Hameg HM 1004-3 Series, Manual

Page: 26 / 36

Subject to change without notice
26
Triggering and timebase
Trigger coupling
Instrument specific information regarding this item can be
noted in the “Data Sheet”. The coupling setting
( TRIG.
MODE (22)) and indication are described under “Controls
and Readout” .
As the automatic triggering does not work below 20Hz,
normal triggering should be used in DC and LF trigger coupling
mode. The coupling mode and accordingly the frequency
range of the trigger signal should meet the signal require-
ments.
AC: This is the most frequently used trigger mode. The
trigger threshold increases below and above the fre-
quency limits mentioned in the data sheet. This filter
cuts off both the DC content of the trigger signal and
the lowest frequency range.
DC: In this coupling mode the trigger signal is coupled
galvanically to the trigger unit if normal triggering (NM)
is present. Therefore there is no low frequency limit.
DC triggering is recommended if the signal is to be
triggered with quite slow processes or if pulse signals
with constantly changing pulse duty factors have to be
displayed.
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 trig-
gering (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 dis-
played 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 dis-
played 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 synchronisa-
tion 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 independ-
ent 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 mislead-
ing display, it must not be activated.
TVF: On the 2ms/div setting and field TV triggering se-
lected, 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 timebase mode the B timebase speed is
increased above the A timebase speed. If the timebase
is set to 5ms/div 2.5 frames are visible. The time delay
for the B timebase 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 be-
come 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 exter-
nal 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 oscillo-
scope 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 (22) .
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 (11) 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