27
Subject to change without notice
HF:
In this coupling mode the transmission range equals a
high pass filter. It cuts off the DC content of the trigger
signal and the lower frequency range.
NR:
Decreases the maximum trigger bandwidth for high
frequency signals. As in DC coupling condition, lower
frequency signal parts are not affected if normal triggering
(NM) is selected.
LF:
The LF coupling (low pass filter function) is often more
suited for low-frequency signals than the DC coupling,
because the (white) noise in the trigger voltage is strongly
suppressed. So jitter or double-triggering of complex
signals is avoidable or at least reduced, in particular with
very low input voltages.
TV-L:
The built-in active TV-Sync-Separator provides the
separation of line sync pulses from the video signal.
Even distorted video signals are triggered and displayed
in a stable manner. This mode is described under
paragraph “Triggering of video signals”.
TV-F:
The built-in active TV-Sync-Separator also provides the
separation of frame sync pulses from the video signal.
Even distorted video signals are triggered and displayed
in a stable manner.
This mode is described under paragraph
“Triggering of
video signals”
.
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.
TVF:
On the 2ms/div setting and field TV triggering selected, 1
field is visible if a 50 fields/s signal is applied. If minimum
hold off time is chosen, it triggers without line interlacing
affects caused by the consecutive field.
More details in the video signal become visible if in
alternate or B time base mode the B time base speed is
increased above the A time base speed. If the time base
is set to 5ms/div 2.5 frames are visible. The time delay for
the B time base can then be used to select any line(s) to
be displayed expanded on the screen. 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.
TVL:
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 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
(23)
.
This trigger mode is present if the trigger coupling information in
the readout displays
“TR:~”
. The symbol can be changed by 180°
by using the trigger slope
(12)
pushbutton. 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 sine wave.
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 maximum
turns of 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 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
(20)
if the preconditions are
Triggering and Time Base
