Cisco Systems UBR7225VXR specifications OL-17309-02

Chapter 4 Connecting the Cisco uBR7225VXR Router to the Cable Headend

Measuring the Upstream RF Signal

Note Refer to the documentation that accompanied your particular spectrum analyzer for detailed instructions on activating and positioning the trigger line.

A known workaround exists for the Agilent 8591C spectrum analyzer. After activating and positioning the trigger line in video mode, you must press the “video” button on the spectrum analyzer once more to enable proper functionality.

Step 8 Adjust the amplitude on your spectrum analyzer so that the uppermost portion of the upstream RF signal is in the top graticule of the analyzer’s display grid and adjust the trigger line accordingly. Your spectrum analyzer will then display an upstream RF signal similar to the one shown in Figure 4-28.

Note We do not recommend using the spectrum analyzer’s “max-hold” feature while analyzing upstream signals in the frequency domain. “Max-hold” readings in the frequency domain can be inaccurate because the analyzer focuses on the peak power of the strongest ranging modem rather than the power levels of cable modems that are operating in a more ideal range.

Figure 4-28 Measuring the Upstream RF Signal—Accurately Measured Amplitude on Spectrum Analyzer

Step 9 Position a marker about 7/8 of the way into the preamble of the signal, as illustrated in Figure 4-28. (The preamble is the regular pattern displayed at the front of the signal and the length of the preamble is a function of the channel width/data rate, modulation format, and DOCSIS burst-profile configurations.) The peak amplitude of the marker, which registers +31.07 dBmV in this case, will be within 1 dB of the true burst power.

Note To verify this reading, you can also measure the power rating with an Agilent 89441A vector signal analyzer (http://www.tm.agilent.com).

If the preamble of your upstream signal is displayed with a significantly lower amplitude than the rest of the RF signal, refer to the “Using the Zero-Span Method with Adjacent Upstream Channels” section on page 4-28for instructions on how to overcome this phenomenon.

	Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
4-24	OL-17309-02