HP 339A - Section III Operation; Operation Introduction and Characteristics; Distortion Measurement Procedures; Voltmeter, Relative Level, and Oscillator Operation

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.