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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
Figure4-36 Vector Signal Analyzer Plot of Upstream Data Burst (Preamble Only)
Figure 4-35 and Figure 4-36 illustrate the benefit of properly adjusting the center frequency of your
spectrum analyzer to enable the power measurement of the preamble to match the power
measurement of the rest of the digital transmission.
Figure 4-35 and Figure 4-36 show how adjusting the spectrum analyzer in the time domain reveals
this frequency domain phenomenon. The spectrum analyzer is unable to capture the data as shown
in the vector signal analyzer plots.
The power level in the upstream channel fluctuates by approximately 1dB between Figure 4-35 and
Figure 4-36. This difference is within both the measurement tolerance of the vector signal analyzer
and the accuracy requirement for any DOCSIS-based cable modem.
c. Switch your vector signal analyzer over to Digital Demodulation Mode. Your vector signal analyzer
displays a set of screens similar to those in Figure4-37.
Using this mode to view your upstream signal allows you to view the same time and frequency
domain information found in Figure4-36, as well as the upstream signal’s phase characteristics,
shown in the bottom-right portion of the vector signal analyzer display screen.