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RF Measurements
Upstream RF Measurement
137Upstream RF Measurement
To measure an upstream signal to the CMTS using zero span mode on the HP8591C CATV
analyzer, follow this procedure:
1. Connect the spectrum analyzer to a cable within th e plant that carries the upstream
signal you are measuring. The signals are received on one of the upstream po rts of the
HFC Connector Module or SIM (US0 through US3).
2. Press the FREQ key and enter the center frequency that corresponds to the upstream
frequency you are measuring. You can view the upstream frequency by issuing the show
configuration command:
user@host> show configuration interfaces cu-virtual-slot/docsis-slot/upstream-interface
cable-options upstream
...
frequency 9m;
...
In this example, the upstream frequency is set to 9 MHz.
3. Press the SPAN key and enter 0 MHz (or select ZERO SPAN). This sets the spectrum
analyzer to zero span mode, which means that signals will be dis played in the time
domain.
4. Press the BW key (bandwidth), select RES BW MAN (resolution bandwidth manual), and
enter 3 MHz.
5. While in the BW key menu, select VID BW MAN (video bandwidth manual), and enter
3MHz.
6. Press the AMPLITUDE key, select ATTEN MAN (attenuation manual), an d enter 0 dB. This
removes all internal spectrum analyzer attenuation.
7. While in the AMPLITUDE screen, select REF LVL (reference level), and enter a value
slightly greater than the maximum power level you a re expecting. The reference level is
the power represented by the top graticule line in the display. Assume the reference level
is set to 5 dBmV.
8. Select SCALE and adjust the scale so t hat the signal spans the entire Y-axis of the display.
9. Press the TRIG key (trigger), select VIDEO, and adjust the trigger line to within one
graticule of the peak of the signal.
10. P ress t he SWEEP key, select SWP TIME MAN (sweep time manual), and set the sweep
time to a value in the range of 80 through 100 microseconds.
DOCSIS specifies that cable modems use TDMA (time
division multiple access) for upstream transmissions,
which means that cable modems are not continuously
transmitting. In order to facilitate the triggering and
capture of upstream signals, the cable modems should be
transmitting long packets as often as possible.