27
Subject to change without notice
voltage point on the trigger signal (trigger point), by the
adjustment of the trigger level control. The control range
depends on the peak to peak value of the signal. This trigger
mode is therefore called Automatic Peak (Value)- Triggering.
Operation of the scope needs only correct amplitude and time
base settings, for a constantly visible trace. Automatic mode
is recommended for all uncomplicated measuring tasks.
However, automatic triggering is also the appropriate operation
mode for the “entry” into difficult measuring problems, e.g.
when the test signal is unknown 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 20Hz. The failure of
automatic triggering at frequencies below 20Hz is abrupt.
However, it is not signified by the trigger indicator 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 peak (value) triggering operates over all variations
or fluctuations of the test signal above 20Hz. However, if the
pulse duty factor of a square-wave signal exceeds a ratio of
100:1, switching over to normal triggering will be necessary.
Automatic triggering is practicable with internal and external
trigger voltage.
Normal Triggering
Information specific to the instrument are given in the articles
NM - AT (15)
,
LEVEL (17)
and
TRIG. MODE (26)
in the section
”Controls and readout”. The time fine adjustment
(VAR.)
and
the hold off time setting assist in triggering under specially
difficult signal conditions. The following description refers to
the Yt (time base) analog mode.
With normal triggering, the sweep can be started by AC signals
within the frequency range defined by the trigger coupling
setting.
In the absence of an adequate trigger signal or when the trigger
controls (particularly the trigger
LEVEL
control) are misadjusted,
no trace is visible, i.e. the screen completely blanked.
When using the internal normal triggering mode, it is possible
to trigger at any amplitude point of a signal edge, even with
very complex signal shapes, by adjusting the trigger
LEVEL
control. If the signal applied at the Y input is used for triggering
(internal trigger source), its adjusting range is directly dependent
on the display height, which should be at least 0.5div. If it is
smaller than 1div, the trigger
LEVEL
adjustment needs to be
operated with a sensitive touch. In the external normal triggering
mode, the same applies to approx. 0.3Vpp external trigger
voltage amplitude.
Other measures for triggering of very complex signals are the
use of the time base variable control and
HOLD OFF
time
control, hereinafter mentioned.
(Slope)
Please note item
(15)
in section “Controls and readout” for
instrument specific information.
The actual slope setting is displayed in the readout. The setting
is not changed by the
AUTOSET
function. The slope setting
can be changed in delay mode for the delay time base trigger
unit if the delay trigger function is active. The previous slope
setting for the undelayed time base trigger is stored and still
active.
For further information please note “Controls and
readout”.
The time base generator can be triggered by a rising or falling
edge of the test signal. Whether the rising or the falling edge
is used for triggering, depends on the slope direction setting.
This is valid with automatic and normal triggering. The positive
slope direction means an edge going from a negative potential
and rising to a positive potential. This 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.
However the trigger point may be varied within certain limits
on the chosen edge using the LEVEL control. The slope
direction is always related to the input signal and the non
inverted display.
Trigger coupling
Instrument specific information regarding this item can be
noted in the “Data Sheet”.
The coupling setting
( TRIG. MODE
(26))
and indication are described under “Controls and readout”.
As the automatic triggering does not work below 20Hz, normal
triggering should be used in
DC
and
LF
trigger coupling mode.
The coupling mode and accordingly the frequency range of the
trigger signal should meet the signal requirements.
AC:
This is the most frequently used trigger mode. The
trigger threshold increases below and above the
frequency limits mentioned in the data sheet. This filter
cuts off both the DC content of the trigger signal and the
lowest frequency range.
DC:
In this coupling mode the trigger signal is coupled
galvanically to the trigger unit. Therefore there is no low
frequency limit.
DC triggering is recommended if the signal is to be
triggered with quite slow processes or if pulse signals
with constantly changing pulse duty factors have to be
displayed.
HF:
In this coupling mode the transmission range equals a
high pass filter. It cuts off the DC content of the trigger
signal and the lower frequency range.
LF:
LF trigger coupling has a low pass filter characteristic. As
in DC trigger coupling, there is no low limit for the pass
frequency range (galvanic coupling) in connection with
normal triggering. The trigger signal is coupled through
a capacitor in the automatic (peak value) triggering and
LF trigger coupling. This results in a low frequency limit
which , however, does not disturb as the limit is below
the repetition frequency of the trigger automatic.
The LF trigger coupling is often more suitable for low
frequency signals than DC trigger coupling because the
noise components of the trigger signals are strongly
suppressed. This avoids or reduces , under borderline
conditions, jitter or double traces especially with very
low signal voltages. The trigger threshold raises
continuously above the pass band.
TV-L:
The built-in active TV-Sync-Separator provides the
separation of line sync pulses from the video signal. Even
distorted video signals are triggered and displayed in a
stable manner. This mode is described under paragraph
“Triggering of video signals”.
TV-F:
The built-in active TV-Sync-Separator also provides the
separation of frame sync pulses from the video signal.
Even distorted video signals are triggered and displayed
in a stable manner. This mode is described under
paragraph “Triggering of video signals”.
Triggering and time base
