(b) TV-V Coupling
(c) TV-H Coupling
(d) Sync Polarity
Fig. 2-3 Using the TV SYNC Separator
2.2.5 Trigger
Options
Triggering is often the most difficult operation to perform on an oscilloscope because of the many options
available and the exacting requirements of certain signals.
Trigger Mode Selection
:
When the NORM trigger mode is selected, the CRT beam is not swept horizontally across the face o the
CRT until a sample of the signal being observed, or another signal harmonically related to it, triggers the
timebase. However, this trigger mode is inconvenient because no baseline appears on the CRT screen in the
absense of a signal, or if the trigger controls improperly set. The AUTO trigger mode solves this problem by
causing the timebase to automatically free run when not triggered. This yields a single horizontal line with no
signal, and a vertically-deflected but non-synchronized display when vertical signal is present but the trigger
controls are improperly set. This immediately indicates what is wrong. The only hitch with AUTO operation is
that signals below 25 Hz cannot, and complex signals of any frequency may not realiably trigger the
timebase. Therefore, the usual practice is to leave the trigger MODE switch (14) set to AUTO, but reset it to
NORM if any signal (particulary one below 25 Hz) fails to produce a stable display.
The TV-V and TV-H positions of the Trigger MODE switch insert a TV sync separator into the trigger chain,
so a clean trigger signal at either the vertical-or horizontal-repetition rates can be removed from a composite
video signal (Fig. 2-3 a). To trigger the scope at the vertical rate (Fig. 2-3 b), set the Trigger MODE switch to
TV-V. To trigger the scope at the horizontal (line) rate (Fig. 2-3 c) set the trigger Mode switch to TV-H. For
best results, the TV sync polarity should be negative (Fig. 2-3 d) when the sync separator is used.
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