27
Subject to change without notice
Triggering and timebase
Trigger indicator ”TR”
The following description applies to the
”TR” LED
. Please
note item (11) under ”Controls and Readout”.
An LED on condition indicates that the trigger signal has a
sufficient amplitude and the trigger level control setting is
correct. This is valid with automatic and with normal
triggering. By observing the trigger LED, sensitive trigger
level adjustment is possible when normal triggering is
used, particularly at very low signal frequencies. The
indication pulses are of only 100ms duration. Thus for fast
signals the LED appears to glow continuously, for low
repetition rate signals, the LED flashes at the repetition
rate or at a display of several signal periods not only at the
start of the sweep at the left screen edge, but also at each
signal period.
In automatic triggering mode the sweep generator starts
repeatedly without test signal or external trigger voltage. If
the trigger signal frequency decreases the trigger automatic
repetition frequency the sweep generator starts without
awaiting the trigger pulse. This causes an untriggered
display and a flashing trigger LED.
HOLD OFF-time adjustment
For instrument specific information please note
DEL.POS.
(22)
in section ”Controls and Readout”.
If it is found that a trigger point cannot be found on extremely
complex signals, even after careful adjustment of the trigger
level control, a stable display may often be obtained using
the holdoff control. This facility varies the holdoff time
between two sweep periods approx. up to the ratio 10:1.
Pulses or other signal waveforms appearing during this off
period cannot trigger the time base.
Particularly with burst signals or aperiodic pulse trains of
the same amplitude, the start of the sweep can be delayed
until the optimum or required time.
A very noisy signal or a signal with a higher interfering
frequency is at times displayed double. It is possible that
trigger level adjustment only controls the mutual phase
shift, but not the double display. The stable single display
of the signal, required for evaluation, is easily obtainable by
expanding the hold off time until one signal is displayed.
A double display is possible with certain pulse signals,
where the pulses alternately show a small difference of the
peak amplitudes. Only a very exact trigger level adjustment
makes a single display possible. The use of the holdoff
control simplifies the right adjustment.
After specific use the holdoff control should be reset into
its calibration detent (fully ccw), otherwise the brightness
of the display is reduced drastically. The function is shown
in the following figures.
Fig. 1 shows a case where the holdoff control is in the minimum
position and various different waveforms are overlapped
on the screen, making the signal observation unsuccessful.
Fig. 2 shows a case where only the desired parts of the signal are
stably displayed.
Delay / After Delay Triggering
The instrument specific information regarding this mode is
part of the section ”Controls and Readout” paragraph
DEL.POS. (22), SEA./DEL. (24)
and
DEL.TRIG. (25)
.
As mentioned before, triggering starts the time base sweep
and unblanks the beam. After the maximum X deflection to
the right, the beam is blanked and flies back to the (left) start
position. After the hold off period the sweep is started
automatically by the automatic trigger or the next trigger
signal. In normal triggering mode the automatic trigger is
switched off and will only start on receipt of a trigger signal.
As the trigger point is always at the trace start position, trace
expansion in X direction with the aid of the time base is limited
to the display on the left of the trace. Parts of the signal to be
expanded which are displayed near the trace end (right side
of the screen) are lost when the time base speed is increased
(time coefficient reduced).
The delay function delays the trace start by a variable time
from the trigger point. This allows the sweep to begin on any
portion of a signal. The time base speed can then be increased
to expand the display in X direction. With higher expansion
rates, the intensity reduces and within certain limits this can
be compensated by the
INTENS
knob setting.
If the display shows jitter, it is possible to select for (second)
triggering after the elapsed delay time
(DTR)
. As mentioned
before, it is possible to display video signals using the frame
sync pulses for triggering
(TV-F)
. After the delay time set by
the operator, the next line sync pulse or the line content may
be used for triggering. So data lines and test lines can be
displayed separately.
Operation of the delay function is relatively simple. Without
delay function set the time coefficient setting
(TIME/DIV)
until 1 to 3 signal periods are displayed. Display of less the one
period should be avoided as it limits the selection of the signal
section to be expanded, and may cause trigger problems.
The
X MAG (x10)
function should be switched off and the
time variable control should be
CAL
position. The signal must
be triggered and stable.
The following explanation assumes that the trace starts on
the left vertical graticule line.
