Cisco 7246 - uBR Router, Руководство по установке оборудования

Страница: 108 / 238

4-12
Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide
OL-5421-03
Chapter 4 Connecting the Cisco uBR7200 Series Router to the Cable Headend
Connecting and Configuring the Downstream
A second signal appears on the spectrum analyzer’s screen, yielding a display similar to that in
Figure 4-11 .
Figure 4-11 Comparison of IF Input Sweep and RF Output Sweep
Step 5
Qualify the 100-kHz phase noise measurement of your upconverter by comparing the shapes of the two
signals. If the signal is substantially higher (with the peak amplitude measurements lined up), your
upconverter might have high-frequency phase noise, and might be incompatible with digitally modulated
signals. Upconverters suffering from excessive gain, power supply difficulties, or design deficiencies can
introduce unacceptable levels of phase noise into your cable headend. If your comparison resembles
Figure 4-11 , your upconverter is operating properly.
Note
If you suspect that your upconverter is “drifting over time,” place two additional traces on your screen,
“min-hold” and “max-hold,” while comparing your two signals. Should either trace change shape, your
upconverter has failed. This method can help you discover mechanical tapping or vibration-related
problems, which can have an adverse effect on upconverter phase noise.
To be certain of your findings, we recommend that you check your upconverter with a QAM analyzer
capable of displaying constellation phase noise impairments.
Step 6
Attach the spectrum analyzer to the output of your cable interface card and clear the saved trace.
Step 7
Reduce the span on the spectrum analyzer from 100 kHz to 10 kHz and allow the spectrum analyzer to
complete at least one full sweep of the RF output.
Step 8
Save this new trace and return to the RF output of the upconverter (reconnecting the upconverter in the
process). Your analyzer should display a signal similar to the one shown in
Figure 4-12 .