Cisco Systems UBR7225VXR specifications Using the Zero-Span Method with Adjacent Upstream Channels

Chapter 4 Connecting the Cisco uBR7225VXR Router to the Cable Headend

Measuring the Upstream RF Signal

The importance of this example is to bring to your attention the need for minimal outside plant noise. Time-varying, fast noise can cause bit errors in packet transmissions, rendering your communication link unreliable, if not unusable.

Figure 4-33 Analyzing the Upstream RF Signal—Outside Plant Noise Included

Note This illustration depicts an upstream RF signal whose carrier-to-impulse noise ratio does not meet DOCSIS 1.0 specifications. The data packet in Figure 4-33was “dropped” due to severe noise interference with a more narrow resolution bandwidth.

Using the Zero-Span Method with Adjacent Upstream Channels

When measuring upstream signals using the zero-span method, a very wide resolution and video bandwidth give very accurate readings, but render your readings susceptible to energy in adjacent channels. As the number of upstream services increases, so does the likelihood of interference from adjacent channels. This section describes using the zero-span power measurement method, with a more narrow resolution bandwidth.

Simply narrowing the resolution bandwidth will not yield accurate readings. See Table 4-2.

Table 4-2	Sample Channel Width and Symbol Rate Combinations with Their Respective Minimum Resolution
	Bandwidth Measurements
				1/2	Center Frequency	Minimum Resolution
Center Frequency		Channel Width	Symbol Rate	Symbol Rate	+/–1/2 Symbol Rate	Bandwidth
20.000		200 kHz	160	80	20.080 and 19.020 MHz	10 kHz
30.000		400 kHz	320	160	30.160 and 29.840 MHz	30 kHz
40.000		800 kHz	640	320	40.320 and 39.680 MHz	100 kHz
25.000		1.6 MHz	1280	640	25.640 and 24.360 MHz	100 kHz
28.000		3.2 MHz	2560	1280	29.280 and 27.720 MHz	300 kHz

	Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
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