See also elog entries 702, 2507, 2709.
I've noticed that the flasher rate (the 8th trigger bit) varies some, due
to triggering on noise. One day the rate was 30Hz with the laser
off! The problem appeared after I amplified the photodiode signal before
discrimination (7/31/03, around run 1810) -- the amplification adds a
negative DC offset which is close to the discriminator threshold. This thr
must be low (25mV) because the Cerenkovs are also in this discriminator.
I realized that the amplified photodiode signal is large enough to use a
LeCroy octal discriminator in a NIMbin. So the amplified signal now goes
through an octal discriminator; the logic result goes to the programmable
discriminator as before. To maintain the timing (time diff between the raw
photodiode and the signal to the XMLU), I had to add 16ns of delay (in the
Philips switch delay in the NIMbin that we already use).
Now the flasher events should all be due to the flasher firing, rather
than noise.
Karen
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