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The SR400 Dual-Channel Gated Photon Counter offers a convenient,
integrated approach to photon counting that avoids the complexity
and expense of older counting systems. No longer is it necessary
to mix and match amplifiers, discriminators, gate generators and
counters. The SR400 combines all these modules into a single, integrated,
microprocessor controlled instrument. Complex measurement tasks
such as background subtraction, synchronous detection, source compensation
and pile-up correction can all be performed easily with the SR400.
Counters
The SR400 has two independent channels that can count at rates
up to 200 MHz. Different counting modes allow you to count for a
fixed amount of time, until a certain number of counts have been
received, or for a fixed number of triggers. Each counting channel
has its own gate generator providing counting gates as short as
5 ns or as long as 1 s. The gates can be set in a fixed position
relative to the trigger signal, or scanned to measure lifetimes
or recover time-varying waveforms.
The actual inputs to the counters can be viewed as NIM level pulses
from the discriminator outputs on the front panel. The discriminated
pulses are negative going from 0 to -0.7 V. The DISC outputs are
very useful when adjusting discriminator thresholds or gate timing.
Signal Inputs and Discriminators
Both analog signal inputs (A and B) are internally terminated into
50 Ω. The inputs accept signals between ±300 mV and
are protected to ±5 VDC. Each input is followed by a DC to
300 MHz amplifier, allowing detection of pulses as small as 10 mV.
If additional sensitivity is required, a remote preamplifier (like
the SR445A) can be used.
Discriminators are provided for each channel with a selectable
threshold from -300 mV to +300 mV in 0.2 mV steps. Pulse-pair resolution
is 5 ns, and pulses of either polarity may be detected. Each threshold
may be programmed to scan in either direction with selectable step
size. This can provide a pulse height analysis (PHA) output and
is useful for choosing photomultiplier tube bias or discriminator
levels.
Count Periods
The SR400 may be programmed to cycle from 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. Consecutive count periods
are separated by a "Dwell Time" which can be set from
2 ms to 60 s. During the Dwell Time, counting is disabled and data
may be transferred or external parameters may be changed. The dwell
output provides a TTL signal which is high during the dwell time.
This can be convenient for interfacing other instruments used in
the experiment.
Outputs
The front panel can display counts up to 109.
Results from both counters can be displayed individually or combined
as A-B or A+B. A front-panel D/A output provides an analog signal
proportional to A, B, A-B, or A+B, depending on the counting mode.
The scale may be logarithmic (1V/decade) or linear.
Computer Interfaces
Built-in RS-232 and GPIB interfaces provide a convenient means
of controlling the instrument and retrieving data. While the SR400
is scanning, each of the count values for the A and B counters are
stored in a 2000 point internal buffer. This buffer can be transferred
on a point-by-point basis, or dumped all at once through either
interface.
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