- To link together the "times" measured with the Neutron Detector Electronics (relative to the L1A) and those measured with the BigBite electronics (relative to the BBtrig), it helps to understand the way the two trigger systems worked. In all cases, the relative time difference between the two reference times (common signals) are recorded by the Lecroy 1875A TDC corresponding to the ~~red:D.ctimeL1A[[0]~~ variable in the TTree.
+ To link together the "times" measured with the Neutron Detector Electronics (relative to the L1A) and those measured with the BigBite electronics (relative to the BBtrig), it helps to understand the way the two trigger systems worked. A diagram of the coincidence-trigger layout can be found in ((GenDocTrigger)). In all cases, the relative time difference between the two reference times (common signals) are recorded by the Lecroy 1875A TDC corresponding to the ~~red:D.ctimeL1A[[0]~~ variable in the TTree.
::-+0.05*D.ctimeL1A[[0] (ns) = L1A - BBtrig+-::
Likewise, to get a BB time relative to the ND trigger to compare against some ND variable, just subtract -+0.05*D.ctimeL1A[[0]+-.
- ===The features of D.ctimeL1A[[0]===
The ((NeutronDetector)) generated triggers using an energy sum over multiple modules; this trigger is designated as __T1__. The BigBite spectrometer generated its own trigger signal, called __T2__, from a threshold placed upon the energy deposited in its calorimeter. The T1 and T2 signals were then passed to a coincidence unit, such that a logical __AND__ of the signals formed the coincidence trigger __T3__. The leading edge of T3 was determined by when the overlap of T1 and T2 occurred. The trigger supervisor would then form the L1A with a time relative to the accepted trigger. The ASCII figure below tries to demonstrate this for a normal "good" T3 event.
+ -=The features of D.ctimeL1A[[0]=-
The ((NeutronDetector)) generated triggers using an energy sum over multiple modules; this trigger is designated as __T1__. The BigBite spectrometer generated its own trigger signal, called __T2__, from a threshold placed upon the energy deposited in its calorimeter. The T1 and T2 signals were then passed to a coincidence unit (see ((GenDocTrigger))), such that a logical __AND__ of the signals formed the coincidence trigger __T3__. The leading edge of T3 was determined by when the overlap of T1 and T2 occurred. The trigger supervisor would then form the L1A with a time relative to the accepted trigger. The ASCII figure below tries to demonstrate this for a normal "good" T3 event.
::__A. Normal "good" event where T3 determined by T1 time, and L1A by T3__::~pp~
|
|__________________________
+
~/pp~
where T3 and the L1A are timed exactly relative to T2.
- To generate the L1A's timing, a copy of the T2 and the T3 signal were sent to a retiming module. If the T3 came "too late" after the T2, the L1A would come at T2 + fixed_offset. So in the extreme case, if the T3 signal was missing or was very late, the diagram could look like this:
+ To generate BBtrig (noted in the trigger diagrams of ((GenDocTrigger)) as "L1A(retimed)"), the reference signal for the BigBite arm, a copy of T2 and the L1A are passed into a retiming module. The retiming module is supposed to generate a signal with a fixed-delay to T2, or if T2 is missing (normally the case for a T1 event) generate a signal with a fixed-delay with respect to the L1A. If the T2 came very early, it is possible for its leading edge to become lost such that BBtrig is aligned with the L1A; this can be checked by looking at the time distributions for detectors in BB (eg: -+B.s.time[[B.s.trpad[[0]+-) for this class of events. In this case, the trigger timing diagram could look like this:
- ::__C. Forced retiming case: timing of T3 determined by T1 BUT L1A is a fixed delay after T2__::~pp~
+ ::__C. Forced retiming case: timing of BBtrig determined by L1A, NOT by T2__::~pp~
T1 ______________________ _____
| |
|___________|
- L1A __________________
|
|__________________
+ L1A ______________________
|
|______________
T2d ___________ _
(delayed) | |
|________________|
+ BBtrig ____
+ |
+ |______________
- |<- ->|
- fixed delay
~/pp~
- This results in D.ctimeL1A and D.bit3 distributions that look like in the following plot.
+ All these effects together result in D.ctimeL1A and D.bit3 distributions that are shown in the following plot of D.ctimeL1A (y-axis) vs. D.bit3 (x-axis). Note that to get the correct "first" recorded T3 time from the 1877 TDC, you have to get the last readout value from the TDC so -+D.bit3[[Ndata.D.bit3-1]+- must be used.
+ {img src=show_image.php?name=L1A_to_T3_to_BBtrig.gif }
| ![[hide]](javascript:icntoggle('mod-mnu_application_menu','mo.png');)
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