HP 8591C User Manual page 273

Test Descriptions
Composite Triple Beat (CTBI Test Description
The beat level is
To accurately determine beat amplitude, the beat level is measured and
measured
compared with the level of the analyzer noise. If the difference between the
beat level and the analyzer noise is less than 10 dB, the beat value cannot be
measured directly; it is instead calculated by using the formula:
where
BP = Beat product f 10
NL = Analyzer noise level -+ 10
With this method of measuring CTB, a preampliEer is not generally needed
for accurate testing.
The beat level is
To find the value for CTB, the beat level is subtracted from the visual carrier
subtracted from the
peak level.
visual carrier peak level
More Discussion about
The carrier-to-beats test is very flexible and can handle a wide variety of
the CSOICTB Tests
testing conditions. It provides feedback messages to assist you in achieving
the optimum test setup.
Like any cable TV active device, a spectrum analyzer has an ideal operating
point which balances C/N and C/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 beat being measured is equal to or greater than 3 dB above the noise
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 cable system noise floor 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.
Beat Level =
floor of the analyzer.