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Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
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Chapter4 Connecting the Cisco uBR7225VXR Router to the Cable Headend
Measuring the Upstream RF Signal
Figure4-35 Vector Signal Analyzer Plot of Upstream Data Burst
The upper graph in Figure4-35 represents the frequency domain and the lower graph represents the
time domain.
In the time domain, the channel power of the preamble of a digital upstream signal is not spread
across the entire channel. However, the channel power of the remainder of the digital transmission
is spread across the entire channel. Even though it may not seem so, the total channel power across
both the preamble and the subsequent data segment remains constant.
b. Narrow the view on your vector signal analyzer to display only the preamble of the digital data
signal in both the frequency domain and time domain.
The upper display in Figure 4-36 is a plot of only the preamble portion of the digital signal in
Figure 4-35. Notice how the amplitude of the signal experiences many “peaks” and “valleys.” When
you are measuring the preamble power using the zero-span method, be sure that you measure the
actual signal energy (a peak), rather than accidentally measuring the power level of a valley in the
preamble.