33
2.2.3.3.
AC/O/DC
switch
The
signals
under
observation
are
fed
to
input
socket(s)
A
and/or
B
and
the
AC/O/DC
switch
is
set
to
either
AC
or
DC,
depending
upon
the
composition
of
the
signal.
As the
vertical
amplifier
is
d.c.
coupled,
the
full
band-
width
of
the
instrument
is
available
and
d.c.
components
are
displayed
as trace
shifts in
the
DC
position
of the
AC/O/DC
switch.
This
may
be
inconvenient
when
small
signals
superimposed
on
high
d.c.
voltages
must
be
displayed.
Any
attenuation
of
the
signal
will
also
result
in
attenuation
of the
small
a.c.
component.
The
remedy
is
to use
the
AC
position
of
the input
switch,
which
employs
a blocking
capacitor,
to
suppress
the
d.c. and
l.f.
signals.
Some
pulse
drop
will
occur
when l.f.
square
wave signals
are displayed.
The
0
position
interrupts
the signal
and
earths
the
amplifier
input for
quickly
determining
the 0 V
level.
2.2.4.
Triggering
If
a signal
must be
displayed,
the horizontal
deflection
must
always
be started
on one fixed
point of the
signal
in order
to
obtain
a stationary display.
The
time-base
generator
is, therefore,
started
by narrow trigger
pulses
formed
in the
trigger unit and
controlled
by
a
signal
originating from
one of the
vertical
input signals, an
internal
voltage
at mains frequency
or an external source.
2. 2. 4. 1.
Trigger
coupling
Three
different
trigger-coupling methods
can be chosen with the
DC/LF/HF switch.
In the FIF and LF
positions, the transfer characteristic
is
limited.
In
position DC the trigger signal
is
passed unchanged.
In
position LF,
a 0 Hz
(7
Hz for
external triggering) to 30
kHz band-pass
filter is
inserted.
This position
can
be used
to
reduce
interference
from
noise.
In position HF,
a 30
kHz high-pass
filter is inserted.
This
position can be
used to reduce interference
from
e.g. hum.
2.2.4. 2. Selecting the
trigger source and setting the trigger level
The trigger signal
is obtained from
channel A (button A depressed),
channel
B (button B depressed), the
COMPosite A and B signals (buttons A and B simultaneously
depressed), an external
source (button
EXT
or
EXT
-j-
10
depressed) or
from
an
internal voltage
at
mains frequency (button EXT and EXT 10 depressed).
The
trigger pulse shaper is
a
dual controlled multivibrator switched
by
the output signals of
a
differential
amplifier.
The trigger signal, together with biasing voltages
which
are
adjustable with the LEVEL potentiometer, fed to
the inputs
of
the differential amplifier.
Depending on the LEVEL setting,
a
certain part of the trigger signal will be
amplified
by the
differential
amplifier.
The multivibrator is thus switched at a fixed point
of the trigger signal
(see
Fig.
2.4.). This
means that, with
the
aid
of
the LEVEL control, it is possible to
scan the shape of the trigger signal (in case of internal triggering
A or B equal
to
the shape of the signal
to
be
displayed) and, thus,
to
choose the
point
where the multivibrator
will be switched.
The LEVEL potentiometer
is
fitted with
a
push-pull
switch
which allows selection
of
the trigger slope.
2. 2. 4. 3. Automatic triggering
When the AUTOmatic button of the AUTO-TRIG-SINGLE
switch is operated, and
if
there are no trigger
pulses available,
the
time-base generator is automatically
free-running,.
The trace
is,
therefore, always visible.
The AUTOmatic mode can be used in all cases where also the
TRIG mode
is usable, except
with signal frequencies lower than 10
Hz
or
pulse
trains
with
an
off time exceeding 100 ms.
As
soon
as
trigger pulses
are
available, the free-running
state of
the time-base generator is
automatically
terminated and the time-base
generator
is
triggered again as
described in sections 2. 2.4.1. and 2. 2.4. 2.
When the
TRIGgered or SINGLE
button is operated,
the auto-circuit is switched off. The LEVEL setting
can also be
used
in
the AUTOmatic mode.
2.2.4.4. SINGLE
sweep triggering
When effects which
occur only once have
to be
observed (usually photographed),
it is often desirable to ensure
that only one
sawtooth is generated, even though several trigger
pulses
might
be produced after
the
phenomenon
of
interest. Of course,
the
single
sawtooth in question
must be triggered
by a
trigger pulse. To
this
end, the SINGLE
button must be pressed. The first trigger pulse
that
appears after the button has
been
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