Application Notes: Proofing and Testing
338
A
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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