Staying Grounded

Still having trouble with the 2nd input-capture datapath (channel #1) today.  I'm routing the comparator outputs to the B2 bus on the scope, so we can see if they're working right, at least.

In our full data run from last week, we got 431 coincidences within 50 ns, and only 12 of them were identifiable "false coincidences" of 30 ns time difference or larger.  The distribution of time differences among the false coincidences was:

30 ns:  ####
35 ns:  #####
40 ns:  ##
45 ns:
50 ns:  #

Most of them were clustered at the low end of the scale, so these are unlikely to be purely accidental coincidences.  Some theories:

1. They represent individual particle pairs in occasional thicker-than-normal EAS pancakes, at least 10-20 ns thick, or perhaps outliers from a normal-thickness pancake.
   
2. Maybe there are multiple showers occurring close together in time?
   
3. There is some unexpected occasional source of up to +/- 20 ns inaccuracy in our time measurements.

By the way, here is the graph of the average pulse rates in that data set (extended a few hours from last time):

Aha, looking at the comparator outputs showed me the problem:  I never properly grounded the AC input node on PMT2; it was just floating.  It was supposed to be tied to the +2.5V supply so that the negative excursions of the pulses would take it past the DAC levels.  We were just lucky that it worked at all before!  I placed a jumper bridge/sheath across J69, and this fixes the problem.

Anyway, let's look at the coincidences with anomolously large time differences in a little more detail.  Here's the first one:

Coincidence #9 with delta = 30 ns:

    3.596D+11    3545.    2.    1.    0.    6.    0.    0.    0.    0.    0.    0.    0.    0.    0.    0.    0. 
    3.596D+11    26549.    1.    1.    0.    1.    0.    0.    0.    0.    0.    0.    0.    0.    0.    0.    0. 

Here are the raw data lines:

Tue Sep 06 17:34:01 2011 + 528 ms: < PULSE,2,3545,359642909738,1,(0,6)
Tue Sep 06 17:34:01 2011 + 527 ms: < PULSE,1,26549,359642909744,1,(0,1)

Not too noteworthy.

Coincidence #10 with delta = 30 ns:

    3.840D+11    3784.    2.    2.    0.    1.    2.    4.    0.    0.    0.    0.    0.    0.    0.    0.    0. 
    3.840D+11    28326.    1.    2.    0.    3.    1.    6.    0.    0.    0.    0.    0.    0.    0.    0.    0. 

These at least are bigger (2-threshold) pulses.