36
Subject to change without notice
Triggering and time base
The maximum input voltage of the input circuit is 100V
(DC+peak AC). The external trigger voltage may have a
completely different form from the test signal voltage, but
must be synchronous with the test signal. Triggering is even
possible in certain limits with whole number multiples or
fractions of the test frequency.
It must be noted that a different phase angle between the
measuring and the triggering signal may cause a display not
coinciding with the slope selection setting.
The trigger coupling selection can also be used in external
triggering mode.
Trigger indicator ”TR”
The following description applies to the ”TR” LED. Please
note item (10) 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 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/TR. POS.
/ HO LED (21) 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 may be 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 over-lapped on
the screen, making the signal observation unsuc-cessful.
Fig. 2 shows a case where only the desired parts of the signal
are stably displayed.
Delay / After Delay Triggering
(Analog mode only!)
The instrument specific information regarding this mode is
part of the section ”Controls and Readout” paragraph DEL/TR.
POS. / HO LED (21) and DEL.MODE / ON OFF (23).
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 a higher intensity
(INTENS)
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 than two
periods should be avoided as it limits the selection of the
signal section to be expanded.
The
X MAG (x10)
function should be switched off in the
beginning but may be activated later. The signal must be
triggered and stable.
The following explanation assumes that the trace starts on the
left vertical graticule line.
