28
Subject to change without notice
Triggering of video signals
In
TV-L
and
TV-F
trigger coupling mode the instrument is
automatically set to automatic triggering and the trigger point
indicator is switched off. As only the separated synchronization
pulses are used for triggering the relationship between the
displayed signal and the trigger signal is lost. In
TV-F
mode
interference may occur if chopped
DUAL
mode is chosen or
the readout is active.
Video signals are triggered in the automatic mode. The internal
triggering is virtually independent of the display height, but the
sync pulse must exceed 0.5div height.
The polarity of the synchronization pulse is critical for the slope
selection. If the displayed sync pulses are above the picture
(field) contents (leading edge positive going), then the slope
setting for positive going edges must be chosen. In the case
of sync pulses below the field/line, the leading edge is negative
and consequently the slope selection must be set for falling
edges. Since the invert function may cause a misleading
display, it must not be activated.
On the 2ms/div setting and field TV triggering selected, 1 field
is visible if a 50 fields/s signal is applied. If the hold off control
is in fully ccw position, it triggers without line interlacing
affects caused by the consecutive field.
The display can be expanded by switching on the
X-MAG. x10
function so that individual lines are recognizable. Commencing
with a frame synchronizing pulse, the display can also be
expanded with the knob
TIME/DIV.
But note that this can
result in an apparently unsynchronized display as each frame
(half picture) triggers. This is due to the offset of half a line
between frames.
The influence of the integrating network which forms a trigger
pulse from the vertical sync pulses may become visible under
certain conditions. Due to the integrating network time constant
not all vertical sync pulses starting the trace are visible.
On the 10µs/div setting and line TV triggering selected, approx.
1
½
lines are visible. Those lines originate from the odd and
even fields at random.
The sync-separator-circuit also operates with external triggering.
It is important that the voltage range (0.3Vpp to 3Vpp) for
external triggering should be noted. Again the correct slope
setting is critical, because the external trigger signal may not
have the same polarity or pulse edge as the test signal
displayed on the CRT. This can be checked, if the external
trigger voltage itself is displayed first (with internal triggering).
In most cases, the composite video signal has a high DC
content. With constant video information (e.g. test pattern or
color bar generator), the DC content can be suppressed easily
by AC input coupling of the oscilloscope amplifier. With a
changing picture content (e.g. normal program), DC input
coupling is recommended, because the display varies its
vertical position on screen with AC input coupling at each
change of the picture content.
The DC content can be compensated using the
Y-POS.
control
so that the signal display lies in the graticule area. Then the
composite video signal should not exceed a vertical height of
6div.
Line triggering (~)
The instrument specific information regarding this mode is
part of the section “Controls and readout” paragraph
TRIG.
MODE (26)
.
This trigger mode is present if the
~ LED
is lit
(26)
. The trigger
point symbol is inactive in line/mains trigger mode as there is
no direct amplitude relationship between the trigger voltage
and the signal voltage.
A voltage originating from mains/line (50 to 60Hz) is used for
triggering purposes if the trigger coupling is set to
~
. This
trigger mode is independent of amplitude and frequency of the
Y signal and is recommended for all mains/line synchronous
signals. This also applies within certain limits, to whole
number multiples or fractions of the line frequency. Line
triggering can also be useful to display signals below the
trigger threshold (less than 0.5div). It is therefore particularly
suitable for measuring small ripple voltages of mains/line
rectifiers or stray magnetic field in a circuit. In this trigger mode
the slope direction pushbutton selects the positive or negative
portion of the line/mains sinewave. The trigger level control
can be used for trigger point adjustment.
Magnetic leakage (e.g. from a power transformer) can be
investigated for direction and amplitude using a search or pick-
up coil. The coil should be wound on a small former with a
maximum of turns of a thin lacquered wire and connected to
a BNC connector (for scope input) via a shielded cable.
Between cable and BNC center conductor a resistor of at least
100
Ω
should be series-connected (RF decoupling). Often it is
advisable to shield statically the surface of the coil. However,
no shorted turns are permissible. Maximum, minimum, and
direction to the magnetic source are detectable at the measuring
point by turning and shifting the coil.
Alternate triggering
This trigger mode can be selected in
DUAL
mode by pressing
and holding the
TRIG.
pushbutton
(23)
if the preconditions are
met (
please note “Controls and readout”). In the case of
chopped
DUAL
mode, selecting alternate trigger mode
automatically sets the instrument to alternate
DUAL
mode. If
the trigger coupling is set to
TV-L
or
TV-F
, selecting alternate
triggering automatically results in
AC
coupling conditions.
Under line/mains triggering condition alternate triggering can
not be chosen. Thus only the following trigger coupling modes
are available in alternate trigger mode:
AC
,
DC
,
HF
and
LF
. The
trigger point symbol is internally inactivated.
With alternate triggering it is possible to trigger two signals
which are different in frequency (asynchronous). In this case
the oscilloscope must be operated in
DUAL
alternate mode
and internal triggering each input signal must be of sufficient
height to enable trigger. To avoid trigger problems due to
different
DC
voltage components,
AC
input coupling for both
channels is recommended. The internal trigger source is
switched in alternate trigger mode in the same way as the
channel switching system in
DUAL
alternate mode, i.e. after
each time base sweep. Phase difference measurement is not
possible in this trigger mode as the trigger level and slope
setting are equal for both signals. 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.
(23)
pushbutton (
see “Controls and readout”), if the trigger
Triggering and time base
