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Chapter 4
Connecting the Cisco uBR7225VXR Router to the Cable Headend
Figure 4-40
To get an even better look at the patterns and dramatic shifts in amplitude within the preamble itself, you
can accelerate the sweep time for your zero-span signal processing.
Tune the spectrum analyzer to the original center frequency (32.608 MHz in this example).
Step 6
Reset both the resolution and video bandwidth of the signal back to 3 MHz, but reduce the sweep time
Step 7
from 200 microseconds to 60 microseconds. The resulting display, similar to
the "tight" pattern of the preamble stretched across three-quarters of the spectrum analyzer display.
Figure 4-41
Step 8
Change the center frequency back to 33.248 MHz and both the resolution and video bandwidth values
to 1 MHz, retaining the new sweep time of 60 microseconds. The peak amplitude is clearly displayed
with approximately 4.25 dB difference between the preamble and the rest of the upstream data
transmission. (See
OL-17309-01
Preamble Amplitude Recovery After Center Frequency Adjustment
Original Preamble Viewed with Accelerated Sweep Time
Figure
4-42.)
Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
Measuring the Upstream RF Signal
Figure
4-41, clearly shows
4-35
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Troubleshooting
