Specifications

Instrument Overview
13
is START, the next EXTERNAL START begins
the first COUNT PERIOD of the next scan.
SCANNING
In all scan modes, a number of parameters may be
scanned. These parameters are the three
discriminator thresholds (Pulse Height Analysis),
the two gate delays (Boxcar mode), and the two
D/A output ports (X-axis of recorder, scope, or
analog control of other apparatus).
After each COUNT PERIOD, each scanning
parameter is adjusted by one step. All changes are
made during the DWELL time so that all values
are stable during the COUNT PERIODS. The scan
limits are determined by the start position, the step
size, and the number of PERIODS in a scan. The
range of a scan is N PERIODS times the step size.
When the counters are reset, all scanned
parameters return to their start positions.
GATING
The gate generators may be disabled by setting the
gate modes to CW. In this mode, the gates are
continuously open and counting is enabled solely
by the COUNT PERIOD. This is appropriate for
CW sources or long counting intervals. If the
signal is pulsed or so weak that synchronous (lock-
in) measurement is required, then the gate
generators are required. The gates prevent pulses
that occur outside the time interval of interest from
being counted and improve signal to noise ratio.
When the gates are scanned, the time behavior of
the signal may be recovered.
The gates are triggered by the TRIGGER input.
The TRIGGER threshold is adjustable from -2V to
+2V and slope may be either RISE or FALL. The
discriminated trigger pulse may be viewed through
the T DISC output when the input to counter T is
set to TRIG. This can be helpful when setting the
trigger threshold.
The minimum insertion delay from trigger to gate
is 25 ns. The additional delay may be programmed
from 0 ns to 999.2 ms with a resolution of 1 part in
1000 or 1 ns, whichever is greater. The gate width
is adjustable from 5 ns to 999.2 ms with the same
resolution. The two gates are output as NIM pulses
from the GATE outputs. The gates are open when
the outputs are low. The gate outputs have a
timing accuracy with respect to the discriminator
outputs of 2 ns. This allows accurate timing of fast
gates relative to the signal.
The two gates have independent modes, delays,
and widths. This allows for a variety of
measurements. For example:
1) "Boxcar" mode. The gates are used to enable
the counters only when the signal is present. If the
gates are scanned, then the time profile of the
signal is recovered. Background subtraction can be
included by using one gate for the signal and an
equal width gate on the background, either before
or after the signal. The signal gate may be scanned
while the background gate held fixed. Gated
source compensation is also possible since
counters A and B may count different sources at
the same time or the same source at different
times. In the compensation mode, the COUNT
PERIOD is determined by the gated output of the
B discriminator and the COUNT MODE is A FOR
B PRESET.
2) "Lock-in" or "Chopped" mode. In this case, a
cw signal is chopped on and off. This can be
accomplished with a light chopper such as the
SR540. The light chopper reference output is used
to trigger the gates. The A GATE is positioned
during the "open" cycle and counts signal plus
noise. The B GATE is positioned during the
"closed" cycle and only counts the noise. The
counter outputs are subtracted to give only the
signal. The widths of both gates should be equal
and less than half of the chopper period. This
prevents frequency jitter in the chopper from
affecting the overlap of the gates with the open
and close cycles of the chopper.
When counter T is preset and its input is set to
TRIG, then each COUNT PERIOD consists of a
constant number of gates. N+I triggers are needed
for N gates per preiod. This is the usual mode of
gated operation. However, a number of other
modes are available. For example, if the input is
set to 10 MHz, each COUNT PERIOD consists of
the number of gates which occur during the
COUNT PERIOD time interval. If the input is a
signal input, then counting is both gated and