Cisco Systems UBR7225VXR specifications OL-17309-02

Chapter 4 Connecting the Cisco uBR7225VXR Router to the Cable Headend

Measuring the Upstream RF Signal

Figure 4-36 Vector Signal Analyzer Plot of Upstream Data Burst (Preamble Only)

Figure 4-35and Figure 4-36illustrate the benefit of properly adjusting the center frequency of your spectrum analyzer to enable the power measurement of the preamble to match the power measurement of the rest of the digital transmission.

Figure 4-35and Figure 4-36show how adjusting the spectrum analyzer in the time domain reveals this frequency domain phenomenon. The spectrum analyzer is unable to capture the data as shown in the vector signal analyzer plots.

The power level in the upstream channel fluctuates by approximately 1 dB between Figure 4-35and Figure 4-36. This difference is within both the measurement tolerance of the vector signal analyzer and the accuracy requirement for any DOCSIS-based cable modem.

c.Switch your vector signal analyzer over to Digital Demodulation Mode. Your vector signal analyzer displays a set of screens similar to those in Figure 4-37.

Using this mode to view your upstream signal allows you to view the same time and frequency domain information found in Figure 4-36, as well as the upstream signal’s phase characteristics, shown in the bottom-right portion of the vector signal analyzer display screen.

	Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
OL-17309-02		4-31