Specifications

Instrument Overview
12
may be programmed to scan in either direction
with selectable step size.
COUNTING
Each counter's input may be selected from a
number of sources. All multiplexing of counter
inputs is done internally to provide accurate timing
and ease of operation. Input cables should rarely
need to be swapped or disconnected to change
measurement modes.
The actual inputs to the counters can be viewed as
NIM level pulses from the DISC outputs on the
front panel. The DISC pulses are negative going
from 0 to -0.7 V. The falling edge is the active or
counted edge regardless of the discriminator slope
setting. The DISC outputs are very useful when
adjusting discriminator thresholds or gate timing.
When the START key is pressed, a START
command is received from the computer interface,
or the EXTERNAL START input is pulsed,
counter T is enabled to count. The COUNT
PERIOD begins with the first pulse from T
counter's input after START. The COUNT
PERIOD enables counters A and B to count their
respective inputs. If the A GATE and B GATE are
CW or continuous, all pulses that occur during the
COUNT PERIOD are counted. If the gates are
enabled, only the pulses that occur during those
gates which fall within the COUNT PERIOD are
counted.
Counter T is presettable from 1 to 9E11. When
counter T reaches the preset count, the COUNT
PERIOD is terminated and counting is halted on
all counters. When the input to counter T is the 10
MHz crystal timebase, the COUNT PERIOD is a
fixed time interval from 100 ns to 25 hours. When
the input is INPUT 2, the COUNT PERIOD is
externally determined and can be as short as 5 ns
and indefinitely long. This mode is used for source
compensation or reciprocal measurements. If the
input is TRIGGER, the COUNT PERIOD is a
fixed number of gates (each trigger provides one
gate). This results in a constant gate aperture time
during each COUNT PERIOD. N+1 Triggers are
required for N gates per period. If the input is B
PRESET, then counter T counts the B DISC
output gated by B GATE. This is for gated ratio or
reciprocal counting.
The SR400 may be programmed to cycle through
1 to 2000 COUNT PERIODS in a single scan. At
the end of the programmed scan, the counters may
be stopped or the scan may be restarted. If the end
of scan mode is STOP, then after the last COUNT
PERIOD in the scan, the counters are halted
indefinitely. A counter RESET is required to
rearm the counters and a START command starts
another scan. If the Number of PERIODS in the
scan is 1, then each START command results in
one COUNT PERIOD. If N PERIODS is greater
than 1, then consecutive COUNT PERIODS are
separated by the DWELL time. The DWELL time
may be set from 2 ms to 60 s. During the DWELL
time, counting is disabled and data may be
transferred or external parameters scanned. The
DWELL OUTPUT is a TTL output which is high
during the DWELL time. At the end of the
DWELL time, the next COUNT PERIOD begins
and the scan continues until N PERIODS have
been measured. If the end of scan mode is
START, one DWELL time separates the last
COUNT PERIOD of the completed scan and the
first COUNT PERIOD of the next scan.
Another DWELL mode is EXTERNAL. In this
mode, the EXTERNAL START input enables
counter T. The COUNT PERIOD begins with the
first pulse to counter T after START. Counting
then proceeds normally until the preset is reached.
At the end of the preset COUNT PERIOD, the
DWELL time is indefinite and the next COUNT
PERIOD starts with the next EXTERNAL START
pulse. In this way, preset COUNT PERIODS may
be synchronized to external events. If externally
determined COUNT PERIODS are desired, then
the preset COUNT PERIOD can be set to a value
much larger than actually required and the
EXTERNAL STOP input used to terminate the
COUNT PERIOD. The COUNT PERIOD then
starts with the first pulse to counter T after
EXTERNAL START and ends with EXTERNAL
STOP. The next EXTERNAL START begins the
next COUNT PERIOD in the scan. When the scan
is finished and the scan end mode is STOP, a
RESET from the front panel or computer interface
is required to reset the scan. If the scan end mode