HP 8591C, Руководство пользователя, Страница: 266 / 373

Test Descriptions
Carrier-to-Noise Ratio Test Description
More about the
MORE INFO
Screen (continued)
Try raising level even more.:
This message assumes the technique mentioned just above was followed but even more dynamic
range is desired. It suggests following the above technique but raising the level 20, 30, or more
T r y r e d u c i n g t h e i n t e r n a l a t t e n u a t o r . :
This message appears instead of the message just above when the attenuator is
The messages above are the analyzer best guess as to what to do given the conditions it can
determine. They are not foolproof and are intended to be taken as suggestions only, not concrete rules
for what to do.
Once the optimum input power level is achieved, it does not need to be
readjusted for each channel. All channels can be measured without further
adjustments to the input level.
The internal preamp is controlled automatically. It is only turned on at
levels low enough to prevent overload. Under some conditions the internal
attenuator is set to 10 to prevent preamplifier overload. When the
internal preamplifier is turned on, a message to that effect is displayed at the
lower right hand corner of the screen.
The carrier-to-noise test has 2 variations. The first is the traditional method
where the video modulation is either left on and the noise is measured at
the lower channel boundary, or the video modulation is turned off and the
noise is measured in the frequency range specified by the FCC.
second
variation is measuring the noise during the vertical interval on a single
horizontal line that has no modulation. This is referred to as a “quiet line”.
The analyzer defaults to line 12, but any quiet line can be used. Line 12 is
quiet on most channels.
Measuring noise on a quiet line is called a “gated” measurement. That is, the
analyzer gates its frequency domain measurement during a specific time slot;
that slot being during the selected quiet line. Option 107 is required to make
this measurement.
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