Two issues as seen with FADC scalers
#1. Disappearing scalers
Occasionally, a bunch of TOF FADC scalers would just stop counting. It always happens in groups of 16 corresponding
to FADC boundaries. In the example above, the first 7 FADC modules are not counting while the last 4 modules are
counting normally. I have seen other combinations of dead/alive modules as well. After some (random?) time, all
of them will start counting again. If this is a bug in the scaler readout program then we can live with it. However,
this may be caused, for example, by some temporary missing control signals to the boards. In such case, these modules
will be empty for random periods of time in the data stream as well.
#2.Two channels with abnormal counts
While all discriminator scalers and 174 out 176 fadc scalers have reasonable rates with beam off, 2 channels
are way out of line. Channel N17 (ch.17 in 1-176 range) FADC scaler rate is ~600kHz, while channel B38
(ch.170 in 1-176 range) rate is ~0.05kHz. First thing to note is that such abnormal rates do not affect the
FADC occupancy for these channels in the data as seen from the plot below (run #51042):
Channel B38
The reason for low FADC scaler rate for B38 is its high baseline at 130 ADC counts (all other channels
have baseline near 100 counts). While not critical, recalibrating the baseline may be desirable.
Channel N17
About half of the time, the shape of North-17 FADC signal appears to be normal as can be seen below:
However, there is a clear sine wave in the other half of N17 signals. It appears to have
a very stable period of about 140ns (about 7.1MHz). I can think of 2 possible causes:
- PMT ringing (why does the discriminator scaler count normally then?);
- Bad cable ground or bad connection somewhere between a splitter and ADC allowing
7MHz noise to be picked up from somewhere.
