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Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
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Chapter4 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.
Figure4-33 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 4-33 was “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.
Table4-2 Sample Channel Width and Symbol Rate Combinations with Their Respective Minimum Resolution
Bandwidth Measurements
Center Frequency Channel Width Symbol Rate
1/2
Symbol Rate
Center Frequency
+/–1/2 Symbol Rate
Minimum Resolution
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 27.720MHz 300 kHz