HP 8591C, Руководство пользователя, Страница: 261 / 373

Test Descriptions
Carrier-to-Noise Ratio Test Description
Initially, the marker is placed at the lower channel boundary. This provides
an indication of the distribution system noise level without having to remove
modulation. The minimum noise level is measured and compared with the
noise level of the analyzer. If the difference between the minimum noise level
and the noise floor is less than 10
the minimum noise value cannot be
measured directly, it is instead calculated by using the following formula:
System noise =
where
DN = displayed noise 10
AN = analyzer noise 10
To find the ratio of the visual carrier level to the noise level, the minimum
noise level value is subtracted from the carrier peak level. The ratio is then
normalized to a 4 MHz noise-power bandwidth.
The message See MORE INFO is displayed if the system noise is within 3 of
the analyzer noise. This can be the case when carrier-to-noise is and
carrier levels are less than 20 Refer to the graphs in the
located in the HP 8591C Calibration Guide.
N O T E
External preamplifier usage can be enhanced by entering amplifier gain and noise figure values in the
Setup
menu. By selecting an external preamplifier in the
Setup
menu, errors caused by the
preamplifier can be accounted for in the carrier-to-noise calculation. When the internal preamplifier
automatically turns on, its gain and noise figure area automatically accounted for and requires no
operator attention.
Averaging minimizes the measurement-to-measurement variation of the
system noise determination.
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