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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
Figure 4-13
Frequencies Lined Up to Illustrate Additive Phase Noise in RF Output
Step 3
Measure the increase in phase noise (the difference in amplitude between the IF and RF output traces)
at 5 kHz from the center frequency to establish a reliable reading. This value is the phase noise
contribution from your upconverter.
Step 4
Compare the value derived in
Step 3
with the minimum specifications for your cable network headend
to get an indication of how well your upconverter is operating.
The spectrum analyzer signals displayed in this procedure are of a high-quality DOCSIS-based
upconverter operating in an “ideal” headend environment. If your particular results reveal significantly
greater levels of phase noise (that is, the skirts of the RF signal are significantly higher than the IF signal
skirts when the peak amplitudes have been lined up), your upconverter might not be suitable for digital
modulation formats.
If possible, double-check your upconverter performance with a specialized analyzer such as a QAM
analyzer or other type of dedicated phase-noise measurement equipment.
Note
If you are concerned about possible upconverter drift or intermittency, try viewing three curves on your
spectrum analyzer: IF plot or “real-time” RF plot; “max-hold” RF plot; and “min-hold” RF plot. If the
IF or “real-time” RF plot differs in shape from the “max-hold” or “min-hold” RF plots, your upconverter
has most likely suffered an intermittency error.
Step 5
Re-establish 64-QAM downstream digital data transmission from the cable interface card by setting the
downstream symbol rate to 5056941 symbols/second on your downstream cable interface. To
accomplish this, enter the following Cisco IOS command in interface configuration mode:
Router(config-if)# cable downstream symbol 5056941
Note
The value 5056941 symbols/second is specified by DOCSIS for 64-QAM transmissions in a 6-MHz
downstream channel plan. For 256-QAM transmission, the value is 5360537 symbols/second.
Step 6
After you determine that your upconverter is suitable for reliable downstream data transmission, proceed
to the following section, “
Completing the Downstream Configuration
.”
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