3.3. OPERATION FROM EXTERNAL TRIGGER SOURCE
In
the
"Ext."
(not
"Single
Trigger
-
Ext.")
TRIGGER SOURCE
mode,
the
4913
will
operate
only
when
a
sequental
or
repetitive
signal is
applied
via
the
EXT. TRIGGER INPUT on the
front
panel.
Provided
that
the
signal is a
derivative
of
the
motion
to
be
studied
and
that
this
is
periodic,
then
the
Stroboscope's
lamp
will
provide
a
stationary
image
of
the
moving
object
even
if
its
motion
frequency
is only
approximately
stable
or
varies
owing
to
alteration
of
the
operating
speed.
Furthermore,
the LED
DISPLAY
of
the
Stroboscope
is
capable
of
providing
a
direct
indication
of
the
external
trigger
signal
frequency
from
5 Hz
up
to
9999 Hz (ie., 600
r/min
up
to
600 k
r/min),
thus
enabling
it
to
be
used a
tachometer.
To
permit
operation
from
external
trigger
frequencies
up
to
9999 Hz,
the
Stroboscope
is
designed
to
operate
in
synchronism
with
the
external
signal.
At
trigger
frequencies
up
to
approximately
11
O Hz,
the
flash
ignition
is
synchronized
with
the
fundamental
of
the
trigger
signal,
whilst
at
higher
frequencies
it
is
synchronized
with
the
highest
frequency
sub-multiple
of
the
trigger
signal
below
110 Hz.
As
a
result
correct
synchronism
is
maintained
with
high
frequency
trigger
signals, even
though
the
actual
flash
rate
of
the
lamp
never
exceeds
approximately
11
O flashes
per
second.
The
minimum
input
signal
level required
for
triggering
is
approximately
100
mV
Pk-Pk.
However,
if
required
input
levels up
to
30 V
Pk-Pk
can
be
accommodated
without
overload
of
the
instrument.
To
prevent
false
triggering
by
noise
and
other
unwanted
components
which
may
be
present
with
the
trigger
signal,
it
is
recommended
that
the
TRIGGER
LEVEL
knob
be
set
approximately
one
quarter
of
a
half
turn
to
the
left
or
right
of
its
centre
zero
mark
"O".
The TRIGGER SENSITIVITY
knob
should
then be
adjusted
by
turning
it
clockwise
from
its
"min."
position
and
stopping
the
moment
the TRIGGER
ERROR LED on
the
front
panel
is
extinguished
and a
stable
trigger
frequency
indication
is
obtained
with
the
LED DISPLAY. To
check
that
triggering
is
correct,
select
the
"Phase
Delay"
mode
and
turn
the PHASE DELAY
knob
from
one
end
of
its
range
to
the
other.
The
observed
image
should
move
through
one
complete
motion
cycle.
If not,
the
TRIGGER
LEVEL
knob
should
be
adjusted
a
further
distance
away
from
its
centre
zero
mark
"O"
and the TRIGGER SENSITIVITY knob
readjusted
until
correct
triggering
is
obtained.
Specific
considerations
concerning
external
triggering
by
other
instruments,
as well as
by
photoelectric
and
magnetic
probes,
are
dealt
with in
the
following
sections.
3.3.1. External Instruments
As
discussed
above
any
electrical
signal which is a
derivative
of
the
motion
to
be
investigated
may
be used
to
trigger
the 4913.
With
the
aid
of
an
accelerometer
and
Vibration
Meter,
for
example,
the
vibration
~f
a
stationary
member
used
to
support
the
moving
object
to
be
studied
may
be
monitored
and
the
AC
output
of
the
Vibration
Meter
used for'
triggering
the
Stroboscope.
A
suitable
output
may
also
be
obtained
using a
Tracking
Filter
in
the
monitoring
set-up
or
where a
Vibration
Exciter
is
employed
for
swept-frequency
vibration
testing
of
mechanical
components
and
structures
etc.,
the
constant
level
output
o·f
the
Frequency
Generator
used
to
drive
the
exciter
can
be
employed.
For
these and
similar
applications,
the
trigger
signal
should
be
applied
between
the
centre
contact
and
outer
screen
of the EXT. TRIGGER INPUT
using
a
single
screened
coaxial
cable
with
BNC
plug
fitted
at
one end. The EXT. TRIGGER INPUT has an
input
impedance
of
50
kn
in
series
with
680 nF and
maximum
input
voltage
rating
of
30 V
Peak-
Peak.
15
To Next Page
Loading...