TDS 620A, 640A, & 644A User Manual
Triggering
This section describes the edge trigger of the main trigger system and ex-
plores, in a general sense, the topic of triggering. This oscilloscope also has
logic and pulse triggers in the main trigger system and a delayed trigger
system. They are described in Section 3.
Triggers determine when the digitizing oscilloscope starts acquiring and
displaying a waveform. They help create meaningful waveforms from unsta-
ble jumbles or blank screens (see Figure 2-1).
Triggered Waveform Untriggered Waveforms
Figure 2-1: Triggered Versus Untriggered Displays
The trigger event establishes the time-zero point in the waveform record, and
all points in the record are located in time with respect to that point. The
digitizing oscilloscope continuously acquires and retains enough sample
points to fill the pretrigger portion of the waveform record (that part of the
waveform that is displayed

before,

or to the left of, the triggering event on
screen).
When a trigger event occurs, the digitizing oscilloscope starts acquiring
samples to build the posttrigger portion of the waveform record (displayed

after,

or to the right of, the trigger event). Once a trigger is recognized, the
digitizing oscilloscope will not accept another trigger until the acquisition is
complete.
The basic trigger is the edge trigger. An edge trigger event occurs when the
trigger
source
(the signal that the trigger circuit monitors) passes through a
specified voltage

level

in a specified direction (the trigger

slope

).
You can derive your trigger from various sources.
Input channels — the most commonly used trigger source is any one of
the four input channels. The channel you select as a trigger source will
function whether it is displayed or not.

Trigger Sources