DISCRLMINATOR SETTINGS

R. D. Cohen and E. A. Beaver
University of California,- San Diego
Instrument Science Report CAL/FOS-050

May 25, 1990

1. [ntroduction

During the period 23-26 February 1988, tests to determine the settings
for the discriminator digital to analog converters (disc DACs) of the
Red (F12) and Blue (F7) detectors were performed at LMSC. The function
of the discrimina.tors is to set a minimum size for an output pulse to
be counted as a detected photon. As most pulses caused by fluctuations
in the preamps (potentially the dominent source of noise) are small;
with a. gaussian distribution, the discriminators can be set to pass
virtua.lly all real counts, while blocking virtually all pulses due to
electron rioise in the prea.mps. By illuminating the detectors and
varying the discriminator settings we ca.n determine an integrated
pulse heigbt distribution, the distribution of all pulses greater than
a given threshold value, set by the discriminator.

The discrimina.tor setting for a given cha.nnel is the product of the
discrirninator reference DAC (REFDAC), which is common to all channels
and the individual disc DAC setting. These individual disc DAC
settings have been prograxnmed into a PROM. The PROM setting cannot be
changed, but can be overridden by command. Changing a disc DAG setting
during actual FOS spaceflight operations requires three FOS serial
magnitude commands corresponding to 9 NSSC1 words. These commands arc
required whenever an FOS detector is turned on from hold mode or
reset.

This report details the status of the discriminator settings and noisy
a.nd dea.d channels during calibration and alignment and documents the
process whereby various changes were made in discriminator
settings. Other reports update this, based on the results of later
tests (CAL/FOS-51 and CAL/FOS-64). The data in this report come from
tapes labeled (at UCSD) 1075 and 1077.