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called
Automatic Triggering.
Operation of the scope
needs, having a constantly visible trace, only a correct
amplitude and time base setting. A LEVEL adjustment is
neither necessary nor possible with automatic triggering.
This simple AT mode is recommended for all uncompli¬
cated measuring tasks. However, automatic triggering is
also the appropriate operation mode for the "entry" into dif¬
ficult measuring problems, e.g. when the test signal is un¬
known relating to amplitude, frequency or shape. Presetting
of all parameters is now possible with automatic triggering;
the change to normal triggering can follow thereafter.
The automatic triggering works above
10Hz
up to at least
40MHz.
The changeover to the break down of the automa¬
tic triggering at frequencies under 10Hz is abrupt. How¬
ever, it can not be recognized by the TRIG. LED; this is still
blinking. Break down of triggering is best recognizable at
the left screen edge (the start of the trace in differing display
height).
The automatic triggering follows immediately all variations
or fluctuations of the test signal above 10 Hz. However, if
the pulse duty factor of a square-wave signal changes so
much that one part of the square-wave reduces to a needle
pulse, switching over to normal triggering and using the
LEVEL control can be necessary. With automatic triggering,
the trigger point lies approx, in the zero voltage crossing.
The time interval, required for the time base start, can be
too short at a steep zero crossing of the needle pulse. Then
normal triggering should be used.
Automatic triggering is practicable not only with internal but
also with external trigger voltage.
Normal Triggering
With normal triggering (AT/NORM, button depressed) and
LEVEL adjustment, the sweep can be started by signals
within the frequency range selected by the TRIG, coupling
switch.
In the absence of an adequate trigger signal or
when the trigger controls (particularly the
LEVEL
con¬
trol) are misadjusted, no trace is visible, i.e. the screen
blanked completely.
When using the internal normal triggering mode, it is possi¬
ble to trigger at any amplitude point of a signal edge, even
with very complex signal shapes, by adjusting the LEVEL
control. Its adjusting range is directly dependent on the dis¬
play height, which should be at least
O.Sdiv.
If it is smaller
than 1 div., the LEVEL adjustment needs to be operated
with a sensitive touch. In the external normal triggering
mode, the same applies to approx. 0.6V external trigger
voltage amplitude.
Other measures for triggering of very complex signals are
the use of the time base variable control and HOLDOFF
time control, hereinafter mentioned.
Slope
The trigger point can be placed alternatively on a rising or fal¬
ling edge of the test signal. This is valid with automatic and
with normal triggering. The selected slope is set with the
+/—
button. The plus sign (button released) means an
edge, which is coming from a negative potential and rising
to a positive potential. That has nothing to do with zero or
ground potential and absolute voltage values. The positive
slope may also lie in a negative part of a signal. A falling edge
(minus sign) triggers, when the
+/—
button is depressed.
However, with normal triggering, the trigger point may be
varied within certain limits on the chosen edge using the
LEVEL control.
Trigger coupling
The coupling mode and accordingly the frequency range of
the trigger signal can be changed using the TRIG, selector
switch. However, this is possible only with the TV SEP.
switch in OFF position.
AC:
Trigger range > 10Hz to 10MHz.
This is the most frequently used trigger mode.
The trigger threshold is increasing below 10 Hz
and above 10MHz.
DC:
Trigger range DC to 10MHz.
DC triggering is recommended, if the signal is to
be triggered with quite slow processes or if pulse
signals with constantly changing pulse duty fac¬
tors have to be displayed.
Always work with normal triggering and
LEVEL
adjustment.
Otherwise there is the possibility in the AT posi¬
tion (automatic triggering) that the trigger point
may change or that triggering may not occur with
signals without zero crossing (e.g. with DC
offset). Sometimes triggering is easier with AC
input coupling,
because the signal then has its
average value exactly at the oscilloscope's ground
potential.
HF:
Trigger range 1.5 kHz to 40 MHz
(high-pass filter).
The HF position is suitable for all radio-frequency
signals.
DC
fluctuations
and
low-frequency
excess noise of the trigger voltage are suppres¬
sed, giving a stable display. The trigger threshold
increases below 1.5 kHz.
LF:
Trigger range DC to 50kHz
(low-pass filter).
The LF position is often more suited for low-fre¬
quency signals than the DC position, because the
(white) noise in the trigger voltage is strongly sup-
Subject to change without notice
Mil 203-7
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