HP 8901B Operation And Calibration Manual page 296

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 f 5 %
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