[Halld-cpp] CTOF TDC issue

[Beni Zihlmann]

Hi All,

I think I understand now the wired behavior of the CTOF TDC data. While the
time difference of the CTOF TDC data makes sense the meantime does not.
See attached my notes on CTOF data for reference.
It demonstrates that the time difference of the CTOF TDC data will lead
to a 20% narrower/better value for the time difference and a position 
resolution
of about 10cm.
However, the mean time is not giving he desired result. When plotting the
ADC time vs. the TDC time one sees a correlation but this correlation with
very broad +/- 10ns. Looking into the TDC configuration of the DAQ system
used to configure and run/readout the TDC modules I found that the 
configuration
parameters for the last TDC used for CTOF readout is missing in all but one
configuration file. And for the one that has it the lines were commented 
out.

My current conclusion is that the clock running on the last CAEN TDC in 
slot 9
is free running and not synced to the DAQ. This means that a 
"time-difference"
between two channels of this module will always give you a good result,
however, any comparison of any single time (or sum of times [like mean 
time])
with a time from an other source, like other TDCs (or ADC) will always 
fail and
only be good to within a clock cycle of the module which is around 43MHz
which is about 23ns.

The bottom line is: The mean time has to be calculated using the ADC 
times to
compare with tracking and determine a reasonable beta.

cheers,
Beni


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