Mode18901B Operation
Audio Distortion and
Level
FUNCTIONS
AUDIO DISTN,
1
kHz
DISTN,
400
Hz
DISTN, RMS, AUDIO INPUT, Special hnction 30 (External
Audio RMS Level)
DESCRIPTION
Measure distortion on either internally demodulated audio
signals
or externally input
signals
with
fundamental frequencies of
1
kHz
or 400
Hz.
(The fundamental frequency must be within f5%
of the nominal
1
kHz
or
400
Hz
for accurate measurements.) Once selected, the measurement
frequency remains unchanged and
is
automatically activated each time the audio distortion (or SINAD)
measurement
is
selected.
Special hnction 30.0 enables the instrument
to
measure the true
rms
level of an external signal
applied
to
the MODULATION OUTPUT/AUDIO INPUT connector. (The connector
is
automatically
switched
to
function
as
an input upon selection of Special F’unction 30.0. The true
rms
level of the
internal demodulated audio cannot be measured.) Connector input impedance
is
nominally 100
kR.
The maximum measurable input level
is
4
Vrms.
PROCEDURE
To measure distortion on the demodulated audio signal, select AUDIO DISTN (Shift., FM).
To measure distortion on an external audio signal applied
to
the MODULATION OUTPUT/AUDIO
INPUT, select AUDIO DISTN (Shift, FM) and AUDIO INPUT. (Only the true
rms
detector can be
used for these measurements.)
To change the measurement bandwidth of the internally demodulated audio signal, Select the desired
filter
keys.
(Refer
to
Audio
Filters
on page 3-179.)
If
measurements are
to
be made on a
1
kHz
audio signal, select
1
kHz
DISTN (Blue Key, PEAK+). If
measurements are to be made on a 400
Hz
audio signal, select
400
Hz
DISTN (Blue Key, PEAK-).
To measure the true
rms
level of
an
audio signal applied
to
the MODULATION OUTPUT/AUDIO
INPUT, select 30.0 SPCL.
HP-IB PROGRAM CODES
AUDIO DISTN=S2
AUDIO INPUT
=
A1
MODULATION OUTPUT
=
A0
1
kHz
DISTN
=
D5
400 Hz
DISTN
=
D6
LIN results
=
LN
LOG results
=
LG
mV units
=
MV
UV
units
=
UV
VOLTS units
=
VL
SPCL
=
SP
MEASUREMENT TECHNIQUE
To measure audio distortion, the instrument compares the input signal level measurement made with
a true
rms
detector
to
the same measurement made with the fundamental component removed. Total
harmonic distortion
is
then calculated as the ratio of the signal with its fundamental removed to the
unfiltered signal.
Audio Distortion and Level 3-201
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