31
Subject to change without notice
time base generator moves the beam from the left to the
right of the screen (time deflection = t).
Normally there are periodically repeating waveforms to be
displayed. Therefore the time base must repeat the time
deflection periodically too. To produce a stationary display,
the time base must only be triggered if the signal height and
slope condition coincide with the former time base start
conditions. A DC voltage signal can not be triggered as it is a
constant signal with no slope.
Triggering can be performed by the measuring signal itself
(internal triggering) or by an external supplied but synchronous
voltage (external triggering).
The trigger voltage should have a certain minimum amplitude.
This value is called the trigger threshold. It is measured with
a sine signal. Except when external trigger is used the trigger
threshold can be stated as vertical display height in div, at
which the time base generator starts, the display is stable,
and the trigger indicator LED lights or flashes. The internal
trigger threshold of the oscilloscope is given as
≤
0.5div. When
the trigger voltage is externally supplied, it can be measured
in V
pp
at that input. Normally, the trigger threshold may be
exceeded up to a maximum factor of 20.
The instrument has two trigger modes, which are
characterized as Automatic Peak and Normal triggering.
Automatic Peak (value) -Triggering
Instrument specific information can be drawn from the items
NM - AT -
(16)
and
LEVEL (18)
in the section
“Controls
and Readout”
.
This trigger mode is automatically selected after the
AUTO
SET
pushbutton was pressed. As the peak value detection
makes no sense in combination with DC and TV (television)
signals, it is switched off automatically in DC, TVL and TVF
trigger coupling conditions as well as in alternate trigger mode.
In this case the automatic is still present, but a wrong trigger
level setting causes an untriggered display.
In automatic trigger mode the sweep generator can run without
test signal or external trigger voltage. A base line will always
be displayed even with no signal. With an applied
AC
signal
the peak value triggering enables the user to select the 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
The description of the most important control elements (NM
- AT -
(16)
and LEVEL
(18)
) is part of the section
“Controls
and Readout”
).
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.
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. 0.3V
pp
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
(16)
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
AUTO SET
function. The slope setting
can be changed in alternate- and B time base mode for the B
time base trigger unit if the delay trigger function is active.
The previous slope setting for the A 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 (27)
) and indication are described under
“Controls
and Readout”
.
As the automatic triggering does not work below 20Hz,
normal triggering should be used in DC, NR and LF trigger
coupling mode. The coupling mode and accordingly the fre-
quency range of the trigger signal should meet the signal
requirements.
AC:
This is the most frequently used trigger mode. The trigger
Triggering and time base
