F2 – Test Menu
Gem-5 User’s Manual 203
To facilitate the test, each Detector Board incorporates a test pulser with control logic
that is controlled by the test program. The amplitude of the test pulses is controlled
by a 12-bit DAC and can be set between 0 and 1 V for the Channel 1 test and
between 0 and 2.5 V for the Channel 2 test. The duty cycle of the test pulse is
approximately 75 µs ON and 75 µs OFF. During the test, analog pulses are sent from
the test pulser directly to preamplifier input, in bursts of 4,369 (1111 hex) pulses.
The counter registers are then read, errors are calculated, and the screen is updated.
This is repeated 3840 times until the final count of 16,777,215 (2
24
-1) is reached. At
each update, the current error is compared with the previous maximum error and the
maximum is updated when appropriate. The Error column displays the maximum
error for the duration of the test, expressed as a percentage of counts sent.
The detector #, counts read, and maximum error are shown for each detector. The
Error should be close to 0 if all circuits are operating properly. The total number of
pulses sent and the elapsed time for the current test are shown at the top of the screen.
Pulse Amplitude and Discriminator (LLD marked as Discriminator and ULD) levels
can be adjusted with the arrow keys (select item) and +/- keys (change value) or
directly by using Ctrl-E. These settings are applied to every detector for the
Amplifier Pulse Test but do not affect discriminator settings used in normal
operation. The discriminator levels used for normal operation are adjusted for each
individual detector on the F8 – Discriminator Plot screen (on page 78).
To prevent an inappropriately large error being reported if the counter wraps around
to zero (counts > 2
24
-1), the number of pulses sent is compared with the counts read
and with (counts read + 2
24
) at each update; only the smaller of these errors is used.
This test requires about 45 minutes completing the 16.7 million counts (150 µs
approximately per pulse plus screen updates) unless the test is terminated early.
The high voltage is automatically set to zero for this test. It may take up to 30
seconds for the voltage to ramp down. A message stating “Ramping HV to Zero” is
displayed and testing does not commence until the voltage has dropped below 50 V
for all detectors.
It should be noted that although the high voltage is switched off for this test, the
detector input is still connected to the preamplifier. Consequently, electronic noise
can result in a few additional counts. Lower discriminator settings increase the
amount of expected noise. Errors below 1% are acceptable.
Note that instead of trying to test them, detectors put in Conditional Service on the F4
– Faults screen (on page 223) show the words “Conditional Service” in place of the
data normally shown. In addition, these detectors are greyed out (also, missing
detectors show channel 1 as grey “n/a”).
In addition, Start Counter Test, Pulse Amplitude and Discriminator settings are
hidden if no detectors use them for a given counter.
Counters not available for a given board are show as “n/a”.
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