Cable TV Measurements (Option K20) R&S FSL
1300.2519.12 2.56 E-11
C/N measurement
This measurement determines the ratio of signal power and noise power (carrier to noise), a very
important figure of merit.
The signal power is normally the peak power of the modulated vision carrier, which is the power of the
unmodulated vision carrier. You can modify this default setting with the Reference Power softkey (for
details refer to chapter 4, section "Cable TV Measurements (Option K20)").
The power of the noise is internally measured with a small resolution filter, and then translated to a
user–defined bandwidth, the so–called reference noise bandwidth. This bandwidth should normally be
as large as the video signal bandwidth, i.e. 4 MHz for M/NTSC and 5 MHz otherwise.
The measurement can be performed in 3 different ways, as shown in the following examples:
• C/N Off–Service measurement
• C/N In–Service measurement
• C/N Quiet Line measurement
The measurement display is split into two panes. The upper pane displays the spectrum of the
measured noise. The lower pane displays the result table for the measurement and whether the limits
are passed or failed.
C/N Off–Service measurement
Test setup:
Refer to the section Analog TV test setup.
Procedure:
1. Press the SWEEP key.
2. Press the Single Sweep softkey.
3. Press the FREQ key.
4. Enter 210.25 MHz for the vision carrier frequency of the input signal.
5. Press the MEAS key.
6. Press the Analog TV softkey.
7. Press the C/N softkey.
8. To change the limits, press the Edit Table softkey.
9. To adjust the input attenuator, press the Adjust Attenuation softkey.
10. Press the C/N Setup softkey to open the C/N Setup dialog box.
Make sure, that Off–Service is chosen as measurement mode.
Set the Reference Noise Bandwidth to 5 MHz (or another value).
Specify the span for the noise measurement in the table: Change the CF value to 2.5 MHz
(denotes the middle of the span relative to the vision carrier) and the Span value to 5 MHz.
If desired, activate the Noise Floor Correction option.
11. Press the RUN key.
12. Turn on the signal when prompted and confirm by pressing the ENTER key.
The R&S FSL will measure the carrier power. This is not visible on the screen.
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