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
De-emphis
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
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