Cisco 7246 - uBR Router Hardware Installation Manual page 139

Chapter 4 Connecting the Cisco uBR7200 Series Router to the Cable Headend
Figure 4-48
Figure 4-47
spectrum analyzer to enable the power measurement of the preamble to match the power
measurement of the rest of the digital transmission.
Figure 4-47
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 1 dB between
Figure
and the accuracy requirement for any DOCSIS-based cable modem.
Switch your vector signal analyzer over to Digital Demodulation Mode. Your vector signal analyzer
c.
displays a set of screens similar to those in
Using this mode to view your upstream signal allows you to view the same time and frequency
domain information found in
shown in the bottom-right portion of the vector signal analyzer display screen.
OL-5421-03
Vector Signal Analyzer Plot of Upstream Data Burst (Preamble Only)
and Figure 4-48 illustrate the benefit of properly adjusting the center frequency of your
and Figure 4-48 show how adjusting the spectrum analyzer in the time domain reveals
4-48. This difference is within both the measurement tolerance of the vector signal analyzer
Figure
Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide
Figure 4-49.
4-48, as well as the upstream signal's phase characteristics,
Measuring the Upstream RF Signal
Figure 4-47
4-43
and