17
Subject to change without notice
HF:
A high pass is inserted in the trigger channel, thus blocking low
frequency interference like fl icker, noise etc.
Noise Reject:
This trigger coupling mode or fi lter is a low pass suppressing
high frequencies. This is useful in order to eliminate hf inter-
ference of low frequency signals. This fi lter may be used in
combination with DC or AC coupling, in the latter case very low
frequencies will also be attenuated.
LF:
This is also a low pass fi lter with a still lower cut-off frequency
than above which also can be combined with dc or ac coupling.
Selecting this fi lter may be more advantageous than using DC-
coupling in order to suppress noise producing jitter or double
images. Above the pass band the necessary trigger signal will
rise. Together with ac coupling there will also result a low
frequency cut off.
Video (tv triggering)
Selecting MODE
>
Video will activate the built in TV sync se-
parator. It separates the sync pulses from the picture content
and enables thus stable triggering independent of the changing
video content.
Composite video signals may be positive or negative. The sync
pulses will only be properly extracted if the polarity is correct.
The defi nition of polarity is as follows: if the video is above the
sync it is positive, otherwise it is negative. The polarity can be
selected after selecting FILTER. If the polarity is wrong the
display will be unstable or not triggered at all as triggering will
then initiated by the video content. With internal triggering a
minimum signal height of 5 mm is necessary.
The PAL sync signal consists of line and frame signals which
differ in duration. Pulse duration is 5 μs in 64 μs intervals. Frame
sync pulses consist of several pulses each 28 μs repeating each
half frame in 20 ms intervals.
Both sync pulses differ in duration and in their repetition inter-
vals. Triggering is possible with both.
Frame sync pulse triggering
Remark:
Using frame sync triggering in dual trace chopped mode may
result in interference, then the dual trace alternate mode
should be chosen. It may also be necessary to turn the read-
out off.
In order to achieve frame sync pulse triggering call MODE,
select video signal triggering and then FILTER to select frame
triggering. It may be selected further whether ”all“, ”only even“
or ”only odd“ half frames shall trigger. Of course, the correct TV
standard must be selected fi rst of all (625/50 or 525/60).
The time base setting should be selected to suit; with 2 ms/cm
a complete half frame will be displayed. Frame sync pulses
consist of several pulses with a half line rep rate.
Line sync pulse triggering
In order to choose line snyc triggering call MODE and select
VIDEO, enter FILTER, make sure that the correct video standard
is selected (625/50 or 525/60) and select Line.
If ”ALL“ was selected each line sync pulse will trigger. It is also
possible to select a line number ”LINE No.“.
In order to display single lines a time base setting of TIME/DIV.
= 10 μs/cm is recommended, this will show 1½ lines. In general
the composite video signal contains a high dc component which
can be removed by ac coupling, provided the picture is steady.
Use the POSITION control to keep the display within the screen.
If the video content changes such as with a regular tv program
only DC coupling is useful, otherwise the vertical position would
continuously move.
The sync separator is also operative with external triggering.
Consult the specifi cations for the permissible range of trigger
voltage. The correct slope must be chosen as the external
trigger may have a different polarity from the composite video.
In case of doubt display the external trigger signal.
LINE trigger
Consult SOURCE
18
in ”Controls and Readout“ for specifi c
information.
If the readout shows Tr:Line the trigger signal will be internally
taken from the line (50 or 60 Hz).
This trigger signal is independent of the scope input signals and
is recommended for all signals synchronous with the line. Within
limits this will also be true for multiples or fractions of the line
frequency. As the trigger signal is taken off internally there is
no minimum signal height on the screen for a stable display.
Hence even very small voltages like ripple or line frequency
interference can be measured.
Please note that with line triggering the polarity switching will
select either the positive or negative half period of the line, not
the slope. The trigger level control will move the trigger point
over most of a half wave.
Line frequency interference may be checked using a search
coil which preferably should have a high number of turns and
a shielded cable. Insert a 100 Ω resistor between the center
conductor and the BNC connector. If possible the coil should
be shielded without creating a shorted winding.
Alternate trigger
This mode is selected with SOURCE
18
>
Alt. 1/2. The readout
will display Tr:alt, but no trigger point symbol indicating level
and time position.
This trigger mode is to be used with greatest care and should be
an exception rather than the rule, because the time relationships
visible on the screen are completely meaningless, they depend
only on the shape of the signals and the trigger level!
In this mode the trigger source will be switched together with
the channel switching, so that when CH1 is displayed in the
dual channel alternate mode the trigger is taken from CH1
and when CH2 is displayed the trigger is taken from CH2. This
way two uncorrelated signals can be displayed together. If this
mode is inadvertently chosen the time relationships between
the signals will also be lost when both signals are correlated!
(Except for the special case that both happen to be square waves
with extremely fast rise times). Of course, this trigger mode is
only possible in the dual channel alternate mode and also not
with external or line trigger. AC coupling is recommended for
most cases.
T r i g g e r i n g a n d t i m e b a s e
