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Model
339A
Section
111
SECTION
III
OPERATION
3-1.
INTRODUCTION.
3-2.
This
section
contains information
and
instructions
necessary
for
operation
of the
Model 339A
Distortion
Measurement
Set.
Included
is
a description
of operating
characteristics, a
description of operating controls
and
indicators,
and
functional
checks
to
be
performed
by
the
operator.
3-3.
OPERATING
CHARACTERISTICS.
3-4,
General.
3-5.
the Model 339A
is
designed
to
measure
Total
Harmonic
distortion
(THD)
of
signals
having
a
fundamental frequency between
10
Hz
and
1
10
kHz.
the
analyzer
section
of
this
instrument
measures
total
harmonic
distortion
levels
from 100%
(0
dB)
full-scale
to
.01%
(-80
dB)
full-scale in
nine ranges
as selected
by
the
DISTORTION
RANGE
control,
to
simplify operation,
the
analyzer
section features
both automatic
"set level"
and
automatic
"nulling".
3-6.
The Auto
Set Level feature automatically
sets
the
measurement
reference
level
over
a
10
dB V
range.
If
the
input
signal
is
outside
this
range,
an
LED
on
the front
panel
indicates
whether
the
INPUT
RANGE
control
setting
must
be increased or decreased
to
be within the
"pull-in"
range
of the
Auto
Set
Level.
3-7.
The Auto
Nulling
feature
is
fully
automatic
when
the
339A
internal oscillator
is
used
as
the signal source.
When
using
an
external
signal source,
an
LED
on
the
front
panel
indicates
which
direction the
FREQUENCY
controls
must
be rotated
to
be within
the
Auto
Nulling
range.
3-8.
The Model 339A
includes
an
AM
detector
which
has
a carrier
frequency range
of
550
kHzto
1.6
MHz.
The
AM
detector permits the
measurement
of
THD
of
a
modulation
signal.
3-9.
The
signal
source used
in
the
Mode! 339A
is
a
"bridged
T"
oscillator
which
provides
a
low
distortion
sine-wave
signal
from
10
Hz
to
1
10
kHz.
The
operating
frequencies of the
oscillator
and
the
analyzer notch
filter
are
set
simultaneously.
The
output
level
of the
oscillator
is
variable
from
1
mV
rms
full-scale to 3
V
rms
full-scale
into a
600
fl
load.
3-10.
The
ac voltmeter
section
of the
Model 339A
measures
the true
rms
value
of
input voltages
from
1
mV
full-scale to
300
V
full-scale
in
twelve
ranges.
Frequency
response
of the
meter
section
is
10
Hz
to
1
10
kHz.
3-11.
True
RMS
VS
Average
Responding
Detection.
3-12.
Since the
339A
employs
a true
rms
converter
to
detect the
measurement
signal,
it
is
less
susceptible
to
errors
than average responding
devices.
Most
average
responding meters
are
calibrated to indicate the
rms
value of
a
pure
sine-wave.
When
reading
a
pure
sine-
wave, both
the true
rms and
average responding
meters
will
give the correct indication.
However,
when
reading
complex
signals the
average responding meter
may
be
in
error.
The amount
or
error
depends
upon
the particular
signal
being
measured.
As
an example;
when
measuring
a
square-wave,
the
true
rms
meter
will
give the correct indication of the
rms
value.
The
average responding meter however,
will
read
1
1%
high.
The
average responding meter
is
also affected
by
signals
with
harmonic
content.
The
amount
of
error
introduced
by an average responding meter due
to
harmonics
is
dependent upon
the
relative
amplitude,
phase,
and
order of
the
harmonic.
The
third
harmonic
usually
causes
the greatest
amount
of
error.
For example,
when
measuring
a signal
with
third
harmonic
content,
an
average responding
meter can
be
in
error
by
+5%
to
-20%
depending
upon
the
amplitude
and
phase
of
the
harmonic,
relative to
the
fundamental
frequency.
Due
to
the errors
inherent
in
average responding
meters,
a
distortion
analyzer
which employs
this
type of detector
will
also
be subject
to the
same measurement
errors.
These
errors
can cause
indicated distortion readings
to
be
as
much
as
1.3
dB
below
the actual
rms
value
for certain
combinations
of
second and
third
harmonics.
The
Model
339A
is
not
affected
by
the errors associated
with average
responding
detectors
and
will
provide
more
accurate
measurement
indications.
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