsequent
events (~150 microsecond time over threshold, with 4-10 kHz of charged particles
passing through the TKR that could make triggers).
The heavy ion rate is ~50 Hz. The layers exposed to the heavy ion are dead for
subsequent triggering until the layer-OR turns off, and the WHOLE TOWER is dead
for subsequent retriggering until its trigger request turns off (no more 3-in-a-row
condition). Thus, with the one-shots OFF, for ~6 ms every second (0.6% of the
time) a fraction of the instrument (~20%) is not retriggerable. [The impact on the
dead area in the readout is effectively nil due to the huge number of strips.]
If the one-shot is OFF, the layer-OR signal will persist for future triggers.
– this is potentially GOOD, because the trigger readout info in affected events
[only those WITHOUT TKR TRIGGERS] will tell us this happened and we can
then monitor the rate of event pileup. Also, we can tell in those events if at
least some hits in a layer are ghosts from pileup. Also, the readout and trigger
information will disagree less than if the one-shot is ON.
• only useful for statistical monitoring at best
– this is NOT GOOD, because use of the TKR information in the trigger logic in
some cases will be incorrect. In particular, the geographic VETO (one of our
two possible veto’s), when engaged, looks for TKR triggers coincident with
lists of ACD tiles. If a subsequent event happens to have a hit ACD tile
associated with the persisting TKR level from a previous heavy ion event, the
subsequent event will be vetoed. The error rate will be small (VETO is only
used for small CAL deposits so backsplash rate is low), and we would be able
to monitor the rate of those incorrect vetoes using the pass-throughs.
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Noise Triggers
• Two categories:
– without the one-shots, after a heavy ion, and AFTER the 3in-a-row condition is no longer true, some of the layers will
still be in saturation for a brief period. During that window,
noise hits in some layers will again make a 3-in-a-row,
resulting in a trigger.
– A noise pulse in the TKR has a very short time over
threshold, typically, so the plane OR is nominally also
short. However, turning on the one-shot will increase the
chance coincidence of noise pulses.
• Noise considerations: best choice is to turn ON the one-shots
but to keep the pulse width short (~1 microsecond).
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Summary
Issue
One-Shot ON
NOISE TRIGGERS
 [clear winner if stretch
One-Shot OFF
short]
 [incremental
TRIGGER/READOUT
TKR INFO MISMATCH
disagreement O(0.1)%]
WRONG VETO

DEAD TRIGGER
AREA
 [<1% effect: layers
INDIRECT PILEUP
MONITORING
[may also have TOT
info?]
TKR TRIGGER
EFFICIENCY
 [w/stretch]
dead instead of whole
towers]
 [small]
 = better choice
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Stretch Value
• need slide from Su Dong
• answer is 14 ticks?
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Conclusion
• Recommendation: We recommend turning ON the one-shots for
nominal operations, with stretch of 14 ticks (0.7 microseconds).
– still important to test the full LAT with the one-shots turned off
to make sure it works as expected. Surprises on orbit may lead
to the desire to turn off the one-shots, at least for systematic
studies.
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