Specifications

Instrument Setups
35
background during the chopper open state and the
other counts the background during the chopper
closed state. The difference between the two
counters is the signal. If A counter is the signal
counter and B counter the background counter, the
two gates should be timed as shown below.
A GATE
B GATE
Noise
Only
Signal +
Noise
Chopper
State
Open
Close
Open
Close
The two gate widths need to be the same width.
The gates are triggered by the chopper reference
signal. The gate delays are used to position the
gates correctly. Since the gate delay can be as
large as the trigger period (minus 1 µs), the phase
of the gates relative to the trigger can be adjusted
to cover the open and closed cycles of the chopper.
The gate widths should be adjusted to be less than
the open and closed periods of the chopper. This
avoids chopper frequency drift or jitter from
disturbing the gate/chopper overlap. Also, if the
chopper aperture does not result in a sharp turn on
or turn off of the signal, the transition region
between open and closed can be avoided.
The COUNT mode A-B FOR T PRESET is used
with T input = TRIG. This displays the difference
between the two channels and ensures that equal
numbers of open and closed gates are counted
during each count period.