User's Manual

Note in this particular example the “shoulders” on the trace caused by the upconverter being
overdriven. The shoulders were eliminated by reducing the signal to the upconverter. This was
done by increasing the value of the attenuator between the W-CMTS and the upconverter. If
overdriving occurs when the upconverter is internal to the W-CMTS, the shoulders can be
eliminated by reducing the Downstream output power.
The final Hub documentation package should include the trace after the adjustment.
5.7.2 Upstream 12 dB Tap points
Upstream 12 dB tap points are installed at the equipment room end of the IF cables from the
Hub Transceiver immediately at the upstream input to the W-CMTS. Spectrum analyzer traces
should be captured and saved at each of these points.
The 12 dB tap point in the same system’s Upstream #5 is shown in Figure 5-19. This upstream
center frequency is 6.4 MHz.
Figure 5-19 is a 30 second Max Hold
build-up of a lightly loaded sector. Note that the individual
modem bursts are not spikes as this display might suggest. Rather, each is a “standard” QPSK
modulation envelope, 3.2MHz wide, centered at 6.4 MHz. They appear as spikes as the bursts
are very short duration relative to the sweep of the spectrum analyzer “window”. Thus only
portions of the modulation envelope are captured in the display.
See the discussion of Figure 5-20 for an explanation of the bursts with energy greater than 8
MHz.
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0
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2
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Spectrum Analyzer
BVT U5
Frequency (1.4 - 11.4 MHz)
Figure 5-19: Upstream (Receive) input to W-CMTS.
5.7.3 Upstream Quiet Sector W-CMTS Input
During the installation process when there are no subscriber units bursting their upstreams at
the base station, there can be a need to check to ensure that the receiver in the Hub
Transceiver is powered up and operational.
June 2003 Page 5-22