Subject to change without notice
26
Triggering and timebase
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.
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 (21)
.
This trigger mode is present if the
~ LED
is lit
(21)
. 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
(18)
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.
Under
TV-L
,
TV-F
and
line/mains
triggering conditions alter-
nate 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
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.
(18)
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 0.3Vpp and should not
increase above 3Vpp. The input impedance of the
TRIG. EXT.
socket is approx. 1M
Ω
II 20pF.
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.
