4
cable, and this type is recommended to ensure
proper termination of the signals.
No input or output connectors need be terminated
when they are not in use.
In any experiment in which it is reasonable to
assume thatthe countrates for start and stop will be
equal or nearly equal, use the signal furnished from
the origin of events into the start input and thesignal
furnished from the response into the stop input.
The 566 will then measure the time difference (T)
from origin to response and furnish an output
amplitude that is some fraction of the selected full-
scale amplitude, proportional to the ratio of T, to the
selected full-scale time range.
In anyexperiment in which the two count rates differ
noticeably, such as one in which fewer responses
than event origins can be expected, use the lower
count rate as the start input to the 566. This assures
that the 566 dead time will be minimized because it
analyzes the time difference only after a start signal
is accepted. When the response is used as a start
signal, furnish the signals from the origin of events
through a delay line into the stop input, and adjust
the delay to match the selected fuII-scale time of
the 566. At each start input signal the 566 will
analyze the time until its related origin signal is
furnished to the stop input. The time measured. is
then delay time minus T, and produces a so-called
inverted time spectrum. The purpose of this type of
system connection is to reduce the number of
conversions and the corresponding dead time.
during the experiment. For each signal accepted
through the start input there must be a conversion,
butfor each signalthroughthestopinputthereneed
not be a conversion. For each start signal that is not
followed by astop signal withinthe selected time full
range, the converter measures a time equal to the
total range, even though no output pulse is
generated.
3.4. LINEAR OUTPUT SIGNAL
CONNECTIONS AND TERMINATING
IMPEDANCE
The source of impedance of the standard TAC
output, with the 0 to 10 V linear range, is about 1
Ω
through the connector on the front panel.
For the front panel circuit, the interconnection to
other modules does not usually require any special
considerations, especially it the interconnecting
cable is shorter than 4-ft in length. Paralleling
several loads on a single output will still not reduce
the 0 to 10 V signal span significantly unless the
combined load is <100
Ω
.
As with any analog instrument, oscillations may be
observedoccasionallywhenunterminatedlengthsof
cable are used. Short cable lengths (up to 4 ft)
need not be terminated. When longer cable lengths
are required for transfer of a linear signal, the cable
should be terminated in a resistive load equal to the
cable impedance to prevent reflections and
oscillationsinthecable.Oscillationsuppressioncan
be effected by either a series termination at the
sending end of the cable or by a shunt termination
at the receiving end. For convenience a BNC tee
can usually accommodate both the cable and a
mating terminator at the input of the receiving
instrument. These units ate available commercially,
includingB.C.terminatorswithnominalvaluesof50,
100, and 1000
Ω
. ORTEC stocks a limited quantity
of all but the 1000
Ω
terminators for your
convenience, as listed below:
B.C. Tee Connector C-29
50
Ω
Terminator C-28
100
Ω
Terminator C-27
Whenashuntterminationatthe receiving endofthe
cable is impractical, consider series termination at
the sending end. For a series termination the full
signal amplitude span is available at the receiving
end only if the input impedance is many times the
characteristic impedance of the cable. For series
termination install the correct resistance between
the actual amplifier output on the etched circuit
board and the output connector. Effectively, the
terminating resistance is in series with the input
impedance of the receiving instrument, and may
result in some loss in signal amplitude. For
example, if the series terminator is 93
Ω
and the
driven load is 900
Ω
, the available signal span will
beonlyabout90%of themaximumsignalamplitude
for each pulse. The termination of a 93-
Ω
cable in a
93-
Ω
load will cause
!
50% loss for the signal.
3.5. LOGIC SIGNAL CONNECTIONS
The start and stop input circuits accept positive or
negative NIM-standard signals. Each of these input
circuits is jumper selectable. Input impedance of
50
Ω
for negative signals or >1 k
Ω
for positive
signals is intended as the proper termination for the
signals.
