Normal Trigger; Slope Selection; Trigger Coupling; Tv (Video Signal) Triggering (Pal) - Hameg Instruments HM 400 Manuel

it becomes hence independent of the amplitude and the shape of the
signal. The duty cycle may e.g. vary from 1 : 1 to 1 : 100 without loss of the
trigger. It may, however, be sometimes necessary to set the TRIGGER-
LEVEL control close to one of its extremes. The next measurement may
require another setting. The simplicity of operation recommends the
auto peak-to-peak triggering for most uncomplicated measurements. It
is also a good start with difficult problems, especially, if the properties
of a signal like amplitude, frequency and shape are unknown.
The auto peak-to-peak trigger mode is independent of the trigger source
and operates above 5 Hz, i.e., if the repetition frequency of the triggering
signal is lower, the time base will freerun.
6.2

Normal trigger

In this mode all settings are up to the user, and there is no visible trace if
there is no sufficient trigger signal. See the chapter Controls for specific
information about the functions of the controls SLOPE
10 16
LEVEL , and TRIGGER to
additional use of the functions Time Base Variable (VAR) and HOLD-OFF
time adjustment.
In the normal trigger mode the trigger signal can be de-
rived from any portion of the rising or falling slopes of the
signal by proper setting of the TRIGGER-LEVEL knob. The
available trigger range depends on the amplitude of the
signal.
If the signal amplitude on the screen is
the adjustment may become critical due to the small range available
and require some care. As mentioned there will be no visible trace if
the TRIGGER-LEVEL setting is false or if the trigger signal is missing
or insufficient. The normal trigger mode allows to also trigger on
complicated signals. With mixed signals it is, however, necessary that
repetitive signal peaks are present which can be caught by careful
operation of the TRIGGER-LEVEL control.
6.3

SLOPE selection

9
With the SLOPE button the signal slope is selected, see the chapter
Controls. This selection is always valid, also in AUTOSET mode. A rising
slope is defined as a portion of a signal which rises from a given potential
to a more positive one, a falling slope correspondingly is defined as a
down slope from a given potential to a more negative one.
6.4

Trigger coupling

See the chapter Controls for specific information about the controls
9 10
SLOPE , TRIGGER-LEVEL
of trigger coupling AC or DC remains unaffected by AUTOSET. See
the specifications for the passbands of the various modes of trigger
coupling. With DC or LF coupling use the normal trigger mode and the
TRIGGER-LEVEL knob. These modes are available:
AC: This is the standard coupling mode. It has a low and a high frequency
limit, below resp. above these limits the trigger threshold rises.
DC: DC coupling is effective from DC to the upper frequency limit. This
mode is recommended for slowly varying signals when triggering
on a definite portion is desired or when the duty cycle of signals
varies.
LF: When LF is selected, a low pass filter is inserted in the trigger path.
In combination with the normal trigger mode there is no lower
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
9
, TRIGGER-
23
. Complex signals may require the
<
1 DIV with internal triggering,
16 23
, and TRIGGER to . The selection
T r i g g e r i n g a n d t i m e b a s e
jitter resp, multiple displays, especially with small input voltages.
Above the bandwidth of the low pass filter the trigger threshold
rises sharply.
LINE: See separate description
TV: See below.
6.5

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
in order to effectively separate the sync pulses. The direction of the first
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 first 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.
6.6

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,
pressing the DUAL 32
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 magnifier 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.
6.7

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.
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,
> 5 mm.
mode button for some time will
15
knob should
Subject to change without notice
35