Chapter 4 Connecting the Cisco uBR7225VXR Router to the Cable Headend
Measuring the Upstream RF Signal
Figure 4-39 Preamble Amplitude Before Center Frequency Adjustment
Note The slight amplitude variations shown in these figures are normal signal level variations between bursts in the upstream channel. Expect modems to vary upstream transmit power by nearly 1 dB between bursts. This is well within the requirements for DOCSIS compliance. The default variation between modems is up to 1.5 dB for most DOCSIS CMTS equipment.
Step 4 Using the examples in Table
In the example, the channel width is 1.6 MHz, which has a symbol rate of 1280 ksym/sec; therefore, the appropriate offset value is 640 kHz.
Step 5 Change the center frequency on the spectrum analyzer by this offset value (33.248 MHz in the example) and check to see that the preamble has regained its lost amplitude by comparing it to the amplitude of the rest of the signal. If so, the spectrum analyzer should display a signal similar to the one in Figure
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