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3.4.3.2.
Trigger source selector,
The main time-base trigger source can be selected with the MTB trigger source selector pushbuttons, A, B,
EXT, COMP and LINE.
When A is selected, the MTB trigger signal is derived from channel A (and B selected, from channel B).
Composite triggering (A + B depressed) is selected for displaying two signals that are not time-related.
In this mode, the MTB is triggered on the signal in turn to be displayed.
Composite triggering (COMP) only functions well in the ALT mode, as the triggering switches over from one
trigger source to the other at the end of every time-base sweep.
For COMP triggering, usethe POSITION controls of channels A and B to set the signals being displayed so
that they completely
range for a stable, well-triggered display.
The POSITION controls influence the triggering in the COMP mode, since the COMP trigger signals are picked-
off after the vertical
When COMP triggering is selected together with TRIG VIEW, the MTB trigger signal derived from channel A
is displayed.
LINE triggering is selected when the B and EXT pushbuttons are selected simultaneously. In this mode, the
trigger signal is derived from the mains voltage; i.e. related to the mains frequency.
When the EXT pushbutton is depressed,the MTB is triggered on an external signal applied to the BNC socket
to the right of the EXT pushbutton.
In the AUTO PP mode, the level rangeis determined by the input signal amplitude. This level is related to the
signal peak-peak level, so triggering will always occur within this peak-peak level range.
The trigger point can only be shifted within this range by the LEVEL control.
In the TRIG mode, there is no AUTO peak-peak level; therefore, the LEVEL control must be adjusted to ob-
tain a well-triggered display.
When the AUTO PPand TRIG pushbuttons are depressedtogether, there is only auto free-run of the main time-
base but no AUTO peak-peak level.
The circuit
functions
The trigger signalis fed to the input of a differential amplifier. The voltageon the other input of this amplifier
isdeterminedby the position of the LEVEL control. If the trigger signalhasreachedthe voltagelevelof the
LEVEL potentiometer, a trigger pulse is generated and the time-base starts. (Seealso Fig. 3.3.).
In this way, the time-baseis started at a fixed point of the trigger signal,which meansthat usingthe LEVEL
control, it is possible to scan the shape of the waveform.
The LEVEL potentiometer also incorporates a push-pull switch +1—SLOPE, which permits selection of the
triggerslope.If theswitchisdepressed, thetime-base startson the positive-going s lope(seeFig.3.3.). If the
switch is pulled, the time-base starts on the
Triggerpulses
I
Sawtoothl
voltage
Fig. 3.3.
Scanning the waveform by means of the LEVEL potentiometer
level control
and auto peak-peak
overlap each other. Overlapping is necessary as they must occupy the same trigger
POSITION
controls
and the NORMAL/INVERT
as follows:
Signal vottage
level.
circuits.
slope of the trigger signal.
Screen
Level
MAT 597
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