33
Subject to change without notice
Even with 180° phase difference between both signals, they
appear with the same slope direction. If signals are applied
with a high frequency ratio (difference), the trace intensity
then becomes reduced if the time base is set to smaller time
coefficients (faster sweep). This happens as the number of
sweeps does not increase because it depends on the lower
frequency signal, but with a faster sweep the phosphor
becomes less activated.
External triggering
The external trigger input is activated with the aid of the
TRIG.
(24)
pushbutton (
see “Controls and Readout”
), if the trigger
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 0.3V
pp
and should not increase
above 3V
pp
. 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 to
not coincide with the slope selection setting.
The trigger coupling selection can also be used in external
triggering mode.
Trigger indicator
The following description applies to the
“TR”
LED
(17)
.
Please note “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. –
HO (28)
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 hold off control. This facility varies the hold off time bet-
ween 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 interfe-
ring frequency is at times displayed double. It is pos-
sible that trigger level adjustment only controls the
mutual phase shift, but not the double display. The
stable single display of the signal, required for eva-
luation, 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 hold off control
simplifies the right adjustment.
After a specific use the hold off 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.(Page 31)
Fig. 1
shows a case where the hold off 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 stable displayed.
B time base (2nd time base) /
Triggering after Delay
Please note instrument specific information in section “Con-
trols and readout”. The most important controls regarding
this mode are
A/ALT
-
B (30), DEL.TRIG. – VAR. (31), TIME/
DIV. (29), DEL. POS. (28)
and
TRS (13)
.
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 time has elapsed 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 the sweep 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 always from the trace start.
Parts of the signal to be expanded which are displayed near
Triggering and time base
