29
Subject to change without notice
coupling is not set to line/mains trigger coupling. Then the
internal trigger source is deactivated. As the external trigger
signal applied at the
TRIG. EXT
socket normally has no relation
to the signal height of the displayed signal, the trigger point
symbol is switched off. The external trigger voltage must have
a minimum amplitude of 0.3Vpp and should not increase above
3Vpp. The input impedance of the
TRIG. EXT.
socket is
approx. 1M
Ω
II 15pF.
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 (16) 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
(Only in analog mode)
For instrument specific information please note
DEL.POS. -
HO (27)
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
(Only in analog mode)
The instrument specific information regarding this mode is
part of the section “Controls and readout” paragraph
DEL.POS.
- HO (27), SEA./DEL. - ON/OFF (29)
and
DEL.TRIG. - VAR.
(30)
.
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.
Triggering and time base
