Operating instructions

occurs (pretrigger---see Fig . 1-3) . The exact amount of the
post-trigger display is indicated by the CRT readout .
Then, without
changing
other
triggering
controls, select
the single-sweep mode by pressing the SINGLE SWP
RESET pushbutton . Set the 7854 to the
Stored
CRT
Display mode and to the AQS (acquire single-shot) WFIVI
Acquisition mode . When the AQ .S button is presed, the
7854 will respond by lighting the "Busy" LED and
blanking the entire screen . Arm the single-sweep
function by pressing the SINGLE SWP RESET pushbutton
again . After the
trigger
event occurs, the stored
waveform will be displayed on screen . A single stored
trace is displayed each time the 7854
AQS
function is
selected and the 71387 SINGLE
SWP
RESET pushbutton
is pressed .
If an Error message is displayed at the bottom
of
the CRT,
the 71387 clock is running faster than the 7854 can
digitize, and the single sweep will not be acquired . To
remedy this error condition, select a slower TIME/DIV
setting and re-acquire the single-sweep display until the
Error message disappears and the
single-sweep display
is stored . If the 7854 does not return from the "Busy"
state or if
it
is desired to abort the AQS before the trigger
event has occurred, press Stop Ext Acquire button
on the
7854 rear panel .
TRIGGER
COUPLING
The TRIGGERING COUPLING pushbuttons select the
method in which the trigger signal is connected to the
trigger circuits . Each position permits selection or
rejection of some frequency components of the signal
which triggers the sweep .
AC . AC
COUPLING blocks the do component
of the
trigger signal . Signals with low-frequency components
below
about 30 hertz are attenuated . In general,
AC
COUPLING can be used for most applications . However,
if the signal contains unwanted frequency components or
if the sweep is to be triggered at a low repetition rate or a
do level, one of the other COUPLING switch positions will
provide a better display .
REV A MAR
1981
Operating Instructions--7887
AC LF REJ . AC LF REJ COUPLING rejects dc, and
attenuates low-frequency trigger signals below about 30
kilohertz . Therefore, the sweep is triggered only by the
higher-frequency
components
of
the trigger signal . This
position is
particularly
useful
for providing
stable
triggering if the trigger signal contains line-frequency
components . Also, the AC LF REJ position provides the
best
alternate-mode vertical displays at fast sweep
rates
when comparing two or more unrelated signals .
AC
HF REJ . AC HE REJ COUPLING passes all low-
frequency signals between about 30 hertz and 50
kilohertz . Dc is rejected and signals outside the above
range are attenuated . When triggering from complex
waveforms, this position is useful to provide a stable
display of the low-frequency components .
DC . DC COUPLING can be used to provide stable
triggering from low-frequency signals
whichwould be
attenuated in other COUPLING switch positions
. DC
COUPLING can be used to trigger the sweep when the
trigger signal reaches a do level set by the LEVEL control .
When using internal triggering, the setting of the vertical
unit position control affects the triggering point .
TRIGGER SOURCE
The TRIGGERING SOURCE pushbuttons select the
source of the trigger signal which
is connected to the
trigger circuits .
INTERNAL . The INT position causes
the 71387 to trigger
on
the trigger signal from the
vertical plug-in unit .
Further selection of the internal trigger
signal may be
provided by the vertical plug-in unit or by the mainframe
;
see the instruction manuals for
these instruments for
more information . For most applications, the
internal
source can be used . However, some applications require
special triggering which cannot be obtained in
the INT
position . In such cases, the LINE or EXT positions of the
SOURCE switches must be used .