35
Subject to change without notice
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 off set 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.
1½ 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
colour 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 recommen-ded, 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/Mains triggering (
~
)
The instrument specific information regarding this
mode is part of the section ”Controls and Readout”
paragraph TRIG. MODE (20).
This trigger mode is present if the READOUT indicates the
“~”
“~”
“~”
“~”
“~” symbol instead of the “trigger source”, “slope” and
“coupling” information. 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 Ohm
should be series connected (RF decoupling). Often it is
advisable to statically shield 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 the aid of
the TRIG. SOURCE (17) pushbutton (please note ”Controls
and Readout”). In the case of chopped DUAL mode, selecting
alternate trigger mode automatically sets the instrument to
alternate DUAL mode.
Under TV-L, TV-F and line/mains triggering conditions 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 not displayed in
alternate trigger mode.
With alternate triggering it is possible to trigger two signals
from different frequency sources (asynchronous). In this case
the oscilloscope must be operated in DUAL alternate mode
with internal triggering and 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.
SOURCE (17) 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 T
T
T
T
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 Ohm II 20pF.
Triggering and time base
