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Measuring System Performance: Carrier-to-Noise Measurements

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The Limit feature allows for comparison of the current scan measurement

with the FCC or other conformance/test limits defined during setup and

configuration. There are two parts to this feature:

• When an out-of-tolerance condition exists, a set of annunciators will

appear below the graph. Updated with each scan update, the

annunciators indicate the following out of tolerance conditions:

<CHAN> Adjacent Channel Error

VIDEO ∨ Video Level Too High or Low

DVA ∧ Delta between video and audio too high or low

• You can display an aggregate result summary by pressing the Limit soft

key. This performs a limit check of all channels contained within the

scan, and reports an overall pass/fail conclusion. The aggregate limit

check is not performed with each scan update; however, you can press

the Check soft key to run the test any time. When Limit is off, the

annunciators do not appear.

• When scanning a digital channel, digital signal limits are checked against

the limits you set in the channel plan.

Although it’s not necessary, it is good CATV engineering practice to use a

band pass filter on the input of the SDA-5000 when making C/N

measurements. This ensures accuracy and extends measurement range. If

a preamplifier is used to boost test point levels prior to measurement, it

should be placed between the band pass filter and the SDA unit.

The C/N measurement is simply a comparison in amplitude between the

video carrier reference signal and the noise (FCC limit: > 43 dB). The noise

measurement should be made at a frequency offset (displacement from the

video center frequency) of least 2 to 2.5 MHz from any other carrier on the

system.

The C/N measurement mode displays the carrier-to-noise ratio of the tuned

channel or frequency. You can set C/N measurement bandwidth and the

frequency offset for the noise measurement from the screen. A proprietary

DSP technique allows C/N measurements, in-service, on modulated carriers

(non-scrambled channels). The C/N measurement measures the video

carrier level and then tune to the offset frequency, searching for a quiet line.

After a quiet line is found, the SDA-5000 measures four consecutive frames,

CARRIER-TO-NOISE MEASUREMENTS

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Wavetek 147 Especificações Página 143