HP 8901B Operation And Calibration Manual page 297

Operation Mode18901B
The broadband audio signal is first measured with a true
rms
detector; then the audio signal path is
switched through a notch filter set to the fundamental frequency (1 kHz or
400
Hz).
The notch filter
has approximately 55.
dB
rejection at the notch frequency with 10% total bandwidth. The distortion is
computed as the ratio of the signal from the output of the notch filter to
the signal without filtering.
The function of the MODULATION OUTPUT/AUDIO INPUT is controlled by the AUDIO INPUT
key. The connector has an impedance of approximately
100
kR when used
as
an input and 600R when
used as an output.
The true rms level (from
OV
to 4V) of any external audio signal (from
20
Hz to 40 kHz) can be measured
using the same rms detector. (In this mode, the MODULATION OUTPUT/AUDIO INPUT connector
I
Auto Distortion and Level Block Diagram
COMMENTS
Audio Level Measurement in Watts
Selection of watts as the unit for audio level measurements results in an error.
Automatic Selection of RMS and PRE DISPLAY
Selection of an audio distortion or level measurement automatically selects the true rms audio detector
and sets
FM
de-emphasis to the pre-display mode. (Refer to FM D e - e m p h i s on page
3-159.)
The SINAD measurement mode (Special Function 29)
is almost identical to the audio distortion
measurement mode. SINAD, as measured by the instrument, is the reciprocal of distortion. (Refer to
SINAD on page
3-203.)
Relative Measurements
If audio distortion or level are to be displayed relative to a reference, enter the value
as a ratio reference
by selecting RATIO. (Refer to Ratio on page
3-215.)
The measured results can be displayed in either
linear units (mV,
%,
etc.) or logarithmic units (dBV,
dB,
etc.) using the LOG/LIN key. The previously
selected units are reinstated whenever an audio measurement is selected.
3-202
Audio Distortion and Level