SRS Labs SR430, Manual

Page: 15 / 146

13
Instrument Overview
polarity may be detected.
The discriminator output is labelled DISC OUT on
the front panel. The DISC OUT is a 50 ½, NIM lev-
el signal showing the actual input to the counter.
The output is active low (-0.7 V) and counting oc-
curs on the falling edge. The output is low when
the signal input exceeds the threshold level with
the correct slope, i.e. the signal is higher than the
threshold with positive slope or the signal is lower
than the threshold with negative slope.The DISC
outputs are very useful when adjusting discrimina-
tor thresholds or determining the exact timing of
the counts relative to the time bins.
The DISC OUT is active at all times. Whenever a
signal pulse exceeds threshold with the correct
slope, a pulse is output at DISC OUT. The unit
does not have to be actively acquiring data. This
enables setting the discriminator threshold without
triggering the unit.
TIMING
The DISC OUT signal is delayed internally by 20
ns so that the DISC OUT and the BIN CLK output
are correct relative to each other. In other words, a
DISC OUT pulse which falls in the first BIN CLK
will be counted in the first bin. However, a DISC
OUT pulse is actually due to a signal pulse arriving
20 ns earlier at the SIGNAL INPUT. This means
that BIN CLK and DISC OUT, when viewed on a
scope using equal length cables terminated into
50 ½, are accurately timed. However, for timing
purposes, each output viewed by itself is 20 ns
late relative to the trigger. Since the insertion delay
for the BIN CLK is 45 ns, signal pulses arriving 25
ns after the trigger will be output at the DISC OUT
45 ns after the trigger and counted in the first bin.
For accurate timing, it is best to use DISC OUT
and BIN CLK, rather than SIGNAL INPUT and BIN
CLK since the latter pair are skewed by 20 ns.
Thus, even though the BIN CLK is output with a 45
ns insertion delay relative to the TRIGGER, the ac-
tual insertion delay is 25 ns (20 ns less). Signal
pulses arriving 25 ns after the TRIGGER will be
counted.
Note that SYNC/BUSY and BIN CLK are only out-
put only during actual data acquisition while the
DISC OUT is active all of the time.
DATA ACQUISITION
To begin acquiring data, the number of bins per
record and the number of records per scan must
be programmed. The record length, N, is the num-
ber of time bins in a single record. Each trigger will
take N data points corresponding to N sequential
time bins. N may be programmed from1024 to
32704 (including Trigger Offset).
Records are accumulated until the number of
records per scan, R, have been acquired. R may
be programmed from 1 to 65536. If R is set to 0
(free run), then accumulation continues without
end. This mode is convenient when timing an ex-
periment since BIN CLK and SYNC/BUSY are out-
put only when data is being acquired. When R is
0, every trigger will output SYNC/BUSY and BIN
CLK without stopping after a programmed number
of triggers.
After the record length and scan length have been
programmed, data acquisition can be started.
When the [START] key is pressed or a START
command is received from the computer interface,
data acquisition begins. The next TRIGGER will
start the first record and count data will be stored.
Successive triggers will acquire records which are
accumulated until the number of records per scan
have been taken. During this time, the unit is
BUSY and the SYNC/BUSY and BIN CLK outputs
are active. After the number of records per scan
have been accumulated, data acquisition is halted.
The unit is now in the DONE state. Triggers which
arrive while the unit is DONE are ignored and
SYNC/BUSY and BIN CLK are inactive.
When the data acquisition is DONE, the final accu-
mulated data is displayed on the screen and may
be analyzed and manipulated. Before taking data
again, the current data must be cleared by press-
ing the [STOP/RESET] key or sending the RESET
command via the computer interface. This will
clear the count memory and the display. The unit
will now be in the CLEAR state. While in the
CLEAR state, the data acquisition setup may be
altered. A new record length or bin width may be
programmed, etc. The [START] key will start data
acquisition once again and put the unit in the
BUSY state.
ADD, SUBTRACT, TOGGLE, and INHIBIT
Normally, records are accumulated by summing
corresponding bin data from successive records.
Records may also be subtracted from the accumu-
lation. The accumulation mode may be set to
ADD, SUBTRACT, TOGGLE, or EXTERNAL.
ADD will sum new records to the accumulation.
SUBTRACT will subtract new records from the ac-
cumulation.