Cable TV Measurements (Option K20) R&S FSL
1300.2519.12 2.58 E-11
10. Press the C/N Setup softkey to open the C/N Setup dialog box.
Make sure, that In–Service is chosen as measurement mode.
Set the Reference Noise Bandwidth to 5 MHz (or another value).
Specify the span for the noise measurement: Change the CF value, which denotes the middle of
t
he span relative to the vision carrier and the Span value in this table. Chose the value CF in a
way, that the span is centered on the gap between 2 analog TV signals. The default value of
–1.25 MHz should be suitable in most cases.
If desired, activate the Noise Floor Correction option.
11. Press the RUN key.
The R&S FSL will first measure the carrier power. This is not visible on the screen. In contrast to
the Off–Service mode, the R&S FSL will not ask you to turn off the signal in the measurement
channel. The noise spectrum is measured in the gap between two active channels. The R&S FSL
measures the noise by sweeping with a small resolution filter in the span you defined in step 10.
12. A marker was automatically set by the R&S FSL. To move this marker, press the MKR key and turn
the rotary knob. The noise density at this marker is measured and translated to the reference noise
bandwidth you specified in step 10.
Fig. 2-39 C/N In–Service measurement
The lower pane shows the final C/N ratio and whether it is passed or failed. It also shows the most
important correction factors used to calculate the C/N ratio.
In In–Service mode the noise reading will be higher and therefore the C/N ratio lower and not as
accurate as in Off–Service mode, due to the active channels. The resulting C/N ratio is just an upper
bound of the real value: If your network passes this quick measurement, it should also pass the more
accurate Off–Service–measurement.
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