Recording Event Times

Now consider the sequence that occurs as an event is captured.

1. Event occurs.

2. Light propagates to the PMTs.

3. A firing signal from PMT propagates to the Front End Card (FEC). Typically this takes dT $\approx$ 130nsec, of which $\approx$ 30nsec is the PMT transit time and the rest is the cable delay ^A.2

4. The signal travels from the FEC to trigger board. This is currently estimated at 70nsec but may change.

5. Next 50MHz tick occurs and, assuming the trigger is satisfied, the signal latches. The latching tick is the global trigger time.

6. A signal is sent back from the trigger card to the FECs to freeze the PMT voltage ramps.

The last point introduces a subtlety. As explained above, the sum of GTR + t is constant, for as the signal propagates from the FEC to the trigger card and waits to latch, t is dropping at the same rate as GTR is rising. However, once the trigger latches the GTR time is defined but t continues to fall until stopped by the returning signal from the trigger card to the FEC. So t is too small by 70nsec. While this does not matter when comparing events, as the offset is constant, it does make a difference to the absolute time of the event, so the equation of PMT hit in 50MHz clock time is:-

GTR + t - 250 + 70 = GTR + t - 180

Ignoring PMT cable delays which will be corrected for. Remember that both the 250 and 70 are interim figures. Also the 70 will depend slightly on the particular FEC. Also note that t starts at 250 but has to drop a minimum of 140 (round trip from FEC to trigger and back) even if the trigger latches instantly so the triggering PMT t will be 110nsec or less. Hits occurring earlier will have a smaller t, and later hits a larger one. A hit that occurs just as the trigger signal returns to the FEC will have t = 250.

The effect of the trigger latching can easily be seen with SNOMAN. Generate events at a fixed time and look at the fitted time. You should see a set of peaks separated by 20nsecs. Each peak represents a different 50MHz clock tick relative to the generated time.