Tv (Video Signal) Triggering (Pal); Frame Pulse Triggering; Line Sync Triggering; Line Triggering - Hameg HM400 Handbuch

T r i g g e r i n g a n d t i m e b a s e
frequency limit, the same as with DC coupling (galvanic coupling).
In auto (peak-to-peak) trigger mode AC coupling will be automati-
cally used, this will cause a lower frequency limit which, however,
is below the functional limit of the auto trigger. For low frequency
signals LF coupling is often the preferred mode, because high
frequency noise is reduced. This eliminates or diminishes trigger
jitter resp, multiple displays, especially with small input voltages.
Above the bandwidth of the low pass fi lter the trigger threshold
rises sharply.
LINE: See separate description
TV: See below.

TV (video signal) triggering (PAL)

When TV triggering is selected, the TV sync separator will be activated,
it separates the sync pulses from the video content and thus allows a
stable display independent of the video content. Depending on the point
of measurement, video signals (Complete composite video signals) are
either positive or negative. It is necessary to select the correct SLOPE
(13) in order to effectively separate the sync pulses. The direction of the
fi rst slope of the snyc pulses is important, the signal display must not
be inverted. If the sync pulses are above the video, negative slope is to
be selected. If the sync pulses are below the video, their fi rst slopes are
negative, hence positive SLOPE must be selected. If the slope selection
was wrong, the display will be unstable resp. will not be triggered,
because it will be the video which generates the trigger. TV triggering
should use the auto trigger function. If internal triggering is selected,
the height of the display must be
The sync signals consist of frame and line pulses which differ in their
duration. In the PAL standard, the line sync pulses are 5 μs of 64 μs
for a full line. The frame pulses consist of several pulses of 28 μs each
with a repetition period of 20 ms for each half frame. Both sync pulses
differ hence in their duration and their rep rate. Triggering is available
from both line and frame pulses.

Frame pulse triggering

For frame synchronization a TIME/DIV setting of 0.2 s/DIV to 1 ms/DIV
is appropriate, at 2 ms/DIV a full half frame will be shown.
Triggering on the frame pulses with chopped dual trace
operation is discouraged because this will cause visible
interference. This is why in TV trigger mode automatically
the alternating dual trace mode will be set. If desired,
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STOP pressing the DUAL
manually change between alternate and chopped modes;
As soon as the TIME/DIV selector is operated, the alter-
nate DUAL mode will be automatically selected.
At the left side of the screen a portion of the triggering frame pulses
will be visible, at the right hand side of the screen the frame pulse for
the next half frame is visible, consisting of several pulses. The next
half frame will thus not be displayed under these circumstances. The
frame pulse following that half frame will trigger again a display. With
the shortest available hold-off time selected each 2nd half frame will
be displayed. Which half frame will be displayed is up to chance. A short
disruption of the trigger may cause triggering on the other half frame.
The X magnifi er X-MAG/x 10 can be used to expand the display in order
to see individual lines. Starting from the frame pulse also the TIME/DIV
knob can be used for expansion, however, this will cause an apparently
untriggered display as each half frame will trigger a display; the reason
is the ½ line displacement between the half farmes.

Line sync triggering

Each sync pulse can trigger a line display; the TIME/DIV
be set between 0.5 ms/DIV to 0.1 μs/DIV. In order to display single lines a
setting of 10 μs/DIV is recommended, appr. 1 1/2 lines will be visible. In
general, the complete composite video signal has a sizeable dc content.
36 Subject to change without notice
>
5 mm.
mode button for some time will
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knob should
If the video content is constant (as is the case with test patterns), the
dc can be removed by selecting AC trigger coupling. If the video con-
tent changes as is normal with any program, DC coupling is required,
otherwise the display will shift vertically depending on the video content.
Use the POSITION control to keep the display within the screen area.
The sync separator circuit is also effective with external triggering. Of
course, the specifi ed voltage range (see the specifi cations) must be
observed. Note that the polarity of external sync signals can be any,
i.e. it can differ from that of the composite signal at the vertical input,
hence the SLOPE must be selected accordingly. In order to check the
external trigger signal, display it by applying it to a vertical input with
internal triggering.

LINE triggering

In the LINE trigger mode a signal from the mains power supply is taken
(50/60 Hz). This mode is recommended for all signals of mains frequency
or synchronous with it. Within limits this also applies for multiples or
submultiples of the mains frequency. LINE triggering will also yield
stable displays if the input signal is very small, i.e. below the trigger
threshold. It is hence especially handy for all kinds of mains frequency
ripple or interference measurements. With LINE triggering the SLOPE
selection will select the positive or negative half wave and not the slope,
hence it may be necessary to pull the mains plug and insert it upside
down if that is possible (not in all countries). In the auto trigger mode
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the TRIGGER-LEVEL will allow to move the trigger point within the
half wave selected. In the normal trigger mode the trigger point can
also be moved outside the selected half wave.
Magnetic interference from the mains can be detected by using a pick-
up coil which allows to determine the direction and the amplitude. The
coil should preferably sport a high number of turns of thin enamel wire
on a small coil former, a shielded cable with a BNC connector should
be used for the connection to the scope. A 100 Ω resistor should be
inserted between the cable and the BNC in order to reduce HF inter-
ference, a ceramic capacitor to ground may be additionally required.
Also, the coil should have a static shield (no short circuit winding). By
turning the coil the minima and maxima of the magnetic interference
are detectable.

External triggering

External triggering is selected by pressing the button EXT
disconnect the internal triggering. The external signal is to be connected
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to the EXT.TRIG/Z-INP BNC connector, see the specifi cations for the
required signal levels. The external trigger signal must be synchronous
to the measuring signal at the Y input(s), but its shape may be entirely
different. Within limits triggering is even possible from multiples or
submultiples of the measuring frequency. A stable phase relationship
is, however, necessary. There may be a phase difference between the
measuring and triggering signals; if the phase difference happens to
be 180 degrees, the other slope has to be selected, else the signal will
be displayed with a starting negative slope although the positive slope
was selected.
The maximum input voltage at the EXT.TRIG./Z-INP
BNC connector is 100 V (DC plus peak AC). The input impe-
dance is 1 MΩ//15 pF.
STOP
The trigger coupling modes are also effective with external triggering.
The only difference to internal triggering is a capacitor in the signal path
(except with DC coupling), the lower bandwidth limit is 20 Hz.

Triggered state indicator LED TRIG'd

The following explanations refer to the TRIG'd – LED – indicator which
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is listed under in the chapter Controls. It will light up if
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, this will
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