Cisco 7246 - uBR Router Hardware Installation Manual page 138

Chapter 4 Connecting the Cisco uBR7200 Series Router to the Cable Headend
Measuring the Upstream RF Signal
Figure 4-47 Vector Signal Analyzer Plot of Upstream Data Burst
The upper graph in Figure 4-47 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.
Narrow the view on your vector signal analyzer to display only the preamble of the digital data
b.
signal in both the frequency domain and time domain.
The upper display in Figure 4-48 is a plot of only the preamble portion of the digital signal in
Figure 4-47. 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.
Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide
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