Wavetek 147 Specifikace stáhnout pdf (Strana 362)

Application Notes: Proofing and Testing

338

range of the field unit is 52 dB. The FCC specification is for CNR > 43 dB.

Most systems design for ~ 48 dB CNR at the end-of-line to account for

house amps, bigger TV screens, and to allow some headroom for digital

loading affects.

Use a bandpass filter and lab amp otherwise, or when doing CSO and CTB.

The FCC specifies that these coherent distortions should be > 51 dBc

(below the reference carrier) for NCTA and > 47 for HRC systems.

NOTE: Stealth firmware versions before 9.3 measured

CTB related to peak distortions when it should be RMS.

This may give a worse reading than anticipated.

CNR can be measured on a modulated carrier, but not scrambled. This is a

gated measurement for in-service tests. If noise is already present on the

signal, or there are no quiet lines, it may be warranted to insert a real “quiet”

line or use another channel.

TIP: Line 25 is recommended for a quiet line insertion

when testing after a setup box.

For more detailed information, log-on to www.tvms.net.

Signal-to-Noise for the Return Path

To calculate the S/N for reverse, find the telemetry level at the headend read

on the field unit, activate the Noise mode and move the marker to the same

frequency. Record the difference.

NOTE: The Spectrum mode RBW is set at 280 kHz and a

VBW > 1 MHz. This is optimized for analog carriers and

burst noise measurements. It has a peak noise detector so

the noise reading may be significantly higher than a normal

spectrum analyzer with the same RBW setting. Don’t do

manual C/N measurements with the SDA unit.

Digital S/N

Set up a "Digital" channel for 280 kHz of measurement bandwidth (MBW).

Set up another "Digital" channel with 280 kHz of MBW at a frequency that is

just noise. Label it "NOIS".

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Wavetek 147 Specifikace Strana 362