Tektronix 5T1A Instrument Reference Book page 91

External Source
When a sampling unit without
signal delay
(such as the
Type 4S2)
is used,
and external trigger signal must be supplied
to the Type
5TLA EXTERNAL TRIGGER INPUT
connector.
External
triggering
also
may be used
instead
of internal
triggering
when a sampling unit
such as the Type 4S1 is used.
'Placing
the
SOURCE switch
in the EXT.
position
connects the
EXTERNAL
TRIGGER
INPUT
connector
through an
ac-coupling
circuit and
through
an isolation
stage
to the
trigger circuit.
The Type
5TLA must
be triggered
at least
55 nsec
before
the
signal reaches
the
samp Ling gate
of the sampling
unit.
The
TIME POSTTION
control
allows
an extra delay
in operat*on,
added
to the
55 nsec
requirement.
Thus,
the time
window
after
triggering that
is displayed
may be moved a considerable
distance
in. time,
allowing
flexibility
in test setup.
Remember
that
as little
as 8 inches
of RG-8A/U cable
can insert
an
extra nsec of delay.
There
are two
methods of
triggering
the Type
DT1A before
the signal
arrives
at the sampling-unit
sampling
gate. The
first
is to
pretrigger
the Type
STLA 'by using
a pulse
generator
that
triggers
the system,
and
some time
later triggers
the
circuit
under
test.
The second
is to insert
some
delay in
the gignal
connections
to the sampling
unit
so that
the signal
will
arrive
at the
sampling
gate after
the Type
5T1A
is trig-
gered (occurs
internally
in Type isl).
The
following
are
Pretriggering
This method
may
be used when
the
circuit
under test
can