Triggered Light; Trigger Mode - Tektronix 7B51 Instruction Manual

applied to this unit from the vertical unit is inadequate or it
is below the lower frequency limit of the AC COUPLING
switch position. If the desired display is not obtained with
the push buttons pushed in, other selections must be made.
Aefer to the following discussions or the instruction
manuals for the associated indicator oscilloscope and verti-
cal unit for more information.
Triggered Light
The TAIG'D light: provides a convenient indication of
the condition of the triggering circuits. If the TAIG-
GERING controls are correctly set and an adequate trigger
signal is applied, the TAIG'D light is on. Under certain
conditions, the TRIG'O light may be off, indicating that
the sweep is not triggered. The cause might be a mis-
adjusted LEVEL/SLOPE control, incorrectly set COU-
PLING or SOURCE switches, low trigger signal amplitude,
or a trigger signal repetition rate outside the acceptable
frequency range. This feature can be used as a general indi-
cation of correct
triggering. It is particularly useful when
setting up the trigger circuits when a trigger signal is avail·
able without a display on the CAT. It also indicates that
this unit is correctly triggered when operating as a delaying·
sweep unit (A).
Trigger Mode
The pushbuttons located under the MODE title (see Fig.
2·4) select the mode in which the sweep is triggered. The
selected mode is indicated by a lighted pushbutton.
MODE
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COUPLING SOURCE
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Operating Instructions-Type 7851
p.p AUTO. When the p.p AUTO pushbutton is pressed,
a triggered display is provided at any setting of the LEVEL/
SLOPE control whenever an adequate trigger signal is
applied. The range of the LEVEL/SLOPE control in this
mode is between about 10% and 90% of the peak·to·peak
amplitude of the applied trigger signal. The LEVELISLOPE
control can be set so the displayed waveform starts at any
point within this range on either slope. In the p.p AUTO
mode, the trigger circuit automatically compensates for
changes in trigger signal amplitude for signals above the
minimum amplitude requirement. Therefore, if the
LEVEL/SLOPE control is set so the displayed waveform
starts at
8
certain percentage-point on the leading edge of a
low-amplitude signal,
it triggers at the same percentage·
point on the leading edge of a high-amplitude signal if the
LEVEUSLOPE control is not changed. This operation is
illustrated in Fig . 2·5.
When the trigger repetition rate is below about 200 hertz
(or outside the frequency range selected by the COUPLING
switch), or when
the
trigger signal is inadequate, the sweep
free runs at the sweep rate indicated by the TIME/ DIV
switch to produce a reference trace (TRIG'O light off).
When an adequate trigger signal is again applied, the free-
running condition ends and a triggered display is presented.
The p.p AUTO trigger mode is particularly useful when
observing a series of waveforms, since it is not necessary to
reset the LEVEL/SLOPE control for each observation.
Also, because of the ease of obtaining a triggered display in
this mode, the p.p AUTO trigger mode can be used for
most applications and the remaining modes used only for
special applications or when stable triggering is not obtain·
able in this mode.
AUTO. When the AUTO push-button is illuminated,
8
triggered display is presented with the correct setting of the
LEVEUSLOPE control (see Trigger Level discussion)
whenever an adequate trigger signal is applied. The range of
the LEVEL/SLOPE control in this mode is fS vertical divi -
sions. The TRIG'O light indicates when the display is trig·
gered.
When the trigger repetition rate is below about 30 hertz
(or outside the frequency range selected by the COUPLING
switch) or when the trigger signal is inadequate. the sweep
free runs at the sweep rate indicated by the TIME/ DIV
switch to produce a reference trace (TA IG'O light off).
When an adequate trigger signal is again applied, the free-
run ning condition ends and a triggered display is presented .
When the LEVELISLOPE control is at a setting outside the
amplitude range of the trigger signal, the sweep also free
runs at the sweep rate indicated by the TIME/DIV switch .
This type of free-running display can be useful when it is
desired to measure only the maximum peak·to-peak ampli·
tude of a signal without observing the waveshape such as in
bandwidth measurements) .
2·7