HP 8591C, User Manual, Page: 264 / 373

Test Descriptions
Carrier-to-Noise Ratio Test Description
More Discussion about
The carrier-to-noise test is very flexible and can handle a wide variety of
the Carrier-to-Noise
testing conditions. It provides feedback messages to assist you in achieving
Test
the optimum test setup.
Like any cable TV active device, a spectrum analyzer has an ideal operating
point which balances carrier-to-noise and carrier beats. When a spectrum
analyzer is operating at its optimum point it has its greatest testing dynamic
range. However, to achieve acceptable results it is not always necessary
to operate the spectrum analyzer exactly at its optimum point. To achieve
acceptable results only 2 conditions are needed:
1. The total power at the analyzer input is below overloading the analyzer.
2. The noise floor being measured is equal to or greater than the noise floor
of the analyzer.
Channel loading forces down the highest acceptable level of each individual
channel carrier which gives rise to the need to preselect the channel of
interest. This is necessary when the noise floor being measured is less than
the analyzer noise floor when the total input power is near overload. The
only way to raise the noise floor without overloading the analyzer is to reduce
channel loading through preselection.
When CARRIER/NOISE is pressed the analyzer measures the total power at
the input, the level of the channel to be measured, and the analyzer noise
floor. Based on this information it decides whether to turn on the internal
preamp, and what the analyzer attenuator setting should be. In addition, it
alerts you when it thinks additional attention is needed. This is indicated by
the presence of message (See MORE INFO) in the lower right hand corner
of the screen. This message occurs when the cable noise floor is within 3
or the analyzer noise floor. It is intended to alert the operator that the
analyzer noise-near-noise correction is kicking in. In addition, the message is
presented in inverse video when the cable noise floor is within 2.2
of the
analyzer noise. This indicates the analyzer is correcting the result by more
than 3
and measurement uncertainty of the result is rising. It is intended
that you raise the input level and preselect if necessary.
8-11