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3.2
GENERAL
SUMMARY
The
signal
to
be
measured
is first
fed to the
LEVEL
measuring
circlets to
determine
the correct
input
conditions for the
DISTORTION
measurement
and
to
provide a
LEVEL
measurement
ty.
^
The
signal
is
fed via
LEVEL
input
attenuator
(SI)
to
impedance
converter
and
thence
to
LEVEL
AC/DC
converter
to
provide a
+5.
25 voltage
for full-scale defie
t:
LEVEL
meter.
With
the distortion
measurement
mode
selected the signal
fcom
the
MVEL
tar,eouslv fed
via
FUNCTION
select
switch
(S2)
to
one
of
the
two
internal
filters.
The
signal stripped
S
is
attenuated
and
fed
via
the
AC/DC
converter
to the
-ratio*
circ^t
together with the signal
from
the
LEVEL
AC/DC
converter.
Thus
both
sig^s
c^be
t2
'ratio*
circuit (as
described
in
Section 3.3.
1
and
defined
in
Section 3.1)
and
their ratio
.indicated
directly
by
the
DISTORTION
meter
(M2).
3.3
CIRCUIT DESCRIPTION
LEVEL EANGE ATTENUATOR
(SI).
Front panel
mounted 6-way
push
button switch consisting
of
one
'straight
through' position
and
five
10
dB
steps.
LEVEL IMPEDANCE CONVERTER
(QlOl,
102).
Large
gain amplifier
wi^
provides high
input
and low
output
impedance
with a
wide
band
frequency
Teepee.
Although
distortion
is
low,
any
dlstLlon
induced
by
the amplifier
is
minimized by
adjustment
of
the
bias voltage
to
QlOl.
LEVEL
AMPUFIER
(QX03
to
Q106).
To
minimize
Induced
gain
and low closed
loop
gain.
The
gain
is
adjusted
for
lo.
4
dB by H119
(LEVEL
GAIN).
The
output of
this
stage
is
also
fed to the
FUNCTION
switch
S2.
LEVEL AC/DC CONVERTER
{QA101/QA102)
uses
diode full-wave
rectification.
To
reduce
the effects
of the
non-iinearity of the
rectifying circuit
a negative feedback
system
also using diodes
is
mcorpor-
The
d. c.
output
to
Ml (LEVEL METER)
is
determined by
the setting of
R134
^
RU9
of the
previous
stage.
The
d. c.
output
required
for
a
full-scale deflection of
Ml
is
+5. 2b
volts.
FUNCTION
switch
(S2)
consists of four
push
buttons.
With
the
LEVEL
push
button selected the output
from
the
LEVEL
amplifier
to
the distortion
measuring
circuits
is
open
circuited.
It
should
be
noted
that
although
the distortion circuits
are switched
out
with
the selection of the
LEVEL
push
button, the level
measurmg
circuits
are always
operative.
Selecting either
of the
400
Hz
or 1000
Hz push
buttons, applies the
total
signal (suitably attenuated
by
the
LEVEL
input attenuator) to the
appropriate
filter.
The
CAL
push
button
when
operated
applies the output
from
the
LEVEL
amp, attempted
by^e
full
60
dB
of the
DISTORTION
range
switch
(S3),
directly to the
DISTORTION
amplifier.
The
levels
are
set
such
that vdien the
LEVEL
meter
reads
Ml-scale
the
reads
full-scale.
The
check
Is
completed
by
reducing
the input signal level
by
10
dB
and check-
ing that
the
distortion
meter
reading varies
by no
more
than 1%,
FUNDAMENTAL
REJECTION FILTERS
are
notch characteristic as
shown
in
Fig.
3. 2.
composed
of active
twin-T
four stage
elements
having a
Two
filters
are
fitted
having fundamental
rejection of
400
Hz
and 1000 Hz,
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