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BRUEL & KJAER 2305 - Electronic System Architecture

BRUEL & KJAER 2305
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Electronic System.
Electronically
the
Recorder
Type
2305
mainly
consists
of
an
input
circuit,
a
Range
Potentiometer,
a
direct-coupled
AC
input
amplifier,
the
specia
l
B
& K
signal
rectifier
arrangement,
and
an
unconventional
DC
output
ampli-
fier
section
which
drives
the
electrodynamic
writing
system
(see
block
dia-
gram
Fig.
1.1).
tnput Circuit.
The
input
of
the
Level
Recorder
is
unsymmetrical.
If
desired
it
can
be
made
symmetrical
by
a
special
Input
Transformer,
see
chnpter
Accessories.
The
signal
is
first
fed
to
the
continuously
variable
input
potentio-
meter
and
then
to
the
calibrated
input
attenuator.
Great
care
has
been
taken
in
the
design
of
the
input
circuit
to
obtain
an
approximately
constant
input
impedance
of
the
Recorder.
The
impedance
varies
between
16
and
18
IdJ,
depending
upon
the
position
of
the
INPUT
POTENTIOMETER,
and
reaches
its
minimum
value
in
the
middle
and
end
positions
of
its
12
dB
range.
Due
to
stray
capacitance
in
the
input
potentiometer
the
frequency
response
will
be
influenced
to
a
small
degree
at
the
highest
frequencies.
Vide
Fig.
1.2
where
the
highest
deviations
for
the
different
positions
of
the
potentiometer
are
illustrated.
The
calibrated
INPUT
ATTENUATOR
attenuates
the
input
signal
in
six
steps
of
10
dB
with
an
accuracy
of
approximately
± 0.2 dB.
Range Potentiometer.
From
the
input
attenuator
the
signal
is
fed
to
the
Range
Potentiometer.
Different
interchangeable
Range
Potentiometers
are
available,
logarithmic
(dB
potentiometers)
as
well
as
linear,
see
also
page
45.
Each
potentiometer
consists
of
precision
resistors
connected
between
216
silver
lamellae
on
which
the
slider
of
the
driving
arm
moves,
again
refer
to
the
block
diagram
Fig.
1.1.
Chopper.
The
voltage
developed
be
twe
en
the
slider
and
ground
is
fed
to
an
electromechanical
chopper.
When
it
is
desired
to
record
AC
signals,
the
c
hopper
arm
is
continuously
in
one
of
its
two
contact
positions.
However,
if
the
Level
Recorder
is
switched
to
re
co
rd
DC
levels
,
the
c
hopper
will
operate
at
twic
e
the
mains
frequen
cy, g
iving
an
AC
input
to
the
following
amplifier.
AC
Input Amplifier.
Th
e
non-chopped
or
chopped
s
ignal
is
applied
to
the
balanced
direct-coupled
AC
amplifier
which
,
in
s
tead
of
a
conventional
AC
amplifier
has
two
distinct
advantages;
unw
a
nted
blocking
of
the
amplifier
due
to
overdriving
(and
thus
overshoot
in
the
recording)
is
eli-
minated,
and
the
innuence
of
variations
in
the
supply
vollage
is
decreased
to
a
minimum.
There
are
several
other
reasons
why
this
type
of
circuitry
is
used.
Firstly,
the
halancing
enables
the
output
from
the
signal
rectifier,
which
follows
immediately
after
the
amplifier,
to
be
symmetrical
wilh
respect
to
ground.
Secondly,
by
using
AC
as
well
as
DC
negative
feed-
6

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