SRS Labs SR430, Manual

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12
Instrument Overview
TRIGGER and SYNC/BUSY
Data acquisition is started by a trigger. The TRIG-
GER input is on the front panel. The trigger thresh-
old is adjustable from -2V to +2V and the slope
may be either RISE or FALL.
The SYNC/BUSY output is a TTL level signal
which serves two purposes. First, it can be used to
synchronize the experiment timing with the actual
time bins of each record. Second, it indicates
when the unit is busy acquiring data and cannot
be triggered.
After a trigger is recognized, the SYNC/BUSY out-
put will go high indicating the start of the first time
bin. The insertion delay from TRIGGER to SYNC/
BUSY is 45 ns. SYNC/BUSY will have 2.5 ns of in-
determinacy with respect to the trigger. As a re-
sult, the entire record starts with 2.5 ns of uncer-
tainty with respect to the trigger. This is not a
problem if the bin width is much longer than 2.5 ns
but may be a problem for shorter bin widths. In
these cases, the SYNC/BUSY output should be
used to trigger the experiment. Since the record is
synchronized to SYNC/BUSY, there will be no in-
determinacy of the time bins with respect to the
experiment. One way to accomplish this is to trig-
ger the SR430 with the experiment's master rate
generator and use the SYNC/BUSY (which is de-
layed from trigger by only 45 ns) to trigger the ac-
tual experiment.
After a trigger, the unit is busy acquiring and accu-
mulating data. During this busy time, no new trig-
gers will be recognized. After SYNC/BUSY returns
low, a new trigger may occur. The busy time is
T
busy
= (N*T
bin
) + (N*250 ns) + 150 µs
where T
bin
is the bin width (5 ns to 10.5 ms) and
N is the number of bins per record (1024 to
32704). The first term is the actual record length
and equals the real time duration of the data ac-
quisition. Counts received during the time N¥T
bin
after the trigger are captured. The second term is
the time needed to accumulate the record, i.e. add
the data to the sum of all previous data. Accumula-
tion takes 250 ns per bin. The last term is a fixed
overhead for processing. For example, if N=1k
bins and T
bin
= 5 ns, then T
busy
= 411 µs. The
maximum trigger rate would then be 2400 Hz.
When the bin width is programmed to be much
larger than 250 ns, then the busy time is almost to-
tally due to the real time data acquisition and virtu-
ally no time is lost processing or accumulating.
SYNC/BUSY is only output during actual data ac-
quisition. Triggers that arrive when the unit is not
acquiring data will not cause a SYNC/BUSY out-
put.
BIN CLOCK OUTPUT
The front panel BIN CLK output is a 50 ½, NIM lev-
el signal showing the bin boundaries. The first
bin,T
1
, is the time period when the BIN CLK is
first low following a trigger. Each rising and falling
transition of the BIN CLK marks the boundary of a
bin. Thus, the second bin, T
2
, is when BIN CLK is
next high, T
3
is when BIN CLK is low again, etc.
The first bin starts synchronously with the SYNC/
BUSY output. BIN CLK may be used with the
DISC OUT signal to determine the exact timing of
the counts with respect to the time bins.
BIN CLK is only output during actual data acquisi-
tion. Triggers that arrive when the unit is not ac-
quiring data will not cause a BIN CLK output.
EXT BIN CLK INPUT
The front panel EXT BIN CLK input is a TTL input
which allows an external source to determine the
bin boundaries. When the bin width mode is pro-
grammed to EXTERNAL, the EXT BIN CLK will
clock the bins. After a trigger, the first rising edge
of the EXT BIN CLK will start the first bin. SYNC/
BUSY goes high at the same time. Each new ris-
ing edge of the EXT BIN CLK will clock the start of
a new bin. The maximum external clock rate is 4
MHz or a 250 ns minimum external bin width. The
EXT BIN CLK is useful if bins longer than 10.5 ms
are required, or when the bins must be synchron-
ized to external events.
SIGNAL INPUT AND DISCRIMINATOR
The analog signal input is labelled SIGNAL INPUT
on the front panel. This input is internally terminat-
ed into 50 ½. The input can accept signals of ei-
ther polarity up to ±300 mV and is protected to
±5V dc. The input is followed by a dc to 250 MHz
amplifier. This allows detection of pulses as small
as 10 mV. If greater sensitivity is required, the
SR440 remote pre-amplifier can be used. The
SR440 can provide gains from 5 to 125 at band-
widths exceeding 200 MHz.
Following the input amplifier is a discriminator.
The discriminator has selectable slope and a
threshold programmable from -300 mV to +300
mV in 0.2 mV steps (referred to the inputs). Pulse-
pair resolution is 10 ns and pulses of either