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Downstream RF Measurement in Spectrum Analyzer Mode

To measure the downstream signal power using the spectrum analyzer mode on the HP8591C CATV analyzer, follow this procedure:

  1. Connect the spectrum analyzer to a cable within the plant that carries the downstream signal you are measuring. The signal originates from one of the downstream ports of the HFC Connector Module or the SIM (DS0 through DS3).
  2. View (or set) the output RF power level of the specific interface to be measured. You can view the output power by issuing the show configuration command:
user@host> show configuration interfaces cd-virtual-slot/docsis-slot/downstream-interface 
cable-options downstream
...
rf-power 61;
...

In this example, the output RF power level is set to 61 dBmV. If the rf-power statement is not included in your configuration, the output RF power level defaults to 61 dBmV.

To set the output RF power level for a downstream interface, include the rf-power statement at the [edit interfaces cd-virtual-slot/docsis-slot/downstream-interface cable-options downstream] hierarchy level:

rf-power rf-power;

The downstream interface power level can be from 50 dBmV through 61 dBmV.

  1. Press the MODE key and set the spectrum analyzer to SPECTRUM ANALYZER mode.
  2. Press the FREQUENCY key and enter the desired frequency (for example, 531 MHz).
  3. Press the SPAN key and enter 6 MHz.
  4. Press the BW key and turn video averaging on by selecting VID AVG ON. The default number of averages is 100. You can change the number of averages by using the numeric keypad.
  5. Press the MKR FCTN key (marker function) and select MK NOISE ON. This sets the spectrum analyzer to read out the power bandwidth, normalized to 1 Hz. The spectrum analyzer display should be similar to the display in Figure 41.
  6. The power shown on the display in Figure 41—shown in the top/right and middle/left—is -19.12 dBmV, at 1 Hz. In order to obtain the power in the 6 MHz channel, a correction factor is required. This correction factor equals 10 log(ChannelBW/measurementBW). In this case, 10 log (6x10^6/1) equals 67.78 dB. Therefore, the actual downstream channel power equals (-19.12 dBmV+67.78 dBmV), which equals 48.66 dBmV. Ensure that this power value is approximately equal to the configured downstream power level, minus any attenuation between the HFC Connector Module or SIM downstream port and the point at which your measurement is taken in the cable plant. In this example, assume the attentuation between the CMTS and the measurement point is approximately 13 dB. Therefore, the expected measured value should be approximately 48 dBmV.

Figure 41: Downstream RF Signal (Spectrum Analyzer Mode)

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