Triggering can be selected on either the rising or falling
edge of the trigger signal depending on whether the
+ / — slope pushbutton is in the out or in position. In the
out position, triggering from the positive-going edge is
selected. The correct slope setting is important in
obtaining a display when only a portion of a cycle is being
displayed.
With
internal triggering
in the
Mono channel mode
on
the Y amplifier, the trigger signal is derived from the
respective channel in use. in the
Dual channel mode
, the
internal trigger signal may be selected from either
Chan
nel I ox ChannelH
using the
CHI/II — TRIG. I
/
II
button; in
the out position, the trigger signal is derived from Chan
nel I. However, it is always preferable to trigger from the
less complicated signal.
For
external triggering,
the small
TRIG. EXT.
pushbut
ton in the X-Section must be depressed. The sync, signal
(0.6-10Vpp)
must then be fed to the
TRIG. EXT.
input
connector.
Coupling mode
and frequency range of the trigger signal
are selected with the
TRIGGER SELECTOR
slide switch
in the X-Section for internal and external triggering. The
HM203-4 has 4 coupling modes:
AC, DC, HF, LF.
The
AC
coupling mode is mainly used.
DC
trigger cou
pling is recommended only, when very low frequency
signals are being investigated and triggering an ap
pointed level value is necessary, or when pulses, which
change strongly their duty cycle during observation time,
have to be displayed. If
DC
coupling is selected, it is ad
visable to use the
Normal Triggering
mode
(AT/NORM.
button depressed), as there is the possibility that, in the
AT
mode, triggering may not be achieved on signals
without zero-axis crossing point (DC offset). However,
automatic peak-to-peak value triggering is not impossible
with
DC
trigger coupling, but it needs a precise adjust
ment of the DC input balance (see page M3).
In the
HF
coupling mode, a
high-pass filter
is switched
into the trigger amplifier. This filter cuts off the DC con
tent and frequency range under 1 kHz of the trigger
signal.
In the
LF
coupling mode, a
low-pass filter
is switched
into the trigger amplifier. This filter cuts off any amplifier
noise and the frequency range above 1 kHz of the trigger
signal.
Frequency ranges of the trigger coupling:
AC
and
DC
to 1MHz,
HF
above 1MHz,
LF
below 1kHz.
If the
video signal of
a television set is to be displayed at
frame frequency,
triggering is generally difficult due to
the presence of the higher line frequency synchronization
pulses contained in the signal. The line pulses can be at
tenuated by switching the
TRIGGER SELECTOR
switch
in the X-Section to
LF.
With
Normal Triggering
and cor
rect setting of the + / — slope button, it will now be
found that the trigger
LEVEL
control can be adjusted to
trigger from either the leading or trailing edge of the
frame pulse. This setting is also advantageous for trigger
ing from other signals that have a recurrence frequency
of 800 Hz or less, as high frequency harmonics or noise
in the signal are suppressed by the presence of the low-
pass filter. However,
TV triggering at line frequency
needs
AC
or
HF
(or
DC
if necessary) setting of the
TRIG
GER SELECTOR
switch in the X-Section. In both cases,
always
Normal Triggering
with
LEVEL
adjustment
should be used.
As already mentioned, simple signals may be triggered
automatically in the automatic trigger mode
(AT/NORM.
button in out position). The repetition rate may also vary
in such cases. However, if the pulse duty factor on
square-wave or pulse signals changes drastically or
deforms to a needle pulse, the
Normal Triggering
mode
with
LEVEL
adjustment may well become necessary.
With composite signals, the trigger facility is dependent
on the occurence of certain periodically recurring levels.
The
LEVEL
adjustment of these signals will require some
care.
If it is found that a trigger point cannot be located on ex
tremely complex signals even after repeated and careful
adjustment of the
LEVEL
control in the
Normal Trigger
ing
mode, a stable display may be obtained using the
TIMEBASE
variable control.
For the purpose of
line triggering
(TRIGGER SELECTOR
slide switch in the X-Section to
LINE),
a (divided) secon
dary voltage of the power transformer is used as a trigger
signal (50-60Hz). This trigger mode is independent of
the signal amplitude or display height and allows a
display below the (internal) trigger threshold. Line trigger
ing is recommended for all signals which are time-related
(multiple or submultiple) to the mains/line frequency or
when it is desirable to provide a stable display of a line-
frequency component in complex waveforms. Therefore
it is especially suited for the measurement of small ripple
voltages from power supply rectifiers or of magnetic or
static leakage fields in a circuit.
In some countries, the standard power plug has sym
metrically arranged plugs (interchanging of Line and
Neutral is possible). In such cases, the + / — slope
pushbutton may indicate the wrong polarity compared
with the display (triggering with falling edge instead of
rising edge). For correction, the power plug of the instru
ment has to be turned.
M9 203-4
