To
measure
the
strain
on
the
test
object
a resistance strain gage should be
used and a Strain Gage
Apparatus
1516
will be
found
ideal as
the
measuring
bridge. The
output
voltage from
the
Strain Gage Apparatus can be fed
directly
to
a Level Recorder with linear
potentiometer
for
automatic
record-
ing.
An example
of
such a recording,
taken
on
a
thin
metaI bar, showing
the
mechanical strain and indicating its resonant frequency,
is
shown
in
Fig
.
5.21.
DDDDDDOOOODDODODDDDDDDODDDJ
50
db
20
10
o
50
100
200
Hz
300
~o
1?DD
I{q
Fig.5.21. Recording
of
mechanical strain in a
bar.
Instrumentation set-up
as
in Fig.5.20.
The
reactance circuit
is
needed
to
controi
the
actual frequency deviation
(modulation swing)
of
the
fixed oscillator by producing an inductive react-
ance across
the
oscillator's
tuned
collector. The deviation can be varied
linearly from O
to
±
250
Hz
where
the
sensitivity
is
approximately
5 mV/Hz.
A
saw·tooth
generator determines
the
frequency
of
modulation i.e.
the
rate
of
change
of
the
frequency swing
of
the
oscillator. Frequencies
of
1-1.6-2.5-4-6.3-10-16-25
Hz
are available. Provision
is
also made
for
modulation by an external generator. Alternative modulation frequencies
or
wave shapes can
the
n be chosen and frequency swings
of
up
to
±
300
Hz
obtained
(but
the
modulation sensitivity varies).
The
signal
from
the
fixed oscillator
is
passed
to
aregulating
amplifier
which controls
the
output
level.
The
regulating amplifier
is
used
to
perform
the
oscillator
stop
and dead zone blocking. A compressor amplifier can
be
switched
in
to
controi
the
regulating amplifier so
that
constant
output
level
is
obtained.
When
the
instrument
is
being used for instance
to
power a
loudspeaker,
the
compressor circuit can be used with a
microphone
to
main-
tain a
constant
sound pressure level.
The
compressor circuit consists
of
an amplifying stage and a full-wave
averaging rectifier stage.
The
signal from
the
rectifying stage
is
then
used
to
varv
the
gain
of
the
regulating amplifier. A variable
potentiometer
(Com-
pressor Voltage)
in
the
input
circuit
of
the
regulating amplifier
can
be used
to
controi
the
output
power from
the
instrument
when
automatic
regula-
tion
is
used.
The
speed with which
the
variation
in
output
level
is
regulated
back
to
normal
depends
on
the
setting
of
the
Compressor Speed knob,
but
also
depends
to
a certain
extent
on
the
amount
of
the
deviation from
the
normal
level. Compressor speeds
of
30
-
100
-
300
-
1000
dB/sec. are available
and are
determined
by
the
integration time
constant
of
the
rectifying cir-
cuit.
The
input
impedance
of
the
Compressor Input
is
approximately
25
kQ
and
the
maximum
range
of
regulation
is
50
dB. Regulation characteristics
for
different
positions
of
the
Output
Level
potentiometer
are shown
in
Fig.3.2.
The
variable oscillator
is
of
similar design
to
the
fixed
one
except
that
it
has a variable capacitor
in
its
tuned
circuit
to
varv
the
oscillation
frequency
between
100
and
120
kHz. A worm gear,
connected
to
the
capacitor
42
11
