HP 8901B Operation And Calibration Manual page 279

Operation Model 8901B
BLOCK DIAGRAM
MODULATION OUTPUT/
AUDIO INPUT
I
C f l l I N J E R
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.. .
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A N 0 DISPLAY
I
LOCAL
OSCILLATOR
DETECTOR
DISTORTION
AUOlO RMS
AUOIO
COUNTER
n
Audio Filters Block Diagram
COMMENTS
Filter Selection Affects Modulation Output
The selected filters are always in the path of the demodulated signal independent of the selection of a
modulation measurement. Thus, unless turned
off,
the output of MODULATION OUTPUT/AUDIO
INPUT is filtered. In addition, the selected filters remain in effect when switching between modulation
measurements.
Automatic Filter Selection
Under certain conditions, filters are inserted automatically. When the 455 kHz
IF is
used, or when the
RF input is less than 2.5 MHz, the 15 kHz low-pass filter
is
inserted unless another low-pass filter has
been selected. (The ALL OFF >200
kHz
low-pass filtering cannot be selected.) The 15
kHz
low-pass
filter selected when operating with the 455 kHz IF is automatically turned off when the 1.5 MHz IF
is selected.
Bandwidth Limitations
With no filters selected, the post-demodulation bandwidth is from less than 20 Hz to greater than 200
kHz. (These frequencies refer to the limits over which measurement accuracy is specified, not the
3
dB
bandwidth.) However, the IF circuits and demodulators impose more severe bandwidth limits. Refer
to each specific measurement in these Detailed Operating Instructions for bandwidth limitations.
3-180 Audio Filters