BK Precision 5105B, Instruction Manual, Page: 17 / 48

17
Subject to change without notice
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
readout 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 first of all (625/50 or 525/60).
The time base setting should be adapted, 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 us/cm is recommended, this will show 1 ½ 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 like
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 specifications 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 vi-
deo. In case of doubt display the external trigger signal.
LINE trigger
Consult SOURCE
22
in ”Controls and Readout“ for specific
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 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 22 > Alt. 1/2. The readout
will display Tr:alt, but no more the trigger point symbol
indicating level and time position. Instead an arrow pointing
upwards will indicate the trigger time position if this lies within
the screen area.
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.
External triggering
In analog mode this trigger mode may be selected with
SOURCE
22
> Extern. In DSO mode it is only possible if
channels 3 and 4 are turned off. The readout will display Tr:ext.
CH4
will be the input for the external trigger, all internal
sources will be disconnected. In this mode the trigger point
symbol (level and time position) will not be displayed, only the
trigger time position will be indicated. External triggering
requires a signal of 0.3 to 3 V
PP
, synchronous to the vertical
input signal(s).
Triggering will also be possible within limits with multiples or
fractions of the vertical input signal frequency. As the trigger
signal may have any polarity it may happen that the vertical
input signal will start with a negative slope in spite of having
selected positive slope; slope selection refers now to the
external trigger.
Indication of triggered operation (TRIG’D LED)
Refer item
23
in ”Controls and Readout“. The LED labelled
TRIG’D indicates triggered operation provided:
– Sufficient amplitude of the internal or external trigger
signal.
– The trigger point symbol is not above or below the signal.
If these conditions are met the trigger comparator will output
triggers to start the time base and to turn on the trigger
indication. The trigger indicator is helpful for setting the trigger
up, especially with low frequency signals (use normal trigger)
resp. very short pulses.
The trigger indication will store and display triggers for 100
ms. With signals of very low rep rate the indicator will flash
accordingly. If more than one signal period is shown on the
screen the indicator will flash each period.
Hold-off time adjustment
Consult ”Controls and Readout“ HOR
30 >
Hold-off time for
specific information.
T r i g g e r i n g a n d t i m e b a s e